Next entry: A spectral riddle in the Rocking Horse

Previous entry: Xi Bootes in Little Cygnus

Eye-relief for my 12-inch telescope
image


Introduction

About one and a half years ago I bought a 12-inch telescope for deepsky observing, an f/5.3 Newtonian telescope from Orion optics UK. The tube is mounted on a dobsonian rocker-box and equipped with the Argo Navis Digital Telescope Computer. During the past 18 months I have put together my set of eyepieces for this telescope. I had four major criteria for selecting the eyepieces:

1. High quality
2. Large true fields of view
3. Weight between 400 and 800 grams
4. Lots of eye-relief

All selection criteria were important, but the most important was number four, eye-relief. In this article I will tell you why these criteria were more or less important to me, which eyepieces I finally chose and why I chose them.

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

image

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

image


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

image


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

image


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.

Posted by Math on 07/13 at 12:54 PM
Equipment • (0) CommentsPermalink

Name:

Email:

Location:

URL:

Smileys

Remember my personal information

Notify me of follow-up comments?