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Hi Carl and all. Only my Second post to this forum so hoping the experienced can help me assess this situation. Back in after 2018 I decided I wanted to build my own telescope, a 10 inch truss Dobsonian and also undertake the fabrication of the parabolic primary mirror. This was all done as a precursor to the final goal, a fully self built 20 inch truss dob to be used for high resolution planetary and lunar imaging. I am an experienced imager of nearly 20 years and have several other scopes that I have used in this time. I spent the first year building a mirror o matic machine before starting my first ever mirror. Of course prior to this I had tried to gain as much knowledge from various sources to give me some confidence of being able to complete this project with hopefully a decent outcome. So in 2021 the 10 was completed and I have been successfully using this for imaging the moon and planets with good results. It star tests well also. Next up a 20 built on a very similar design as the 10. This brings me to the current day and the reason for this post. I am hoping you folks may be able to help assess a problem I’m seeing during star testing my recently completed 20 inch. Some specs for the mirror are as follows: · Material: Supramax 33 from Schott · Back ground flat to 320 grade · FR, f4.4 · Clear diameter 502mm (3mm bevel after removing some TDE) · Edge thickness, 39mm (started at 41mm) · ROC 4420mm · Secondary mirror is a 4” minor axis Antares 1/30wave. · Mirror cell: 18 point floating design based on Plop analysis. Points are nylon, approx 12mm diameter. Edge supports are Whiffletree design placed at COG. · Triangles are 8mm thick aluminium on spherical bearings. · Single large primary fan placed at rear on centre. Two front face fans. Most final figuring work carried out by hand on the mom turntable. The turntable is 3 layers of 19mm mdf and was bonded together on a granite surface plate. It is flat to a few thou and sealed. Support is a layer of false turf. All testing during figuring carried out using Foucault moving source slitless tester and Ronchi 100lpi grating. Also utilising the matching Ronchi test from Mel Bartels site. Over the course of several months I worked this mirror to the best of my ability for more hours than i care to remember and aimed high with my goal for the final figure. At times it was really painful but with a strategic approach it finally started coming together. The finished mirror is the best I think I can achieve, I am really hesitant to put lap to glass again but if I need then so be it. So the problem seems to be significant overcorrection showing in the star test. I have been using an 8mm Baader Hyperion eyepiece with and without Teleview 2 x barlow. I see the same overcorrection in either situation. I have also used a 2.5x Powermate but same result. I realise I need to remove any barlow/Powermate as I don’t know how much error this could be contributing but as of now I can’t purchase any other premium eyepiece. I do understand overcorrection is normal for a large cooling mirror as the edge shinks more than the centre but so far for me the condition remains even after lengthy cooling (4hrs). Mirror is kept indoors at the moment which is approx 21deg C. Is this amount of cooling still too little? I have been measuring the air temp and rear face of the mirror using thermocouples. Last time out the rear face got to within 0.1 degree of ambient but of course I can’t determine the inner parts of the mirror which could be warmer. The front of the mirror was exposed to the breeze, its not a deep mirror box design. Seeing is usually poor where I am in the UK, the scarce good night may be weeks or months away so I have to make do. But I can easily see the classic signs of overcorrection through any turbulence. Any residual TDE being the source of the problem I have hopefully eliminated with an aperture mask. The inner and outer defocused star images still show overcorrection. Unfortunately, I don’t yet have images of the star testing. What I’m seeing is fairly clear rings outside of focus with brighter outer ring and indistinct spiky rings inside of focus. So I would really appreciate the vastly more experienced amongst you to advise me if possible. My assumptions are that the matching Ronchi images show a fairly well corrected parabola. Would you agree with this assessment in respect to matching ronchi? Please see attached pics of matching Ronchi images for the finished mirror. There is still some tde, I’m not too worried about this and can see a complete diffraction ring albeit not 100% balanced. If I need to increase the bevel a little more at some point I will or use the mask. I realise there could be other several factors contributing to this scenario but the main reason for this post is to obtain some idea if indeed the primary mirror is the likely cause and I need to re-figure (scary thought). The in focus images look good considering the seeing conditions. Stars seem pinpoint, the moon shows good levels of detail with a crisp edge. Jupiter when I last looked was also respectable given the average seeing. I am an experienced imager but in mirror making still inexperienced so this is why I seek your help. I hope I have provided enough information for assessment. One final point also, I can’t detect any astigmatism at all thankfully. All advice greatly appreciated. Pete.

