At observere med en 20″ (på engelsk)

Owing a 20” F/5 Obsession Telescope.

 

The Dream of the Ultimate Astronomical Telescope – My Personal Experience

By Claus Bagger, Birkerød, Denmark

Last updated February 3rd, 2020

 

 

 

Content :

 

1. Introduction

2. The choice between a refractor and a reflector

3. The choice between equatorial or alt/azimuth drive system

4. The choice of optics

5. Mechanical characteristics

6. Potential telescopes

7. The final choice of a telescope

8. Economical aspects

9. Purchase and transportation to Denmark

10. First assembly

11. Using the 20” Obsession

12. Problems experienced with the telescope system

13. Conclusion

 

 

 

1. Introduction

I have been fascinated by seeing astronomical objects with my own eyes since I was 15 years old. Despite the ease of making astronomical images with a camera, the pleasure of picking up small or faint details with your naked eye has always been thrilling to me. That’s why visual observations of deep sky objects and planets will always have my first priority.

After having own telescopes for more than 30 years, I decided to go for the ultimate telescope of my life. This telescope should be able to satisfy my requirements and wishes for the rest of my life. No more, no less.

The following describes the major considerations I made before I chose to purchase my large telescope. It describes the purchase of the scope and some of the experiences I have had since I got it, good as well as bad.

If you have not done so already, I hope the description can also inspire you to purchase your own dream scope, be it an Obsession or another telescope.

 

2. The choice between a refractor and a reflector

People have various reasons for their choice of a telescope. Some prefer the aesthetic appearance of a beautifully crafted telescope, even for a small telescope. Some prefer the point like star appearance that a refractor displays, even though the stars may display some degree of colour away from the optical axis.

Amateur astronomers soon discover that the larger the telescope they use, the  more stars and nebula they see. Unfortunately, a refracting telescope becomes exponentially more expensive with the diameter of the lens. Furthermore, they are usually made with F-ratios of 10 or more, making them quite heavy and bulky relative to their diameter. Contrary to this, reflecting telescopes with large apertures can be made at reasonable prices, and even very large aperture telescopes are sold with F-ratios as low as 4. Such low F-ratios makes a telescope lighter and more compact relative to its diameter.

Furthermore, when the optical surface area increases, you get a better optical resolution, which enables a higher magnification and the possibility of viewing smaller details.

So, all in all, weather you are interested in observing planets, double stars or deep sky objects, a larger telescope will always have the potential to give you more and smaller details.

 

3. The choice between equatorial or alt/azimuth drive system

If your main interest is astro photography, you will need a very precise and sturdy equatorial mount equipped with a precise clock and control system. This will set certain demands, but also limitations to the size of your telescope, because there is a natural limit to the weigh of a telescope a certain mount can hold. Due to the heavy weight of a sturdy mount, most people who photograph will place their telescope system in an observatory.

If you are interested in visual observations, the requirements to a study mount decrease. At the same time, you don’t need a telescope with an equatorial mount since you don’t need the precise tracking ability. This opens up the possibility to operate the telescope in azimuth/altitude mode, and with the development of modern type computer control systems, the telescope can operate with full GO TO functionality and good tracking stability. This is where modern type Dobsonian telescopes come into the picture. These are telescopes that are operated entirely in azimuth/altitude mode, are relatively sturdy but still easy to assemble and disassemble and enable a good ventilation of the optics for improved equilibrium with the surrounding air. Furthermore, the optics is placed in a very stable, but still easily adjustable mirror cell.

Based on the above mentioned considerations, I choose to look for a good reflecting telescope.

Quite a few considerations were made before I purchased and they are discussed in the following.

 

4. The choice of optics

4.1. Resolution

One basic requirement for my choice of optics was that it should be diffraction limited. This requirement is easy to fulfil for smaller optical systems, but much more difficult for larger optical systems. Diffraction limited means that the following measures or calculated values should be satisfied :

  • Root sum square (RMS) < 0,075 (≈1/14 λ, the wavelenght of light).
  • Strehl-ratio > 0,8
  • Peak to valley < 0,25

 

4.2. Diameter of optics

My major wish was a telescope with a large, optical surface with a diameter in the range 16 to 25 inches. The main reason for this was my wish to observe as faint deep sky objects as possible. The telescope should potentially enable observations for the rest of my life. Don’t forget that in Denmark we only have 60-70 clear nights a year, giving a total of  70 times 25 years equal to 1750 observations in total. The number 25 is the maximum number of years I anticipate that I will be actively observering after the telescope purchase.

