Posts Made By: Michael Connelley

Hello:
Probably #7, but nights that good are quite rare (i.e. good seeing, no wind, etc.). I use a 12.5" classical cassegrain and observe from 9,300 ft on Mauna Kea, Hawaii. I have been able to see Red Jr, as well as the different sizes and colors of the moons, but I have not been able to visually see surface details on the moons. There was a night were I may have been able see as well as #8. We were on the summit at 14,000 ft, there was no wind and the seeing was 0.3". I was able to split a 0.4" binary (gamma Andromedae BC), but alas Jupiter was not visible at the time.

Cheers
Mike Connelley

Hello:
First, I'll disagree with the previous post regaring the optical perfornance of our eyes with very small exit pupils. I agree that our eyes are not that good (optically speaking), I have quite a bit of astigmatism, so it's particularly bad for me. But with a very small exit pupil, we are only using a small part of the cornea/lens, so that little patch is much closer to being optically perfect than the whole thing. It's the same idea as using an aperture mask to improve image sharpness.

As far as why the image "breaks down", I agree that the eyepiece doesn't affect the image made by the telescope. All the eyepiece does is magnify the image that is there. Given a perfect telescope, the image would start to break down when you magnify so much that the resolution is limited by diffraction. In most cases, the resolution is limited by seeing. I also think that the eye can see more detail with more light (part of the eye-brain image processing miracle mentioned in the previous post). I remember looking at Saturn once on a night with very good (0.3") seeing. At 400x it looked great. At 800x, no so wonderful. It was dimmer, and didn't look as sharp. So, I thought the image was "better" at 400x. But was it? It looked sharper since the magnification was lower, and brighter for the same reason, but the two images presumably had the same detail. I think that at 800x the image resolution was limited by seeing, making the image more blurry, and dim enough that the eye/brain was having a harder time pulling out detail.

Cheers
Mike Connelley

Hello:
I use a Sky90 on this mount. I bolted the tube ring to the dovetail bar using two short 1/4-20 hex bolts. The tube ring has two holes in the bottom, whereas the dovetail bar has three holes (one tapped in the middle, and a clear hole at each end). I put a bolt through the front hole of the tube ring into the clear hole in the front of the dovetail bar, and the other bolt though the rear hole of the tube ring through the tapped hole in the middle of the dove tail bar. This makes the scope sit a bit forward (i.e. front heavy) on the mount, so that it balances when I put a heavy eyepiece in.

Cheers
Mike Connelley

Hello:
That looks very nice. How much does it weigh? For my 17.5", I'm going to try to keep the top end weight below 4.5 lbs (without eyepiece). I noticed that you have the Kineoptics helical focuser. How do you like it? Have you had a chance to use one? I am tempted to get one since they're so light, but I'm skeptical as to how well that design will handle heavy "hand grenade" eyepieces.

Cheers
Mike Connelley

Hello:
If you're thinking about a Takahashi, how about the astrographs? The Epsilon 180 is nice and small. The BRC-250 is supposed to have a nice wide and flat focal plane and is roughly the same price as the Mewlon 250. The Mewlon series is mostly intended for high contrast visual observing. The Dall-Kirkham design of the Mewlon is easier to make well, but has more coma than a regular Cassegrain. Thus, I would recommend against the Mewlon for deep sky CCD imaging. Another option would be a 10" Richey-Chritien from RC optical systems, which is about that price too. You could get started with a nice, wide field refractor (Stellarvue, Pentax, etc), and have money left over for the CCD, filters, guide scope, etc.

Cheers
Mike Connelley

Hello:
Omega Centauri blows it away. It's not even a close contest. I don't know if the individual stars are easier to resolve in one or the other, but in both it's pretty easy. I don't know how it'll look from where you are, especially if it'll be much closer to the horizon than M22. From here (+19 degrees latitude), both get high enough for a good show. One thing we like to do when we have visitors from the mainland is to show M13 (nice, especially under dark skies), THEN go to Omega Centauri. If they haven't seen it before, they usually have no idea how big it is.

Cheers
Mike Connelley
Who still hasn't seen 47 Tuc in a telescope

Hello:
You don't want to use a split ring equatorial mount for a refractor. The split ring is only suitable for a newtonian (with tube rotation) or very large cassegrains where you'll only have cameras on the back (i.e. the Mayall 4m at Kitt Peak, Blanco 4m at Cerro Tololo, 4m AAT at Siding Spring). What these scopes have in common is a short tube with the center of gravity near one end, and you don't have to have your head near that end. For a refractor, you need to support the scope pretty high to get to the eyepiece, and have access to the bottom. As such, the German equatorial mount is your best option. As has already been mentioned, Josef Fraunhofer invented the german mount for his refractors. German mounts are hard to make on your own, but there are several available commercially. For a scope the size of yours, something like the Losmandy G11 would work well.

