Category: Equipment

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Criteria choosing my eyepieces

1. High Quality
I wanted high quality eyepieces, for two reasons. The first reason was that good eyepieces stay with you forever, even when you buy and sell telescopes in the future, the eyepieces stay with you. The second reason was I had the money for buying them.

2. Large true fields of view
If you have a push-to telescope (you follow the object by pushing the telescope manually) you want fields of view that are as large as possible. The higher the magnification the more important this becomes. If for instance you watch the Eskimo Nebula at a magnification of 600x, it virtually flies through your field of view, so the larger the field of view is, the longer you can concentrate on observing before having to move the telescope again.

When I bought the Orion Optics 12-inch, I only had a set of Vixen lanthanum LV eyepieces with an apparent field of view of 50 degrees. Compare that to a Pentax XW eyepiece with a field of view of 70 degrees, you will see that the Pentax's field of view is twice as big, in other words, at the same focal length and magnification, it will take an object twice as long to cross the field of view when using a 70 degree eyepiece instead of a 50 degree eyepiece. So I was looking for high-quality eyepieces with large fields of view. These two criteria already narrowed the search down to Televue Naglers and Panoptics, Pentax XW's or Vixen Lanthanum LVW's.

3. Weight
The Orion-Optics UK dobsonian is a lightweight instrument. I wanted eyepieces that were in more or less the same weight-class to prevent problems with balancing the scope and adding an optional friction brake, which has to be adjusted when switching eyepieces of different weight. So I wanted eyepieces that weighed between 400 and 800 grams.

4. Eye-relief
The last criterion for me was the most important, lots of eye-relief. I always observe wearing glasses. First of all, I have astigmatism. The Televue Dioptrx was no option for me. I tried one for a while, but they can only be used on Televue eyepieces. On top of that, every time I want to look in my star-atlas or make a sketch of the object, I have to put on my glasses anyway, so I want to keep them on all night while observing. With glasses you need eyepieces with lots of eye-relief, preferably 17mm or better (some people find 15mm also OK).

The four criteria narrowed down the search, and in fact only a handful of eyepieces met all my wishes. So in the end, the choice wasn't that difficult at all.


The set of eyepieces I chose



In the first column you will find the eyepieces I chose for my Orion Optics 300mm Dobson. In the second column you see the apparent field of view in degrees, in the third column the magnification of the eyepiece when used in combination with my 12-inch f/5.3 telescope (focal length 1600 mm), in the fourth column the true field of view in arc minutes and in the last column the eye relief in millimeters.

The 35mm panoptic is the eyepiece with the lowest magnification and a large field of view. The three Type 4 Naglers are for medium magnifications. The three Pentax XW's are for high power viewing.

Why I chose these eyepieces

The 35mm Panoptic


This eyepiece is for low power viewing. When searching for objects or observing large objects, this is my favorite eyepiece. The 35mm Panoptic is a wonderful eyepiece. The views of larger objects like the double cluster in Perseus or the Rosette Nebula are simply stunning. A flat field of view with sharp and crisp images.

Another candidate for low power was the 31mm Nagler, but it was too heavy for this telescope and the eye-relief was not as good as with the 35mm Panoptic (19mm vs. 24mm). The true field of view however is a bit wider in the 31mm Nagler.

The 22mm, 17mm and 12mm Nagler Type 4


These eyepieces really surprised me. The first I bought from this series was the 12mm. At first light a had a lot of problems with the kidney-bean effect, large blackout areas, due to the fact that my eye was placed at the wrong position. But once I understood how to use the adjustable eye-cap, I was completely convinced that this eyepiece would stay.

The 12mm Nagler Type 4 produces very bright and clear images, and the field of view is simply overwhelming. It really looks like you are in space. It is not possible to take in the whole field of view at once. You have to move your eye around more or less to look around the rim of the field of view. The spacewalk experience with this 12mm Nagler Type 4 convinced me completely and without hesitation I ordered the 17mm and 22mm Naglers Type 4 as well. These three eyepieces are at the core of my set of eyepieces for my 300mm Dobson and they are the most used magnifications. They are simply wonderful. Open clusters like M 35 or M 37or the Orion Nebula with the 22mm, M81 and M82 with the 17mm, M13 with the 12mm, it al looks just right.