Table 1 is from D. Krieges very informative book about building a Dobsonian telescope. As you can see, the number of observable galaxies in telescopes in the 16-25” range is 10,000 or more. Enough for the rest of your life. From the table it can also be seen that you actually don’t gain a lot in limiting magnitude by increasing the aperture by e.g. 5” from a 20” telescope.

Due to the reasons mentioned earlier, only reflecting telescopes are realistic to consider in the size range from 16” and up. Realistic alternatives are therefore Newtonian principles or Schmith Cassegrain telescopes for visual astronomy.

A Schmidt Cassegrain telescope in the 16” range is only manufactured by Meade, but I quickly realised that this solution would be too expensive for me.

 

4.3.  Focal length

Only a handful of deep sky objects require a field of view of more than 1 degree. With a Nagler eyepiece of 31 mm focal length and a telescopic focal length of about 2 m, the sky field of view is close to one degree, enabling nice observations of the majority of deep sky objects in the heaven.

Having given up the idea of a large Schmidt Cassegrain system, my next though was to look at Newtonian telescopes based on the Dobsonian design. With this design, it is easy to look up the physical dimensions of various telescopes from vendors of this design. You quickly realise that you have to climb high in the air with a very large Dobsonian. How high are you willing to climb in the dark ? This can help you sort out what focal length fits you best.

An important aspect in connection with focal length is the resultant F-ratio of the telescope. An F/4 or faster optical system will result is severe coma, whereas an F/6 or longer has quite little coma. A small F-ratio telescope will also require a larger secondary mirror to capture all light from the main mirror than a longer telescope. A larger secondary mirror will decrease the (aspect) ratio between the main mirror and the secondary mirror, decreasing the contrast and slightly decreasing the resolving power. Thus, if you are interested in observing planets, a small secondary mirror is preferable.

 

5. Mechanical characteristics

 I have been observing with an mechanically unstable Dobsonian telescope for several years, so I had a clear requirement for my next scope : it should be so mechanical stable that I was able to observe at high magnification with only minor vibrations visible when the telescope is touched or moved.

I also wanted a collimation principle which enables easy and precise collimation of both the secondary and primary mirror. The optics should also be shielded from stray light with a shroud.

Finally, I wanted a mechanical system that is easy to transport from one place to another.

 

5.1. Control system

Since I was used to observe with a Dobsonian telescope without any control or drive system, virtually any control system would be an improvement and therefore this topic was not given much attention at the time of order/purchase. Besides having the ability to track stars, my only wish was to have a hand pad with the possibility to fine adjust the movements.

 

5.2. Build or buy your dream telescope?

Not just the prices of telescopes matters when considering the telescope of your future. I can warmly recommend anybody interested in Dobsonian Telescopes to read the book “The Dobsonian Telescope”  by David Griege and Richard Berry (William Bell, 2001). This book explains many details about owning and building your own telescope. Originally, I considered building my own telescope, which I was sure I could also do after having read the book. I also figured out that this accomplishment would consume much more time than I was willing to spend. Adding all the expenses for optics and standard component like spider vane etc. up, I came to the conclusion that I would rather pay for this workmanship.

If you choose to build your own Dobsonian, you can add digital setting circles and a control system afterwards, e.g. with Argo Navis and ServoCat, respectively. You may also choose to use control software available free of charge on the Internet, e.g. Mel Bartels software (https://www.bbastrodesigns.com/BBAstroDesigns.html#Computer_Operated_Telescopes). You have to supply the hardware yourself (stepper motors, setting circles, etc.), and build the mechanics, but then you are up and running for only a few dollars.

Again, I chose the easy solution, to buy something on the market.

 

6. Potential telescopes

Based on the above mentioned considerations, I chose to concentrate of an 20” Dobsonian with an F-ratio of 5. This would give a huge light collecting area with an acceptable level of coma.

At the time of purchase consideration (beginning of 2005), I found only a relatively few interesting alternatives to choose between. The following companies sold Dobsonian telescopes with computer control : Starmaster, Johnsonian, Obsession and Reginato. The first three are American telescopes manufactures and the last one Italian.

 

Starmaster offered a 20” F5.4 at a cost of approx. 7000 $ plus 2500 $ for the control system.

