Posts Made By: Michael Connelley

Hello:
Probably the most common thing I've done is to get a plastic toolbox and use it to carry my eyepieces and actual tools. For my 12.5" dobsonian, I bought a black bedsheet to use for the shroud. I also use a large vinyl flower pot to protect the mirror and also to block stray light. The dob is a minimalist/lightweight design, so the mirror is rather exposed and close to the ground and I wanted a way to protect it from dust, dirt, rocks, shoes, etc. To use the flower pot, I cut a hole in the bottom large enough for the collimation screws to fit in, so the mirror sits in the bottom of the pot and the pot is sandwiched between the mirror cell and the back of the telescope. I have also use lots of soda cans for light baffle material. They're 2.5" in diameter, thin, light, and opaque. This makes them perfect for a focuser baffle on a dobsonian, and now I'm using this for the primary and secondary baffles on a cassegrain.

Cheers
Mike Connelley

Hello:
It doesn't sound like much of a problem to me. I've left my truss poles unpainted and haven't seen any ill effects from it. The truss poles are generally so far from the light path that it's pretty hard for light to reflect/scatter off and go into the focuser, especially with a shroud on. As fas as I've heard, the main reason for putting pipe insulation on the poles is to dampen vibrations and so your fingers don't get so cold pushing the scope around. I say let the paint fall off, I doubt you'll notice it when you're looking in the eyepiece, and who cares what the scope looks like in the dark anyway?

Cheers
Mike Connelley

Hello:
I don't think you're doig anything wrong. Generally it's hard to see much color in the sky. Color films and color CCD images have different sensitivities to colors than our eyes. Also, it is very common for the color saturation to be enhanced in photos to make the photos more appealing. A good example are the Voyager images of Jupiter. Anyone who has looked through a scope at Jupiter knows it doesn't have bright red bands and spots, but that's the way they processed the image for visial impact. Cassini released a very nice image of Jupiter that they processed to reflect the colors as seen by the human eye, and it is very interesting to compare the two images. Asside from this, the eye loses some of it's color sensing ability when it is looking at something very bright, such as a planet. For example, people often commentt hat Mars doesn't look that red. Part of that is because, espectially in a large scope, it's so bright. So I don't think you're doing anything wrong. For the record, in my scope (a 12.5" cassegrain) I see the ball of Saturn as an yellow-ish off-white with light tan bands, a white B ring, and a slightly grayish A ring. The colors and shades are subtle, and higher powers may help by reducing the brightness of the planet and rings.

Cheers
Mike Connelley

Hello:
I put together a 12.5" cassegrain where the cental hole is 54mm in diameter but there the optimum baffle diameter is 70mm. To make my central baffle, I used soda cans stacked together with the ends cut off. The back end of the baffle is tapered a little, so that it can plug into the cental hole in the mirror. The front of the baffle is held in place with fine wires, which make a little diffraction on very bright stars but nothing too bad. So, although it is not necessary to have a central hole larger than the baffle tube, it would have been nice! In the case of this 6", it may be difficult to find a cutting tool with just the right size, so a hole a little too big or small may be necessary.

Cheers
Mike Connelley
Also interested in aplanatic gregorians...

Hello:
Without a doubt, the bible of mirror making is "How to make a telescope" by Jean Texereau, which is available through Willmann Bell publishing. If you want to make a mirror, this book is worth every penny. Willmann Bell also sells mirror making kits, with everything you need. I think are a bargain since they provide the novice mirror maker quality materials that are hard to find in small quantities separately. Texereau's book is nearly 50 years old, so while all of his techniques are still very good, some of the materials he uses can be hard to find today. Get it, read it, and use it. When I made my mirrors I had it open next to me as I was working away.
For your first mirror, I'd recommend sticking with pyrex for the reason that it is more thermally stable. This can help make a better mirror not just because it is more stable as your mirror cools down at night, but also as you polish. Polishing generates a lot of heat, which you don't really notice since the water in the slurry soaks it up, but still a lot of heat is there. As the glass warms up during polishing, the pyrex will expand less, making it easier for you to get a good figure on your mirror. With plate glass, you have to be very careful. A warmer area will expland more, which will then tend to be polished out, which will leave a divit in your mirror when the glass cools. You also have to wait longer between polishing and testing to ensure that the mirror has cooled.