Other eyepieces I tried in the 22-12mm range were the 24mm Panoptic and the 22mm Panoptic (smaller eye-relief and smaller field of view than the Naglers) , the 21mm Denkmeier (smaller field of view, but a very good eyepiece), 17mm Panoptic (poor eye-relief) and the 13mm Type 6 Nagler poor eye-relief, kidney bean effect, pin-cushion distortion).

The 7mm, 5mm and 3.5mm Pentax XW


For high power viewing, the Pentax XW eyepieces are the only high-quality eyepieces with an eye-relief of 20mm and a field of view of 70 degrees. Of course the Nagler Type 6 have a much wider field of view, but the eye relief again is rather poor. It is possible to use eyepieces with an exit-pupil smaller than 2mm without glasses, and still get a sharp image, but as I said before, I want to be able to read, write and draw at the telescope, so I want to keep on my glasses. That's why I chose the Pentax XW over the Naglers in this range. The adjustable eye-caps make me decide for the Pentax over the Vixen LVW eyepieces, which are also very fine, with their 20mm eye-relief and 66 degree field of view. With the Pentax's adjustable eye cap it is possible to adjust the position of the eye, making it easy to take in the whole field of view without getting the kidney-bean effect. It is possible to take in the whole field of view at once. For planetary nebula, small galaxies, globular clusters and double stars, these eyepieces provide the high magnification, but of course to seeing has to be stable to use the high powers.

Conclusion

The choice of this set of eyepieces for this particular telescope was based on my personal wishes (criteria 1-4) and I only can say that all the eyepieces I've tried, were of an excellent quality. Often it was eye-relief that tipped the balance in favor of a certain eyepiece. If you do not wear glasses, the range of eyepieces to choose from becomes much wider. The best advice I can give you is that when you have the chance, try out the different eyepieces before buying them. Also try to write down the things you expect from your eyepieces (price, weight, focal length, eye-relief, which telescope etc.) This will narrow down the number of eyepieces to choose from. In the end the right choice will give you a lot of satisfaction when using them, and probably save you some money as well. Should you have any questions about the eyepieces or my telescope, please feel free to contact me.

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After reading many reviews, test reports and discussing equipment with fellow observers, I decided to go for the 300mm f / 5.3 Dobson from Orion Optics UK. The OTA (with cradles) and the rocker box both weigh about 15 Kilograms (without the alterations and additions that were made). The tube is about 155 Centimeters long and is ideal for sit-down observing.

I ordered the telescope from the factory with four upgrades:

- Hilux coating
- Primary mirror 1/6 lambda (in the end I got almost 1/9 lambda)
- 9 x 50 finder scope
- 2-inch Crayford focusser

After 8 weeks the telescope arrived. It came in two boxes, one small box containing the rocker box, and a big box with the complete tube assembly. The Crayford focuser was slightly damaged, probably during transport. It was replace by Orion Optics UK immediately. (You can enlarge all images in this article. Just move the mouse-pointer over the image and click)




Rocker Box and tube assembly and tube-rings
The rocker-box is made of aluminum about 10mm thick. The setup (only Allen screws) took me about 3 minutes. It looks very stable and sturdy. The tube-cradles are attached to a pair of large rings that fit into the rocker box. The rings can be adjusted in all directions using setting screws. This way the rings can always be adjusted to fit the rocker-box. The rings rotate in the rocker-box on four pieces of Teflon. The base of the rocker-box rotates on three Teflon disks that are fitted to the three legs of the base.