Johnsonian made several types of poncet platforms, but in 2005 offered a very interesting GO-TO telescope, see two pictures at right. The telescope beams were made of carbon which is lightweight, yet very strong, and was made in such a way that it could be collapsed to take up virtually no space  as shown at right! Scopes were made in versions of 15”, 18” and 22”. The price tag for a 18” was somewhat higher than a 20” from Starmaster (or Obsession) and since super transportability was not that important for me I decided not to go further with this concept.  These nice scopes have since been taken off the marked.

Reginato

This Italian company made some interesting motorized alt/azimuth telescopes with apertures in the range 16 to 30”. In 2005, the price in Europe for a 20” F/4 was 18,000 € or about 20,000 $  (about 135.000 DKK, including customs and taxes)

Obsession offered a 20” F/5 at a price of 6000 $ plus 2900 $ for the control system, i.e. a little bit cheaper than the Starmaster system.

When adding Danish customs and taxes for goods purchased outside EU (32 %), the prices for a telescope from Starmaster and Obsession was considerably lower than Reginato. Thus, the final choice had shrunk to a combat between Starmaster and Obsession.

 

7. The final choice of telescope

When you read Obsessions Telescopes homepage, it is difficult not to be influenced by the enormous number of positive comments about Obsession Telescopes from their respective telescope owners. These comments alone could make me bye an Obsession Telescope. When you study the claims about Obsession Telescopes and their advantages over other types of telescopes, many other advantages can be mentioned :

1)    The telescope itself is beautifully and aesthetically crafted and assembled. Only the stiffest types of plywood are used. All wood is painted with a strong, frost resistant lacquer.

2)    All effort has been put into making the telescope as easy as possible not only to use, but also to transport.

3)    The main mirror and secondary mirrors are made by some of the best manufactures in the field with not only very good specifications, but also actual superb performance.

4)    The main mirror is fixed on a rock steady back plate, fixed on 18 point on 6 laser cut stainless steel plates.  Still very easy and precise adjustment is possible.

5)    The secondary mirror is easy to adjust precisely with the finger screws attached to it. The mirror can be heated with a wire, so dew is avoided.

6)    The range of F/5 telescopes were sold with main mirrors from stock, which means that a telescope was delivered within 2 months from the time of order. Very quick.

 

My final choice was a 20” F/5 Obsession with a focal length of 2575 mm. The main mirror has a 96% aluminium coating, whereas the secondary mirror is 80 mm (minor axis) and has a multi-dielectric aluminium coating with a 98% reflectivity at 550 nm.

The telescope weights 51 kg, but the effective lifting force is only 12 kg due to the handles that come with it. I ordered it with a 2” feathertouch eyepiece holder, which has a 1:10 gearing and a 7×50 Antares 90 degree view finder with field erected conversion (you see it erected as seen by the naked eye, except at an angle of 90 degrees).

The telescope was furthermore ordered with Argo Navis digital setting circles with 8000 point encoders and ServoCAT GoTo control system.

 

8. Economical aspects

Below, the various expenses are outlined (2005):

Obsession 20” F/5 Dobsonian telescope 5995 $
Light shroud 159 $
View finder, 7×50 Antares 150 $
Focuser Feathertouch 325 $
GoTo drive , ServoCAT with Argo Navis system 2895 $
Total amount (excl. shipping and crating) 9524 $
Transportation (DHL) 1110 $
Total expences before import to Denmark 10634 $
Total expensesn after import to Denmark (32 % tax) 14035 $ (equal to 87.000 DKK)

 

 

9. Purchase and transportation to Denmark

Generally, the level of service from Obsession was good. David Kriege responses personally and very quickly to your questions.

Since the cost of transportation was very high, I asked Kriege about the possibility to transport by cheaper (and of course slower) means. That was not an option, I was told, which I think was too bad. Of course, the advantage with the very high transportation cost was the fast delivery to Denmark, only four days, but in Denmark you also pay customs and taxes on transportation expenses.

 

10. First assembly

The telescope was picked up at the customs at Copenhagen Airport Kastrup and the six large carton boxes driven to Birkerød with a trailer. Here I spent an evening opening up the solid boxes.

All parts were thoroughly packed with foam injected into the boxes around the telescope parts so all gaps were filled out.

