Posts Made By: Michael Connelley

Hello:
I've heard various opinions regarding tube and tube lining materials, specifically whether it's better to have a thermally conductive or insulating tube material. I've read that some people have lined their tube with stuff like cork to have the inner wall be an insulator.
My situation is as follows. I'm making a tube for a 12.5" cassegrain. It will be 1/16" thick aluminum, mainly since it's easier and more practical to get a sheet of aluminum and roll it versus learning how to fiberglass on a limited amount of spare time. I was just going to paint the inside of the tube black to reduce stray light. Unlike a Newtonian where part of the tube is in the field of view of the eyepiece, in a Cassegrain there is no part of the tube that can be seen from the eyepiece so it's not worth spending a lot of time (IMHO) getting the inside of the tube really black.
So, have people actually done experiments to see if it makes a difference to have conductive or insulating tube materials, or is this mostly based on hearsay? What are the arguments in favor for or against using thermally conductive or insulating tube materials? It seems like the arguments would be based on which material would minimize tube currents. With my scope, I think the limiting factor for the effect of tube currents will be the mirror itself. As it is nearly 2.5" thick, it'll take a LONG time for it to cool down (I'm planning on using a few fans to speed things along).

Cheers
Mike Connelley

Hello:
Since it says observing conquests should go here, I will do so. A I just had what was easily the best night of my life. A friend of mine and I were observing with a pair of 12.5" scopes from the 9,300 foot level of Mauna Kea. The sky was very dark and transparent, and the seeing was quite good. My goal for that night was to check the optical quality of a 12.5" f/15 cassegrain I had just put together. I did not yet have a primary baffle tube, so the sky background in the cass was bright, making faint galaxy largely impossible.
The real fun started when the moon came up around 1am. We broke down, and a call to the summit found that there was no wind, it was warm (4C), and the seeing was 0.3" or better! It didn't take long for us to head to the summit and set up the cassegrain between Gemini and the UH88" observatories. Due to the above mentioned moon and baffle issues, we stuck to planets, the Moon, binary stars, and a few planetaries. Most of our observing was done at 710x, which works out to about 56x per inch. In many cases, we could have used more magnification since the image wasn't about to break down.
Looking at the Moon and Mars had that "out the window of a space ship" feel. If there was any seeing, I couldn't tell. Even though Mars is far from opposition, at 710x it's pretty big and full of detail. On the Moon, crater chains and rilles just popped out everywhere, or so it seemed. Copernicus (I think) took up about half of the field of view on its own.
We knocked off as many close binaries as we could think of, which brings us to the conquest part, All the way up to the summit, I figured our first target would be gamma Andromidae, since that 0.4" companion to the B star had eluded me so far. Once set up, we went to the star and put in the 710x eyepiece. We were greeted by a pair of textbook diffraction patterns that wavered only very slowly. I had never seen a 12.5" scope be diffraction limited, until that night. Close inspection of B showed it to in fact be elongated, though not cleanly split. My friend saw it before I did, but I definately saw an elongation in the B component when compared to A. So, I'll mark it off in my book as resolved. In other news, the trapezium in M42 easily showed 6 stars, and the eastern most belt star of Orion was cleanly split without puting up much of a fight.
Towards the end of the night, we looked at Saturn. Though only about 20 degrees up, it was still very steady, with shadows and belts easy to see on the planet, and the Cassini division also easily seen (no Encke gap unfortunately). Mercury, about 5 degrees up, was also very steady, and we could see a phase, though we mostly saw a low resolution spectrum of it! Needless to say, the telescope itself passes muster, and is a definate keeper. Good observing nights for amateurs on the summit of Mauna Kea are few and far between (no wind, not too cold, etc.), and we were very fortunate to be up there on a really good night. At sunrise, we were very tired, but also very happy.