The tube is made of aluminum and sprayed with bright white paint. On both ends there is an aluminum black ring, held in place by a few small screws. The finder scope is mounted with a dovetail and bracket. The bracket and the finder scope are in the same black and white color as the tube. The 2-inch Crayford focuser is black. The finishing of the whole tube-assembly looks very good. All in all, the entire telescope looks nice (aesthetically). The initial setup of the rocker-box and tube-assembly takes only 5 to 10 minutes. To give you an idea of the size and the looks of the telescope, here it is, a proud owner included




The mirrors / collimation
The secondary mirror is held in place with a 4-vane spider. You can center the secondary in the tube by tightening and/or loosening 4 large setting screws on the outside of the tube. On the back of the mirror cell you will find 4 screws. With the central screw you can move the secondary “up” and “down” through the tube. This way you can center the secondary in the focusser. When the central screw is loosened you can also rotate the secondary. Once you centered the secondary ( in the tube and in the focusser), you can fasten the central screw, and collimate the secondary, by using the other three screws. (For an image of the secondary and the 4-vane spider see image below under "flocking paper in the tube")

The primary mirror is held in place by a three-point mirror cell. Collimation of the primary is very easy, and must be done manually with the help of some collimation tool (laser, Cheshire, sight tube etc.). You simply loosen three Allen screws by hand. You then “collimate” the mirror using three large setting screws. This is also done manually. When the collimation is finished, you simply fasten the three Allen screws “finger tight”. These three screws hold the collimated primary in place.




As for the quality of the mirrors, I can only tell you about the primary. You get a detailed test-report with your mirror, telling you the different values like Strehl, PV wave front, RMS etc. as well as the serial number of the primary. I do not know a lot about optics, so I rely on others if it comes to explaining the graph and the data on the datasheet. According to Harry Rutten (author of Telescope-optics), whom I showed the test-report, the mirror is of an excellent quality. The Strehl ratio is 0.986, the PV wave front is 0.115 wave and the RMS is 0.019 wave. To see the test report, click on the image below.



Finder-scope
I have tried the finder-scope a few times. I always wear glasses (astigmatism) when observing, and I noticed that the eye-relief is far from sufficient for me. I only see a very small part of the field of view. If I take of my glasses, the eye relief is ok. The finder scope will be replaced with a Telrad or some other one-power finder in the near future, because I will use it only for aligning the telescope on two or three bright stars. The rest is up the Argo Navis Digital Telescope Computer.

Crayford focuser
The Crayford focuser works fine. Focusing is no problem, even at high magnifications. It runs very smooth, and the friction can be adjusted simply by turning a setting screw. With a second screw the focuser can be locked. The only downside is that I have to use an extension with all eyepieces I tried (2-inch and 1.25-inch).



Alterations/additions
In the last few weeks, together with Leo and Joop from the local astronomy club, we managed to make a few alterations and additions to the telescope and mount:

1: a fan for creating laminar airflow in the tube;
2: digital setting circles DTC Argo Navis;
3: power supply for computer and fan;
4: tube extension to keep out stray light;
5: flocking paper in the tube.

Together with these alterations and additions, the tube assembly weighs 18 Kg and the rocker-box 17 Kg.

1: the fan
I added a fan to the backside of the tube, which covers the complete aperture. The fan draws in the air from the top of the tube. The air goes through the tube and past the mirror, and exits behind the mirror, through the fan. This way a laminar flow is created within the tube, and turbulences due to difference in temperature inside the tube disappear within a few minutes. I tested the fan 4 or 5 times until now, and even when I moved the scope from a heated room into my backyard, just a few minutes after the fan was switched on, the turbulences inside the tube were gone. It works perfectly. Leo had tested this system on two other Newtonians, and it worked, so we expected it to work just fine on my new scope.

How did we construct this fan? Leo used a piece of very sturdy foam about 1 inch thick. We got a fan that is used for cooling PC’s with the option to have variable speed. We cut an opening in the centre of the foam in which the fan fits nicely. After that Leo connected an external pot-meter to the fan so that the speed can be varied from absolutely zero to fast. I finished the fan with flocking paper and 4 strips of Velcro to attach the fan to the backside of the tube.



