Posts Made By: Gary Weber

I have a used, but newly acquired 12.5 f/6 dob. It has (to the best of my abilty to figure) a secondary size of 2.6". It occurs to me that this may be larger than necessary, but finding easy to understand information on making an absolute determination on secondary size seems difficult.

NEWT (the software) indicates I can get a full field illumination with a secondary of 1.83". Other designs of similar size have used 2.14".

My question is...What is the smallest secondary I can use for a fully illuminated field for a UO 40 mm Konig (AFOV 70*)? Is this different from what would be considered to be the optimum size for the same result?

Any help will be appreciated.

Gary

I live in Nova Scotia, but travel 2000 miles to Alberta a few times each year. I recently purchased an 8” f/8 Discovery to leave at the Alberta location so I could have a decent scope to use when I am visiting the area. I did a number of common modifications to the telescope which included installing three small fans on a board, which is mounted just below the primary mirror cell. The fans suck air down the tube and exhaust out the bottom of the tube. The fans are operated off a small 12V gel cell with a rheostat to control their speed. I have calculated that with the fans operating at their lowest setting, I am getting two tube changes of air every minute.

Over my Christmas visit, I spent about five nights observing with the scope. The temperature varied, but was always below zero (Celsius) which meant there was a significant amount of ice crystals in the air making the transparency quite poor for this location (NELM from ~4.9 to 5.8). I found the little gel cell did not operate well in the cold weather and I didn’t always use the fans. I was normally viewing planets using a 7mm Pentax (230x) and when conditions allowed, I used a 5.2 mm Pentax (310x). The views at 310x were usually sharp only for perhaps 60% - 70% of the time as the sky steadied for a few seconds.

During the evening of December 27, the clear sky clock predicted good seeing (5 out of 6). My brother and I set up to do some planetary observing just before midnight. The temperature was -21° C (-5° F) and never changed through our observing session. I did install the battery and I turned the fans on to their lowest setting. Over the next 15 or 20 minutes, we observed Saturn and slowly increased the power as conditions allowed. As during the previous sessions, we began at 230x and moved to 310x after about 10 minutes. After another 10 minutes, the image was very steady and I decided to try more power. I placed a 9mm Nagler (T6) in a 2.5x Powermate for 450x. Much to my surprise, the image was steady and very crisp. Over the next 20 minutes we traded views and marvelled at the wondrous image of Saturn.

At some point while my brother was viewing, I leaned around and switched the fans off. A few minutes later, he reported that he couldn’t get the image to focus. I told him that I had shut the fans down about 5 minutes earlier. We tried to turn the fans on again, but the little gel cell would not power them. We decided to take a break and go in for tea and I placed the gel cell on a trickle charger in the garage.

Gary Weber

Dan this looks like a very rewarding project. If you took lots of photos, this would be worth writing up for Cloudy Nights. I think it would make a good article for others to see what and how you did what you did.

Gary Weber

Herb, I just want to say kudos on all the work to upgrade Astromart. A few bits of polish here and there and it will be just fine...that'll come I'm sure.

I have used Astromart very little for transactions, but when I do, it always impresses me how well it works.

Keep up the good work, this is clearly one of the finest services within any hobby.

Gary Weber

Astigmatism is something I have, and I view generally without glasses. I can notice the astigmatism in my 13mm EP and anyhting with a longer focal length.

However, in order to find that astigmatism is present, I have to de-focus the image. Now like most astronomers, I actually view the sky while the image is in-focus.

So what deterioration to a focused image does astigmatism cause?

Gary Weber

For all the measured aspects of our equipment, there seems to be a very high level of personal subjectivity into what constitutes a good eyepiece. I think much of it is based on physical comfort, but there is such a variation in our own eyesight. Aside from the telescope used, different people notice different things and they all affect our impressions of the EP in different ways.

More than any other piece of equipment, eyepieces are much more personal, in terms of how they work for us. The point "try before you buy" is critical for eyepieces as one person's ideal is another person's piece of overpriced junk. Fortunately we have many, many choices and if you acquire one you don't like, you can sell it on Astromart for very close to what you paid for it.

Gary Weber

The Veil is around 3° of true field. With your scope, to get 3°, you need a long focal length EP. I would suggest the 40mm Pentax XL, which would give a true field of 3.2°. With patience, these can be found on Astromart and are fine EPs. A UO 40mm Konig would also be fine in your scope, however the Pentax has slightly better contrast.

To use filters such as the UHC and/or the OIII, you need to keep your exit pupil quite large (2.5mm or larger). If you get the exit pupil smaller than about 2.5mm, the image dims too much for picking out detail.

I have used my 17 Nagler in my 4" refractor (exit pupil 2.8 mm) with good success, but could not drop to the 13 Nagler (exit pupil 2.2 mm) without losing detail.

Gary Weber

I have read a number of posts recently that speak about the application of 2" collimation devices.

I have always thought that one was better off collimating with 1.25" tools because it was only at high power where one would notice the precision of collimation (or lack of it). High power for most telescopes is normally achieved with 1.25" EPs.

My thoughts were always try to achieve the best collimation with the accesories that you will use to view at high power. This avoids any potential alignment differences between the 2" barrel of the focuaser and the 1.25" insert.

I am curious about the thoughts of others on this.

Gary Weber

I spent a great deal of time in the Canadian arctic doing various kinds of work centered around wildlife management with a lot of field work. Here is a landscape shot from a hiking trip on Victoria Island. This is the second largest island in Canada and this is taken in late July at about 71° 30' north.

Gary Weber

This is about 100 miles down Frobisher Bay from Nunavut's capital, Iqaluit (SE Baffin Island). The boat in the foreground is a travelling buddy in his 24 foot freighter canoe. That mountain is on a small island and is about 890 feet high.

Gary Weber