Cheers
Mike Connelley

Hello:
Welcome to the club. I'm wrapping up a similar project. Probably the most important thing you'll need are the designs for the cassegrain and gregorian, e.g. the mirror separations and back focal distances. I didn't have this and wish I did. I wouldn't recommend trying to make one tube for both a Gregorian and Cassegrain since the position of the secondary mirror is quite different. You'd end up with the Cassegrain secondary mirror at the end of a long rod which probably won't be that stable. Cassegrains like these are VERY sensitive to collimation errors. There isn't that much to gain by having both a Gregorian and Cassegrain since the f/ratios are close. I'd pick one and stick with it. Swapping the secondaries between the Newtonian and Cassegrain mirrors can be done, but you need a way to collimate the scope at night. It will be very hard to make the mirror mounts so stable that you won't need to collimate when you swap the mirrors unless you want to machine kinematic mounts.

As far as parts, you'll need a lot. For starters:
- tube: You can make either a round solid tube or a truss tube. I made a round solid tube since it's easier to attach it to a german mount, and the scope is compact enough that I don't need to disassemble the tube to fit it in the car. To make the round tube, I got a piece of sheet aluminum 1/16" thick, got it rolled into a tube, then riveted it together.

- focuser(s): Any well made focuser will be good. I have ones from Astrosystems and Moonlite. If you want to use both Newtonian and Cassegrain foci, you'll need two focusers. Get a 2" focuser, since you'll want as wide of a field of view as possible with the f/16 or f/24 beams. For that matter, get an eyepiece with as big of a field stop as you can. Your Cassegrain focuser should have a lot of travel.

- spider: I got mine from Astrosystems, and it works quite well. If you want to use a Nazmuth focus with your Cassegrain, you'll need another spider and another diagonal mirror.

- primary mirror cell: Almost any old mirror cell will work, as long as it has a hole in the middle. Make sure it can hold the mirror at all orientations, especially roll angle. I'd recommend a mirror cell from Novak, but I don't think they're in business anymore. The ones from Parks cost an arm and a leg. You can put an add on Astromaart and see what pops up.

- secondary mirror cell(s): For the Newtonian secondary mirror holder, there are a lot of places, such as Astrysystems. For the Cassegrain mirror, you may have to make it on your own. The article on CloudyNights describes this.

- Back end plate: Connects the primary mirror cell to the tube. I fabricated by own from 3/4" plywood.

- Mount: For the cassegrain configuration, I'd recommend a german mount or making a dobsonian on a tracking platform and a Nazmuth focus. There was recently a very good article on CloudyNights with a scoope like that. You definately want something that tracks. I was using my 12.5" f/15 cass without tracking, and it was a major pain. For my scope, I got a G-11 mount. But watch the weight, since the G-11 can only hold 60 lbs, and a 12.5" Cassegrain can easily exceed this.

I found that putting the scope together takes alittle trial and error, especially with the Cassegrain. The backfocal distance is proportional to the primary to secondary mirror separation and the square of the secondary mirror magnification, if I recall correctly. A shift of 1/32" in the secondary mirror position will have quite a noticable effect in where the focus is, so you'll need to have a way to shim the secondary mirror position to get the focal point to be within the range of your focuser. I put the spider a little farther back than necessary, then put washers behind the secondary mirror holder until the focus was where I wanted it. The mirrors were made for a specific separation, so you should build the scope to that design or else you'll have spherical aberration.

Cheers
Mike Connelley

Hello:
Here's an idea I've been kicking around a little while, and I'm wondering if anyone has thought of, or actually, made something similar. It seems like a Schmidt camera is the way to go for fast, wide field imaging. Making and using one is a bear. A Schmidt-Newtonian has the advantage that the focal plane is not in the telescope, which makes focusing and alignment easier, and allows the use of larger camera bodies and CCDs. It occurred to me that the Meade Schmidt-Netwonians are almost what you need. In theory, all you'd have to do is put the corrector plate at the radius of curvature of the mirror (from what I've read, the correction for spherical aberration does not depend on the separation between the primary and corrector plate, although the coma does), and make a field flattener. If the primary mirror is the same size as the corrector, then it'd probably be best to stop down the aperture at the corrector so the pupil is at the corrector plate. Naturally, all of this is easier said than done, but I suspect it's within the realm of possibility for many. This is all the more attractive since 8" and 10" Schmidt-Newtonian OTAs are available on Astromart fairly cheap. Is there anything fundamentally flawed about this scheme? Has anyone tried this out?