Obsession supplied a good manual for assembling all parts and during the assembly evening everything was put together. The secondary mirror holder requires special attention, so it is positioned correctly relatively to the mechanical axis of the telescope. A wire for turning the altitude axis has to be fitted around the motor and the altitude encoder has to be assembled. A black dot for placement over an etched cross at the centre of the main mirror was supplied and put in place.

The same evening I enjoyed “first light” with a grand view of the Ring Nebula. The weak central star was seen pin point sharp. But it took a couple of days before the drive system was up and running and I could enjoy high power magnification observing.

 

11. Using the 20” Obsession

The overall system consists of three vendors products :

1)        Obsession telescope

2)        Argo Navis digital setting circles

3)        ServoCat computer control with GO TO functionality

 

When purchased from Obsession, these parts come pre-assembled and with default settings for a 20” telescope, so everything works more or less from the beginning. A sign had to be reversed in the Argo Navis encoder settings and I had to discover that two star alignment should be done with the first star being the most westernly positioned.

The three parts works well together. One disadvantage of having three separate products is that the documentation is clearly made by three different partners (more on this later).

The responsibility for spare parts is not always clear, e.g. concerning delivery of cables between the various parts.

 

11.1. Mechanical performance

 Mechanical performance consists of overall stability and ability to resist vibration, mechanical performance in connection with finding an object and ability to track an object when found.

 

Mechanical stability

The telescope is very stable and even at very high magnification it can easily withstand relatively strong winds for visual observations. The telescope dampens within a few seconds if struck by a wind. When the telescope is focussed, no noticeable vibration is seen in the view.

The telescope came very fine balanced from Obsession, and no further balancing has been required. But this only accounts for a system where the motor is engaged. Both motors are very strong, and the altitude motor can easily hold a heavy Nagler eyepiece plus a Powermate. If the altitude motor is not engaged, the telescope loses its balance if you observe close to the horizon and if you use the above mentioned eyepiece combination.

The mechanics around the secondary mirror is well built, so collimation is not necessarily needed between two observing sessions, if only low magnification observing is performed.

 

Pointing stability

The mechanical pointing stability is mainly determined by the mechanical system, and it performs quite well. With a 31 mm Nagler eyepiece (50 arc minute field of view), the object is normally within 10 arc minutes or better from the centre of the field of view.

The Argo Navis software also determines the precision of pointing. With this software, you can improve the precision by pointing to dozens or even hundreds of stars and compensate through mathematical modelling performed by the software. I have only done this once with 30 stars around the sky, and I did experience some improvement in pointing stability.

 

Tracking stability, hysteresis at fine adjustment

The ability to track objects is a combination of the smoothness of the mechanics and the software. For photography, closed-loop tracking is possible with Argo Navis and Servocat in combination, otherwise tracking is open-looped. I have been positively surprised by the incredible stability you can obtain with this system after some fine adjustment with the small ServoCat PC-program. Even at 1000 times enlargement, the object will stay close to the centre for about 10 minutes before it starts to drift away.

Another surprise for me is the stability of tracking. If you observe or photographs planets with a web-cam, you can see variations (slack) in the drives in the order of 30”. For these purposes, very accurate two-star alignment is required. For deep sky purposes, this tracking variation is negligible.

Due to the relatively simple mechanical principle of turning the telescope, it has some back-lash when you change the slewing direction. For visual observations the level is acceptable and is not noticeable until you use several hundred times enlargement. Software compensation enables some reduction of this problem (see description under software).

 

11.2. Optical performance

 

General optical performance : contrast, coma

Below, the measured optical quality of the main mirror is shown. As seen, the values are as follows :

RMS   0.031

Strehl ratio  0.96 and

Peak-to-valley 0.24

 

These value are all a little worse than the average values for a 20” Omi Torus mirror with values of RMS 0,03, strehl ratio 0,97 and peak-to-valley 0,18.

 

 

Resolving power

The resolving power of the telescope has been critically assessed by observing double stars, please refer to this post. These tests showed that the telescope can completely resolve stars only 0″.5 apart, which is very satisfying.

The theoretical resolution is 0.28 arc seconds and this value will never be possible to obtain by visual means due to weather conditions and bad seeing. An indication of resolving power may be obtained by photographing with a web-camera, but a lot of other factors also influence the result here. At least the minimum resolving power can be estimated this way.

