Posts Made By: Doug Scobel

For the third time in the past several years, I'm attempting to refigure an 8" f/8 mirror. Every time I have tried I've run into the same problem. No matter what I do, no matter what stokes I use, I end up with a hill in the middle of the mirror. Sometimes broad, sometimes narrow, but always there, and always too large to ignore (I'll be using a 1" minor axis diagonal).

The mirror is completely polished out to the edge. I'm hand polishing on a channeled full sized lap (on Pyrex tool), mirror on top. Gugolz #73 pitch. Facets are 3/4", spaced 1/4". I use netting to microfacet the surface of the lap for better contact/conformity. Polishing with cerium oxide. The basement where I'm working is usually between 62 and 64 degrees F., with between 70 and 75% relative humidity.

I don't think that lack of contact is the problem. I always start the session by pressing. Earlier in the project I would leave the mirror on the lap overnight and wrap them with wet towels to prevent drying. Now I've been starting out by hot pressing, and commencing work before all the heat has gone out of the mirror/tool. In either case, I would polish for short periods, 5 minutes at most, then cold press for 5+ minutes before polishing some more, to maintain contact. There's no slipping and grabbing - the feel is good and even. No ripple nor dog biscuit either.

1/3 W or center over center strokes always leave the mirror severely oblate spheroid with turned up (yes, up) edge. Longer (1/2 dia. or longer) W or center over center strokes tend to flatten the edges, but I still end up with a broad hill about 4" in diameter. Really long parabolizing strokes with lots of overhang side to side tend to leave the center of the mirror too shallow, and with a smaller (maybe 1.5 - 2") hill in the middle.

Getting to a sphere has been all but impossible. Use of a lot of overhang, and doing most of the polishing in the overhang position reduces but does not eliminate the hill. To get it to go away completely I have to go so far over to the edge that I invariably dig a ragged hole.

Another curiosity is that no matter how much long strokes I use, I have never turned the edge. I always have a good diffraction ring all the way around. In fact, too much short strokes (1/3 - 1/2 dia) always leave the edge turned up.

It's almost as if the only part of the lap that is working is the extreme edge. I can control the width of the hill by varying the amount of overhang side to side that I use. Using more overhang will leave a narrower hill - less overhang leaves a broader hill.

I theorize that my pitch is too hard? That would explain why I have not turned the edge. Is there anything else that could explain why I'm getting the hill? In all the books I have (Texereau, Thompson, Howard, Ingalls, Berry), none of them explain what might create a central hill, only how to treat it (which never seems to work as described).

Now that I've finished my 8" f/8 mirror (yippie!), I need to start thinking about the tube assembly. First things first, I need a tube. Anyone have any knowledge regarding what's better, aluminum, fiberglass/resin, or fiber (Sonotube)? Factors to consider include thermal properties, dimensional stability, ease of finishing, etc. I will use this telescope primarily for planetary and high power observing, and so I am going with a small, 1" MA diagonal. To keep the distance from the secondary to focal plane to a minimum, I want to use a 9" tube. With a 64" fl, the tube will have to be close to six feet long.

Sonotube may not be an option, since it comes in 8" and 10" IDs only. Does anyone know about Kwiktubes?

Thanks in advance,
Doug

This has bugged me for a long time. Do anti-reflective coatings really increase the amount of light going through a lens? The simple answer is, “if less is being reflected, then there must be more going through”. But my understanding (which could be faulty) of how anti-reflective coatings work is that the coatings are made to create two reflections that are half a wave out of phase so that they interfere destructively. So the reflections are still there, you just can’t see them.

Certainly less internal reflection in an eyepiece will increase contrast, but my question remains – if the reflections are really still there, then how can the throughput be increased? I suppose that there’s a quantum-mechanical related answer that I may be unaware of. Anyone know?

Hi, all -

Had first light in my new homemade 8" f/8 Newt yesterday morning. Seeing wasn't great, I'm sure partially because here in SE Michigan Mars is culminating at only around 34 degrees altitude these days (and getting lower BTW). There were very brief moments when the air would settle and I could see more detail, but they were too few and far between to record reliably. Here's the sketch I made.

Doug

I finally got a night of decent seeing Friday morning. I was out Thursday morning, and the seeing was the usual 6, maybe 7. Couldn't sketch because it was so damp and dewy, when I tried using the eraser it was tearing up the paper! Had to give up. Anyway, set the alarm for Friday 2:00 a.m. (had to use the travel alarm, because we got caught in the blackout, but Mars observing must go on!), and boy I'm glad I did. Seeing was probably 8, maybe better, plus no dew! Here's the sketch. I got the SPC a little fat N-S, otherwise I'm happy with the sketch. I noticed that Mare Sirenum (to E) and Mare Cimmerium (to W) appeared to be connected, although they are not in any of the maps I've seen. Has anyone else noticed this?

Regards,
Doug

I got up and observed Mars again this morning. Seeing was the usual 6-7, except for about five minutes when it jumped to 8 or better. I have not had time to identify all the features, but Solis Lacus (the "Eye of Mars") was obvious to the east, and Mare Sirenum to the west.

It doesn't get any better than this! If you haven't yet, then get out there!

Doug

This is the first of two Mars sketches I made at the Black Forest Star Party in north central Pennsylvania (Cherry Springs State Park) on the morning of Aug 31. The sky was clear but with poor steadiness Thursday night until it hazed over, it rained off an on all Friday night, but it was clear and steady (although HEAVY dew) Saturday night, when I made two observations. This sketch was before midnight local time, the second one was in the wee hours.

Doug

Here's the second one.

Doug

At the risk of re-hashing a previous thread, I’m wondering what others’ experience is regarding atmospheric steadiness (or lack thereof) during this Mars apparition. I’ve been out at least a dozen times since mid-July, and I have yet to have what I would call a really steady night. Three or four of those times the images were so bad that I did not even bother to attempt a sketch. The seeing was more or less steady during rest of the sessions, some better than others, but never what I would call rock solid. Sure, there would be brief moments when I could see some really fine detail, but it was always very fleeting. So I’m sure my scope is capable of better images, but the atmosphere just won’t let me.

I’m observing with an 8-inch f/8 Newtonian. It has a fan that blows across the mirror, and out vents in the opposite side of the tube. I always let the fan run a good half hour before I start observing, which typically takes a half hour more. The fan is rheostat controlled, and I turn it down to about half speed while observing. (Star images at high magnification do not elongate so there is no vibration induced by the fan.). So I don’t think that it’s a tube current/boundary layer problem.

I always try to observe when Mars is transiting, to ensure that I’m looking through as little air as possible. But here in SE Michigan (~42 N), Mars is barely reaching 32 degrees altitude now. Not very high. Even at the BFSP (about the same latitude) this past weekend, we saw the same thing. In my back yard, I’m looking between houses, although there are a couple homes on a cul-de-sac a couple hundred yards away in the next block. After that it’s just woods and fields. But every time I have observed, the image always has some motion to it, enough to degrade the detail I can see.

Is anyone else experiencing this? How many of you have had at least one really steady night? Am I expecting too much from my latitude? Should I move to southern Florida before it's too late?

Doug

Here's my latest Mars observation from the morning of Sep 6 (Saturday).

Doug