Cheers
Mike Connelley
If you don't learn patience making a mirror, you never will

Hello:
Can you colimate the lens and/or the focuser? You mention that you used a Cheshire collimation tool, so it sounds like the focuser is not pointing at the lens, You may be able to fix this with a few shims. If it doesn't affect the image quality, then I say leave it alone. With an f/ ratio that long, it should be fairly forgiving of a little misalignment. Otherwise, it may be worth it to get a new tube. While I never checked the collimation, for a few years I used a 16" Clark refractor that was destroyed in a storm around the turn of the century. Alough the lens survived, the tube was really bashed in. The tube was hammed out, and a reinforcing ring put on were the tube was crushed, and the scope worked very well. With a tube that big, it is quite a bit easier to hammer out the dents than in your scope.

Cheers
Mike Connelley

Hello:
What was the dramatic and real differenc between your 2 scopes? Contrast? How faint of a star you could see? Planetary detail? The difference between 1/4 and 1/40 wave optics is (visually) quite small, and would most strongly affect the structure of the PSF. WIth a better figure you may get a higher strehl, which may allow you to see fainter. All of this assumes that the seeing is steady enough to allow your 15" scopes to be diffraction limited. If not, then I doubt the difference is in the figure of the mirror. One mirror may have a smoother polish or better coatings. Surface roughness (either from incomplete polishing, microripple, etc.) can quickly rob a lot of contrast without affecting the optical figure of the mirror. The baffleing of one telescope may have been better than the other.

Cheers
Mike Connelley

Hello:
In your case, it should work. From what you say, your secondary mirror is a flat. In this case, the telescope performance doesn't depend on the tilt of the flat. With a Newtonian configuration, you should be able to get your central obstruction to less than 20%. The one possible problem I see is that (as far as I know) the meniscus lens must be a specified distance from the primary mirror. Hopefully the Newtonian secondary can be closer to the primary than the lens, or else you'll have to punch a hole through your lens or grind the lens to a different prescription.

Cheers
Mike Connelley

Hello:
I gave it a shot last weekend, which turns out to be a good move since the weather forecast is not good here for the weekend. I used a 12.5" dob and got 108 targets, missing M52 at dusk and M30 at sunrise. It was partly cloudy at sunset, so I didn't seriously get started until it cleared off around 8pm. If it had been clear at sunset, I may have been able to get M52. There was clearly no chance at M30, and I think I'd have to wait a few weeks to have a good shot at it. Happy hunting.

Cheers
Mike Connelley
Hilo, Hawaii

Hello:
To maximize your field of view, you could go with the 31mm Nagler, 41mm Panoptic, or 55mm Plossl (if just to mention what Televue offers), which all have about the same true field of view. The 31mm is the most expensive, but IMHO it's worth it. A friend of mine has a C-11, and is always asking to borrow the 31mm! Since the C-11 has a fairly long focal length, it's very useful on most deep sky objects. If you get it, you'll use it a lot. The 55mm has a brighter sky background relative to the 31mm, and less of a "wow" factor, but it's also good, especially as a finder eyepiece. There are many cheap wide field eyepieces on the market that tend to work okay at long f-ratios, so you might want to look into these as well if you don't want to cough up $600 for an eyepiece.
For planetary observing, I use a few Meade Plossls and am very happy with them. Since the C-11 has a long-ish focal length, you can get high magnifications without getting super-short focal length eyepieces (esp. with a barlow) which have very short eye relief. Compared to a Nagler of the same focal length, the Plossl showed better contrast and detail.