If the web-cam images of Mars taken during the 2005 apparition are studied, see image at left, details as small as about 0.5 arc seconds can be distinguished.

An enormous amount of detail can be seen on Jupiter. I have not had a chance to observe Jupiter high in the sky, so the drawing at left was made with Jupiter hanging just 20 degrees above the southern horizon. Still, very small details could be seen.

Another evident way to judge the resolving power of the 20” Obsession is the high magnifications this telescope can operate with and still attain razor sharp images. In the beginning, I was hesitant to go much above 500 times magnification. But as you discover the potential of the telescope, you find out that sharp images are possible at magnifications above 1000 times. Only once have I used 1290X, but I regularly observe at 800 times with my 13 mm Ethos in combination with a 4 times Powermate.

 

Limiting magnitude

In order to determine the limiting magnitude of the telescope, I have used a reference star map with visual magnitude indications around M57. At 368X, a star of magnitude 16.8 has been spotted several times, and the limiting magnitude is therefore close to 17th magnitude, very satisfying.

 

Optical stability due to thermal non-equilibrium

My telescope is permanently stored in an open ventilated carport. For high power deep sky observations, the sky turbulence is normally the limiting factor, not thermal equilibrium. But for planet observations, even a slight temperature difference of e.g. one degree between mirror and surrounding air becomes very evident. This can be studied directly by de-focussing the planet. De-focussing in one direction will show the main mirror air turbulence and de-focussing in the opposite direction will show the turbulence in the atmosphere.

If quick changes take place in temperature, e.g. shortly after dawn, the large volume of the main mirror cannot cope with this un-equilibrium state and will constantly be emitting heat. In critical situations, the main mirror should be cooled from behind with the pre-installed fan that runs with power from a battery.

 

11.3. Software

The Argo Navis and ServoCat work well in combination. Basically what they do is that the Argo Navis determines a position (either the present position or requested position) and the ServoCat can then move to this position, typically through a GO-TO call on the hand pad. The ServoCat database contains thousands of deep sky objects and you can download up to 1000 user specified objects. Because it does not contain the PK or PN catalogue of planetary nebula, I have downloaded the ESO catalogue of planetary nebula onto the hand pad.

You may also use a planetarium program to generate object positions through the ASCOM interface protocol. I use the planetarium program SkyTools professional 3 from SkyHound  and the telescope can GO-TO a position generated by this program. In the program you can see the present telescope position indicated.

 

Argo Navis software (firmware)

The software and control unit from StellarCat comes pre-installed and works in default mode for your telescope size and type.

The software is easy to use and well documented in the supplied manual. Basic as well as complicated matters are systematically explained. The firmware can be updated with new software releases, but the process is not straightforward and does not use standard PC-oriented approaches. A thorough reading of the manual is therefore required.

In Firmware version 2.0, complicated types of compensation for pointing errors can be modelled and computed. You can pick any number of brighter than 6m star for building up the model, the more stars, the more accurate.

I have only tried this once with 30 stars and without performing any systematic analysis of the effect, I have experienced a more accurate pointing ability. Usually the object is within 10 arc minutes of the centre or better.

 

ServoCat software

A small PC program is supplied with the ServoCat. As explained in the manual, a strict order of actions is required to make it communicate with your PC through a special serial cable supplied. This way you can fine adjust the tracking speed to accommodate for your telescopes tolerances. You can also control the step size and level of back-lash compensation. This can be controlled in single steps of movement for each axis in both directions.

 

12. Problems experienced with the telescope system

12.1. Finder Scope

Generally, the supplied Antares finder scope display a clear image with sharp and faint stars. But, the scope is not easy to fix in the snap lock fixed to the telescope. A better guide in the snap should have been made.

After just one year of operation, the backside of the diagonal became rusted, which is aesthetically not nice to look at comparing to the rest of the telescope. Now it is completely rusty.

 

12.2. Argo Navis

As mentioned earlier, it is best to star align with the western most star first, otherwise the second alignment star may fail to register probably. This has been experienced several times. If this happens, you will need to re-collimate the optics since the scope has to be moved to vertical position for initialization.

During use, the screw to the altitude encoder may loosen, and then the encoder does not register any signal when the telescope is mowed. If you cannot star align after several attempts, this may be the problem.

During the first year of operation, the encoder signal cord lost signal transmission ability completely. It was replaced at no expense by Obsession Telescope.