2: digital setting circles DTC Argo Navis
On my EQ-6 I used Argo Navis digital setting circles for locating objects. Because the 300mm Orion Optics UK will be my deepsky telescope from now on, I decided to transfer the Argo Navis from the EQ6 to the Orion Optics. I ordered a set of new (10.000 steps) optical encoders and a few cables from Wildcard Innovations, the company that produces the Argo Navis. Joop Wiersma managed to attach the decoders and the Argo Navis to my new dob just wonderfully. He already had some experience because he build the Argo Navis onto his own Orion Optics 300mm and with my scope he even made a few improvements to the whole construction. He needed some small parts like bolts, nuts and a few pieces of aluminum (total price 10 Euros) and what he constructed in a few hours is amazing. It not only looks very good, it works 100%!

Below you will find some images. From left to right you see the Argo Navis, the altitude encoder and the azimuth encoder. If you like to know about the complete construction in detail, you just have to wait a little. It will be published in my blog, as a separate item. Anyway, the construction is made in a way that when I want to separate the tube from the rocker box, loosening one screw (manually, no screwdriver needed) and disconnect one encoder is enough. A very convenient and user-friendly solution. However, for transport I remove the bracket that secures the altitude encoder to the rocker-box completely to prevent it from getting damaged. A few yards of Velcro were used to keep all the cables in its place.



And here's a shot of the complete construction...................



3: power supply for computer and fan
Of course the Argo Navis and the fan both need some power. I chose to attach two sealed led-acid batteries to the floor of the rocker-box again with some Velcro. One is connected with the fan, the other with the Argo Navis. I used cigarette lighter plugs to connect the fan and Argo Navis to the batteries. This makes it very easy to unplug everything after an observing session. Because of the weight of the batteries, everything feels even more stable. The rocker-box is the best place for the batteries. You can turn the scope in whatever direction, all the cables stay in exactly the same place. A simple but perfect solution.

4: tube extension to keep out stray light
Another addition I made is a tube extension that both keeps out stray light and prevents the secondary from dewing. The tube was made from 1mm thick cardboard and yes, a few feet of Velcro again. I did not have a chance to test it because the extension is a bit heavy, and I need to attach a counterweight to the tube to keep it balanced. I simply will add another cradle to the tube to act as the counterweight. It can easily slide up and down the lower half of the tube, to adjust the balance.

5: flocking paper in the tube
I added some flocking paper to the inside of the tube, but only to the part of the tube that I see when I look through the focuser directly without using an eyepiece. I hope the flocking paper prevents reflection of incoming stray light directly into the eyepiece. I added the flocking also to the inside of the tube extension.



To be continued………..
So far for the hardware. My next article will be about “first light” and my experiences with the telescope under the stars. Will the additions and alterations work? I hope that the weather will help a little in the next few weeks, so that I can give you some detailed observing reports. Until then I wish you all clear skies!


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Why use the Zeiss for lunar imaging anyway?

TAL 200K
My 8-inch TAL 200K is an f/10 (2000mm focal length) telescope, and is only fit for use with 1.25 eyepieces. I achieve the lowest possible magnification using the 32mm Televue plossl. This eyepiece gives me a magnification of 62.5x, and with a 50-degree apparent field of view, this results in a true field of view of 0.8 degrees or 48’. The Moon has an apparent diameter of 30’, so visually the full Moon fits nicely into the field of view, but it fills the view almost completely. There is not much “space” left around the lunar disk.

However, shooting a picture of the full Moon (or nearly full Moon) with this setup is impossible. When the camera is connected to the 32mm eyepiece and the TAL 200K, the view will suffer from severe vignetting (a kind of tunnel-view). I can get rid of this problem by zooming in on the Moon using the camera’s optical zoom, but then I cannot fit the lunar disk completely into the field of view of the camera. At the bottom and the top, a part of the lunar disk is “cut off”. So the TAL 200K is excellent for shooting detailed images of the Moon, better than all other telescopes I own, but for imaging a (nearly) full Moon, I need another telescope.

