Posts Made By: Doug Scobel

Please, everyone. They're vanes, not veins!

Just had to get that off my chest. I feel better now. 8)

Doug

David, with the few shots that I've done I've just brought the eyepiece to a focus by eye, attached or hand-held the camera, and then let the camera auto-focus. I've not tried setting the camera to a set focus yet, so I don't know if it is focusing "near" (macro) or "far" (infinity), or somewhere in between.

Sounds like something to experiment with.

Doug

Hi, Jeremy, I’m currently at work so I’ll have to keep this short. A few comments:

1) The must puzzling thing is that the star test is indicating undercorrected and the Foucault test is indicating overcorrected. This cannot be – one of them is lying to you. You may be misinterpreting the Foucault test results. When you say the center is “low” do you mean that the measurements (i.e. radius of curvature) near the center are too long relative to the parabola? If so, that really means that the center is “high”, because the radius is too long. Still, if the outer regions are good, then that is more important because there’s much less area in the center 2 inches than in the outer 3.

2) Make sure the “overcorrection” you’re seeing in the star test isn’t really a turned down edge. Double check that section of Suiter’s book.

3) The astigmatism you’re seeing may be the result of miscollimation or a not flat secondary. First ensure that everything is collimated – an f/5.5 will be quite susceptible to misalignment and it may be the culprit. If you eliminate that, then it could be the secondary. First make a note or drawing of the direction of the astigmatism you’re seeing. Then rotate the primary in its cell 90 degrees, re-collimate carefully, and then try again. If the astigmatism is still in the same direction, then it’s being caused by the secondary. If it has rotated 90 degrees, then it’s in the primary.

4) You need to take your 1/8 – 1/12 wave numbers with a grain of salt, especially since this is your first mirror, no offence please. It takes testing quite a few mirrors to really get good at doing the Foucault test right, and even then it’s not a “perfect” test. One thing to look out for with a fast mirror like yours is when to judge that the shading in the zone openings are equally “gray”. Make sure you judge them when they are just starting to darken, not after they have gotten really noticeably dark. You want the knife to be just encroaching into the converging beam, not well into it. If you compare when the mask openings are too dark then you’ll get erroneous readings, most likely leading to an undercorrected mirror (I think). That being said, if the mirror truly is 1/8 – 1/12 wave at the wavefront, then that’s really good. Are your numbers peak to valley or wavefront? Plus I don’t know what you mean when you say “although there are outlier data sets that have it at 1/3 wave”. That's a big discrepancy.

5) Almost no mirrors will pass the star test without some defects being seen. It’s that sensitive. There really is no such thing as a perfect mirror in practice. The real test is what does the diffraction pattern look like in focus. Try the double double again at really high magnification, say 400x or higher, seeing permitting. If you can see clean Airy disks with diffraction rings then your mirror is probably good, especially considering it’s your first mirror. Also the double double isn’t that good of a test – you might want to try something closer – Zeta Bootis at 0.7” in your 10” might be good, but you may not have good enough seeing to split something that close.

The bottom line is that the star test will nearly always reveal something wrong with any mirror. But if in-focus images are good, then the mirror is indeed good.

Hope this helps,
Doug

That's great, Floyd. Congratulations!

That's encouraging for me. I just spent the weekend installing an Antares 1/25th wave 1.30" MA secondary and Protostar single arch secondary holder in my 8-inch f/8. I've been using a generic old one, probably 1/8 wave, with a four-vane Novak spider. I'm eager to see if there's any difference. I'll probably try Zeta Bootis as a test.

All I need is a clear night. Last night was crap. Stay tuned.

Doug

Hey, Lee, I was thinking of using your cell design in my next project. But I was thinking of fixing the mirror to the rear plate (using spacers of some sort for air circulation around the mirror), and using the three outboard bolts that fix the rear plate to the one attached to the tube as the collimation screws. I would sandwich some weather stripping foam between the plates to make an air-tight seal yet allow for movement when collimating. Any thoughts on this? Did you ever consider it but reject it for one reason or another?

Thanks,
Doug

Tom, sorry to hear what happened, but by all means follow the good advice others have given and just cover it up with tape. You'll be amazed.

When I was refiguring my originally lousy circa 1984 13" Coulter mirror I brain faded and dropped a small pressing weight on the face of the mirror. It too left a small ding (smaller than yours) and some stress around it, but the mirror still performs well. I haven't even covered it up. It's visible out of focus but I can't attribute any image degradation to it in focus. But I did feel like a moron when I did it. Of course it probably would have felt worse doing it to a Zambuto rather than a Scobel. :S

Don't worry. Be happy.

Doug

Duane, I just bought a Pontiac Vibe (GM has some really good incentives on 2004s right now), which is really a Toyota Matrix under the nameplate. It's a small wagon, but with the rear seats folded flat there is a LOT of room, more than enough for that sized dob. My 13" solid tube dob (with 16" tube) will fit in it easily. A friend of mine transports a 12.5" Portaball in his Matrix.

My $.02

Doug

Walt, in addition to the other suggestions, perhaps you are working in a too dry environment? Relative humidity should probably be in the 70% range or so. If it's too dry then the slurry may dry out too fast, plus you may end up with evaporative effects giving you problems when figuring. Squeaking doesn't sound good to me.

Good luck,
Doug

Welcome to my world, Dannon. :C

Doug

As an Astrofest attendee for most of the past 15 years, I thought that a common complaint was the lack of decent skies, due to the proximity to Chicago. Is that not why they moved it?

Anyway, two years ago a group of us from Ann Arbor (Lowbrows) started going to the Black Forest Star Party, in north central PA, which is often about the same time as Astrofest. Longer drive, but much better skies, and not quite as crowded. We miss the Astro Dogs tho. At least there's no one yelling "Crystal!" all night.

Doug