The encoder PCB’s (printed circuit boards) are placed too unprotected on the telescope. The small board holds the signal cord and this board should be better protected. When you secure the cord to the board, make sure the cord cannot twist the board any way if by mistake you pull the cord. For some reason I am not able to explain, I managed to pull the azimuth cord so hard that the PCB-board broke !   Somehow, the cord has been pulled during transportation or use of the telescope.

A replacement was easy to get from StallarCat, but you need to take the entire bottom plate apart in order to change it.

 

12.3. ServoCat

After having used the cord between the ServoCat unit and the remote control for about a year, the socket on the ServoCat was slowly worn out and it became loose.  Normally, you send a MOVE signal to the ServoCat when you press the remote and the telescope will then move until you stop pressing the remote. But if the cable socket becomes loose, signal transmission is lost. When this happens, you may send a MOVE signal to the ServoCat and when you stop pressing the remote, the telescope continues to move !    This is really an awful experience for several reasons. First, you may completely loose the object you have spent lots of time to identify at high magnification. Secondly, when I was once observing an object close to the horizon and wanted to move downward, the telescope tubes simply crashed into the rocker box because it could not stop. This happened at medium moving speed and shook the entire telescope seriously. Fortunately, no damaged occurred.

Due to these cable and socked problems, I have now changed to a cordless remote control.

Over the years, the connectors on both motor axis have malfunctioned and have had to be replaced by new. Each time, the PCD with the connector brazed to it has to be sent to ServoCat for repair which has always been easy, but of course it is annoying to miss your telescope during the repair period. Apart from the connectors not being outdoor friendly, the climate in my outdoor storage room is very humid. Furhermore, I live only 10 km from the coast and Denmark is surrounded by sea on all sides, so the air contains salt residuals.

 

12.4. Omi Torus main mirror

The same day the main mirror was received, I noted that the coating process had left an irregular blue pattern on top of the mirror surface. Later on, I discovered that this does not affect reflectivity, but if the mirror becomes wet from moisture or dew, the pattern is very evident and aesthetically not pleasant to look at when you expect to see a completely blank surface.

Due to normal temperature changes during the day and night, dew may develop on the surface of the main mirror. If larger amounts of dew develop, it may start to flow down the surface and collect in small “pools” around the surface. Eventually these may solidify, leaving concentrated sulphur and other acids left on the surface that corrodes the coating. Unfortunately, I have experienced the result of several of these islands of acids on my mirror, because they leave a coating-free surface on the glass ! A terrible experience.

A way to avoid this it to install a small lamp inside the rocker box that heats the mirror to above the dew point. After I did this about a year ago, the corrosion process has seized to develop.

Obsession has designed their telescope to being able to observer objects down to the horizon when used without computer drives. Due to the wire used for moving the altitude axis, the attachment of this wire restricts the movement to about 6 degrees above the horizon. To counteract this, the telescope may be placed on an inclined surface, and this way I have been able to observe objects to about three degrees above the horizon.

 

 

13. Conclusion

Overall, the computer controlled Obsession telescope system consisting of Obsession mechanics and optics, ServoCat motor control system and Argo Navis digital setting circles works very well and is a great pleasure to use. Since purchased in August 2005, the system has given me more than a thousand positive observations and many memorable and exiting experiences with cheer joy.

The entire system is easy to use and has the flexibility of being ready for use within 5 minutes without computer control for simple observation or 25 minutes with the computer system attached.

For visual observations, the system is a marvel and performs exceedingly well mechanically as well as optically. It tracks fine and precisely at above 1000 times magnification without problems or any indication of vibration. Incredible fine details can be seen, so the telescope is perfect for planetary observations or observations of small objects. At the same time, it can show extremely faint details in deep sky objects and many objects look like their photographic equivalents like the Veil nebula, Swan nebula and many globular clusters and open star clusters.

A few problems have been experienced along the way. The encoder PCB’s should be better protected so it does not accidentally brake. The sockets for the ServoCat cord should also be improved so it is not so easily worn out. It would be nice with some clear recommendations for protecting the main mirror against acid corrosion, e.g. by using a heat element after observation and/or during the morning.

With the few available clear nights in Denmark each year (about 60), the telescope has the potential for observations for the rest of my life. And without reservation I can warmly recommend anybody to purchase a computer controlled Obsession Telescope. I have already obtained my dream telescope.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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