Cheers
Mike Connelley

Hello:
I think you're worried about some things more than necessary, and haven't addressed some issues. First off, some guys have figured out how to make really good fast mirrors, so that getting good performance out of an f/4 mirror is quite feasable. You'll certainly get good image quality out of an f/6 mirror, but for mirrors above 20" this becomes a very long telescope. I also firmly believe that the problem of the size of the secondary obstruction is over rated, especially for big telescopes where you're very rarely diffraction limited. For example, the secondary obstruction for HST is not small. You'd also probably be shocked at how dirty big mirrors get before they get cleaned. Sure, a dirty mirror scatters light, but other issues like a smooth polish and good baffleing tend to me much more important. Most eypieces won't vignett with steep light cones, but they will have problems with astigmatism. Fast mirrors also have more coma. If you want to use both eyes, it seems to be much easier to build a slightly bigger scope and use a binoviewer than to build binocular telescopes. Not only do you need to build two telescopes, you need to align two telescopes, AND align them to eachother. Also, binocular telescopes are very heavy and awkward. Or, you can build one bigger scope that can be used in bino OR cyclops mode and use a conventional (i.e. simpler, easier, quicker, cheaper) design. Cyclops mode can still be necessary to give the widest field of view since many wide field eyepieces, such as the 31mm Nagler, are too big to put side-by-side for binocular observing. For a scope around f/5, the 31mm Nagler is the eyepiece that will give the widest field of view without producing an exit pupil that's too big.

Cheers
Mike Connelley

Hello:
This reminds me of the video of Subaru's recoating, which is on a DVD. In the case of Subaru, there are a few scary moments, such as then the mirror is being lowered through the dome floor to the coating room floor (about 4 floors below) by the crane. At that moment, the whole mirror is hanging by a steel cable. After the mirror is washed and the aluminum is stripped, baking soda is spread over the mirror with big brooms (the kind janitors use to sweep hallways). The mirror is then inspected for damage, such as from falling wrenches (one which hit the mirror). During this time, people walk on the mirror surface in order to get a good look at it!

As much as these mirrors are pampered, they take some abuse. Many of the mirrors on Mauna Kea have had things dropped on them. The Subaru mirror had some glass pulled out of the back side by one of the hard points. The CFHT mirror had mercury leaking onto it. The IRTF mirror has about a 6"x4" chunk taken out of the mirror surface around the central hole (which is not in the light path...whew!). I'd bet most amateurs out there would be shocked to see how dirty some of these mirrors get before they are washed!

Cheers
Mike Connelley

Hello:
I have a few scopes, among them a 12.5" cassegrain, a 12.5" and 8" dobsonian, and a 90mm refractor. Since the cassegrain was built, it's been getting nearly all of the action. It's optics are great, it's on an equatorial mount (a plus versus the dobs), and it has good aperture. It's field of view is only about 1/3 that of the 12.5" dob, but for most objects this isn't a big issue. I've found that the higher magnification and tracking are a big help in hunting down little galaxies. If the seeing is bad, I just keep it to low powers, which pretty much means just one eyepiece since it has a 5m focal length.

I recently had a chance to look through a very nice 5" apo refractor that was, well, perfect (a star test and ronchi test found nothing to be desired). It was clearly as good as a 5" gets, but I liked the image better in the 16" dob next door simply because the images were bigger and brighter. Granted, we didn't try a formal showdown on a planet or high contrast binary, but that's the impression I got after looking at a few nebulae and globulars. In the end, aperture wins.

One thing I'm really not convinced of is how larger telescopes are affected by seeing. Lots of people say that big telescopes are harder hit by seeing. I think it's just more visible with larger telescopes since they a) gather more light, so all of the crap out in the wings of the image are easier to see b) the diffraction disk is smaller, so it's harder to be diffraction limited, and c) larger telescopes tend to be reflectors, with their associated mirror cooling issues. I can almost always see a diffraction limited image with the 90mm, bit I've only seen this once with my 12.5". The reason is that the 90mm only needs 1" seeing to be diffraction limited (quite common) whereas the 12.5" needs 0.3" seeing (quite rare). If bigger telescopes are harder hit by bad seeing, then I should be able to take my 12.5" scope out on a night of bad seeing, stop it down to 4", and see the image improve. I doubt this will happen. Also, if larger scopes are more adversely affected by bad seeing, then how can 8m telescopes get 0.5" images in the optical without AO?

My advice would be to keep the Mewlon of course, but don't worry about getting a refractor unless you really want a wider field of view. Even in nights of bad seeing, you can still go for fainter deep sky targets that will look better than in any 5" refractor. Unfortunately, bad seeing is bad seeing, and there isn't much we can do about it but try another night.

Cheers
Mike Connelley