Cheers
Mike Connelley

Hello:
I have a Losmandy G-11 with a 50lbs 12.5" cassegrain on it. This scope is pretty close to the limit of what the mount can handle; it is both long and heavy. So, it's not too surprising that the scope is less than rock steady on the mount. I would still like the scope to be as steady as possible. To this end, I intened to install a strut from the end of the counterweight shaft to the end of the tube.
However, the main source of flexture is in the RA clutch pad area. I have adjusted the gear mesh to minimize backlash. When I wiggle the scope back and forth (not enough to make the clutch slip, just enough to simulate wind shake) in the RA direction, the motion seems to be at the interface at the clutch pad. When I wiggle the scope while my thumb is pressed over the RA setting circle and vernier, I can feel a little differential motion. The motion has a resonant frequency of about 2Hz. I have found that reducing the friction on the clutch (with he knob at the back of the RA shaft) reduces this instability, but then the scope is free to move on its own. My question is: what can I do to reduce or eliminate this instability? Will replacing the clutch pads with the ultra-high molecular weight stuff help? Is there some home brew fix that I could try? I suppose I could get a heavier duty mount, but I'd prefer to try the less-than-$10000 options first!

Cheers
Mike Connelley

Hello:
There is a good review over on CloudyNights of planetary eyepieces. By thier comparisons, a Nagler didn't even come close compared to eyepieces such as a Televue Plossl or even an Edmund RKE. Inspired by this report, I did a side-by-side test between my 17mm type4 Nagler and a 15mm Meade Plossl. The telescope involved is a 12.5" f/15 cassegrain, so these eyepieces were giving about 300x. I did most of my testing on the Moon, looking for craters and rilles on the floor of larger craters. After a bit of back and forth, it was clear that the Plossl was showing more detail. Details were a bit more crisp and contrasty, and features on the edge of visiblity were easier to see in the Plossl. The Nagler is no slouch, and I use it a lot for higher power deep sky work were the extra field of view is useful. But for planetary viewing, I now reach for the Plossl.

Cheers
MIke Connelley

Hello:
I would like to try to use a Celestron Neximage webcam-like imager with my mac laptop. I've downloaded Keith's Astroimager and another application called macam, but so far no joy. Macam notices that I have a camera plugged in, but I just get a black screen. Keith's Astroimager doesn't seem to notice the camera at all. So, my question is: has anyone successfully used this (or other similar) webcam with thier mac, and if so, what did you do to get it to work? One solution, of course, it to just install a windows emmulator on my mac, but I'd like to avoid such drastic actions as much as possible.

Cheers
Mike Connelley

Hello:
Can anyone recommend a good coater? I'm thinking about getting my 12.5" mirror recoated. I've heard good things about Spectrum Coatings. I've tried to contact them, but haven't heard back. What other companies are out there have do good work? I've thought about stripping the mirror myself, and having the recoating done locally, but I'm a bit afraid of doing this myself. These mirrors are quite good but the coatings are old and may not come off easily.

Cheers
Mike Connelley

Hello:
I may have an opportunity to focault test a large and fast mirror. Suffice it to say, it's larger than 1 meter and faster than f/4. Now, I have a little focault testing experience on the kind of mirrors amateurs more commonly come across, but nothing like this. Making such a large Couder screen is cumbersome, so we may do the focault test photographically and fit the null zones in image processing. Thus, instead of moving the knift edge to put the null at a certain radius, we'll find what radius the null is at for a given knife edge position. So my questions are:

1) Does anyone out there in astromart land have experience focault testing such fast mirrors?
2) Are there any particular "gotchas" or things to look out for?
3) Are there any special considerations is making and using the focault tester?
4) What kind of light source do you use? A slit, pin hole, fiber optic?
5) Any other adivse that's helpful?
Thank you

Cheers
Mike Connelley

Hello:
Here's a rather dark image of my scope taken at tonight's star party. It's a 12.5" f/15 classical cassegrain on a Losmandy G-11 mount.

Cheers
Mike Connelley