TAL 100RS
Since two years I have been using the TAL 100RS, a 4-inch f/10 (1000 mm focal lenght) achromatic refractor, to shoot my overall lunar images. With the 32mm eyepiece I get a true field of view of of 1.6 degrees or 90'. Using this telescope got me some satisfying results, but the problem with achromatic refractors is that you get a more or less bright, somewhat colored circle around bright objects like the Sun, the Moon and the Planets. When processing the images I shot with the TAL 100RS, the bright fringe around the Moon becomes even more apparent. I simply need yet another telescope for shooting the overall lunar images. The solution to the problem came from a totally different instrument, a birding scope.

ZEISS DIASCOPE 85MM
During the last few years my girlfriend and I have gradually taken on the hobby of birding, not only during vacations, but also throughout the year, around our home and in our backyard. In 2005 we decided it was time for a birding scope. The choice was rather easy. There are three different top-class birding scopes available, Zeiss, Leica and Swarovski. These have all been tested many times, and you can find enough test reports on the Internet.

We chose the Zeiss because we simply liked the views we got from it, and we liked the professional finish of this wonderful instrument. Because of the high quality and the short focal lenght, this little scope is not only perfect for birding, but also for imaging the full Moon. The Zeiss Diascope 85 is an f/5.5 (focal lenght of 500mm) apochromatic refractor. With the 32mm Plossl eyepiece, the true field of view is almost 3 degrees, or 180'. So there's lot's of space around the Moon, and no colour fringe! However, for shooting detailed lunar images I will allways use the TAL 200K (shows much more detail on the surface because of higher resolution).

The eyepieces
For birding we only use the Zeiss 20-60 Zoom eyepiece. You cannot change eyepieces when you've got a bird in focus, because birds have the strange habit of moving around, especially when you want to shoot an image! Sometimes it seems like they don’t like being photographed.


Luckily the Moon moves a bit slower across the sky (and not in all different directions like those.... birds). Using an adaptor enables me to connect all astronomy eyepieces I currently own to the Zeiss. The 32mm Televue Plossl and the whole range of Vixen Lanthanum eyepieces, they all come into focus, even the 2.5mm Lanthanum, which results in a magnification of 200x. I am convinced that there are a lot of astronomy eyepieces on the market, that will not come into focus with the Zeiss Diascope, but that’s no wonder. The Zeiss Diascope is a birding scope, not an astronomical instrument. So be careful when considering this instrument for backyard-astronomy!

The setup for birding and for imaging the Moon
Shooting images of birds or the Moon has two problems in common: how do I get the object into focus, and how do I shoot the images without touching the camera, resulting in vibrations and .......blurred images. To make life a little easier I chose for a complete adaptor-set from Eagle Eye OpticZooms in the UK:

1. A digimount adaptor for connecting the camera to the eyepiece;
2. A Shutter-release arm and wing 4500 bracket;
3. A 10-inch shutter-release cable;
4. A micro fiber lens cleaning cloth.

On the images below (click on the images to enlarge them) you can see how everything is connected to each other. The shutter release cable works mechanically. You do not have to touch the camera to shoot your images. The X-tend-a-View Pro is just great. It magnifies the 1.5-inch TFT color monitor on the backside of the camera with factor 2. The view is so detailed you can even see the different colors of the individual pixels. It is a great help for focusing the camera/telescope on a bird, the Moon, or whatever other object. With the shutter release arm and the wing 4500 bracket, both the shutter release cable and the X-tend-a View Pro are connected to the coolpix 4500.







Click on images to enlarge


So this is the setup I use for birding. The setup I use for imaging the Moon is basically the same, only the eyepiece and the eyepiece adaptor are different. You find more info on how the Televue Plossl and Vixen Lanthanum’s are connected to my camera on my old website, backyard-astro.com. Just follow this link to get there. The shutter release cable and the X-tend-a-View Pro are used for shooting the lunar images as well.

Anyway, I want to finish this article with a close-up (award-winning) shot of a sparrow, shot with the birding set-up. In a contest from the Dutch Bird Protection (Vogelbescherming Nederland) my girlfriend was one of the ten prize winners with this image. Not bad for a beginner . I hope that in the next months these everlasting clouds will disappear over the city of Landgraaf, and I will be able to shoot a few lunar images with the Zeiss to present them to you in this blog.

Clear Skies to all of you!


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