Greetings, folks. Paul Valleli caught my attention in a private email to a gentleman who is building a polishing machine. It may be covered here in general, but I'm taking a particular segment of text and starting a new subject. The reason I'm doing this is because someone of unusual experience in a career in industrial optics made mention of the subject. So as a start, I'm going to insert the text that caught my eye, then the answer that came from Paul when I sent it to him, which was posted here in the most recent posting. I intend to make a serious subject from this and hopefully to have material that can be codified for the amateur community. This is only the start, I will have much more to say on all this in the coming weeks and months. I would like to attempt to keep this thread as clean as possible, meaning I hope it stays on topic. Here is the starting correspondence: "We still had to return to hand-figuring to smooth out the surface roughness and reduce slope errors to less than one arc-second. That was done with colloidal cerium oxide to reduce light scatter in the short wavelengths of blue, violet, and UV regions. Dale Eason had added that display feature to DFTF testing for the Interferometry group. We did the final figuring with the optical workpiece mounted in the test setup for interferometry." --------------------- Private email for Paul V: So Paul, this is the first time I’ve seen slope errors addressed in any meaningful fashion…ever. This is what the Zambuto method and theory are all about, and no one else talks about it in the amateur telescope industry. Starting a discussion on this subject could be of value, and what I mean is, I might find corroboration from the industrial side, because these concepts are lost on the amateur public, who continue to ask what the Strehl rating is, thinking its going to define the quality of the optic. My whole thing is, the entire industry is hung up on measuring HEIGHT errors, and have no concept whatsoever of the effect of SLOPE errors, and how they contribute. As a mention at this juncture, everything we are about here is SLOPE, and I concern myself with such errors on every level, from the polish, to the large scale measurable surface. When one considers the Seven Criteria, such can be noticed by the thinking individual, if they are really paying attention. Suiter addresses this right in the introduction, and it has been overlooked by EVERYONE as far as I know. I have never seen a discussion on slope issues anywhere. Suiter’s text is copied below. Carl From Star Testing Astronomical Telescopes chapter 1, Introduction: "Another factor often neglected in statements of optical quality is the slope of the error. If sharp channels, turned edges, or roughness appears on the optics, the overall wavefront error can often be contained within the expansive Rayleigh tolerance. The anomalous slope does not persist over the whole aperture. The sharply sloped fault still diverts light out of the central spot to pollute the rest of the image, but the optics are still "officially" perfect." And then here is Paul's first response (again) from the previous thread here on this forum: Carl: The one arc-second slope tolerance was introduced to my knowledge of tolerancing accuracy requirements by an Ultra-Violet Astronomer at NASA's Goddard Space Flight Center. They had learned much from Astronaut Don Pettit who comes from a long line of astronomers. Another was Norm Remer , optical engineering contractor in CT that cut his teeth as a graduate student working on building UltraViolet Telescopes at Univ. of Wisconsin. His books on Refractor lens design and Interferometry for the ATM were suggested by Perry Remaklus at Wilmann-Bell publishing company. Dale Eason of the Interferometry group added a feature to the DFTF analysis software to indicate excessive slope in the color yellow on the surface contour maps. I doubt many amateurs pay attention to it. Yes, the Strehl Criterion does not take into account slope errors or scattered light that surrounds bright star images as an aura. It lowe

Sorry if this is off-topic. I am curious as to why there seem to be no amateur Korsch telescopes for sale? Is it just too difficult to produce so that the high prices are prohibitive? I attach the layout of this telescope, with 3 mirrors and 1 flat reflecting plate. I see that several government organizations recognized the elegance of this type of telescope and had them made, such as the James Webb telescope. This telescope seems ideal with its correction of the common optical aberrations such as coma, etc. Randall

Returned to astronomy after a 45 year hiatus. In 1973 ground an 8" f8. In 1975 a 6" f6 that I still have. So has only been 50 years since I have done any mirror work. Recently I obtained a red tube Coulter 13.1" scope for free. I knew going in the optics ranged from okay to not so good. However I like free stuff and worst case could get rid of it should it prove unusable. Got it home and cleaned it up. Pulled the mirror and removed dead mice and a bunch of disgusting crud. Coating was missing/worn in multiple areas. Put it together and got it sort of collimated. Star tested it and it actually did not work too bad. Yes it has coma. Stars at center focus quite sharp but squish out different directions inside and outside focus. Not unexpected as the focuser made from plumbing parts and only 1 primary bolt worked. At this point stripped the coating. Mirror has no scratches or defects I can see. Built a simple focault tester to evaluate the primary. That is where I am at right now. Got everything aligned and reading up and playing with the tester to reacquaint myself with things. Looks like a parabola, hyperbola or oblate spheroid. As near as I can tell! However there is something on the mirror face. No idea what it is. Hoping someone here can tell me. Before we get too far, my intended use for this scope is to have it at friends in northern Minnesota. It will be in a pole barn and wheeled out for use. Perhaps 10 times a year. I have a good 10" dob and plan to buy a used premium 14" to 16" next year. So I am looking to spend minimal time working on this project. The back of the mirror has a very orange peel surface. The lighter stuff on the mirror face. (Looks like an alien world) . Took this handholding my phone. Was suprized it turned out. However the shape presented not what I saw. It was more pronounced and even. I hope to hear what I am dealing with so I can make a desicion on next steps. Regards, John Baligrodzki Saint Paul, MN

I thought I would add this, because I have something printed right now I don't need to use. Once we have the parameters I get out a 2D CAD program (very, very old on an old Mac) and draw circles. The program is able to lock centers of objects, so that really works nice. The software locks all the circles together on center. Then I print out the result on a laser printer, and use that as the base for the mask. We usually reinforce the back side with plastic shipping tape, and we leave the face untouched because we don't want it to be reflective. Attached is an image of one half of a mask we made this week for a special project. What you're seeing was drawn in Claris Draw 1.0 (they never updated and stopped selling it). Then I copied the objects to Appleworks in Draw mode and printed it on an HP laser jet printer. Once that's done we lay it out and check the zones with an engineer's ruler that measured as small as .01 inches. It can be plus or minus .01 or .02 and not harm the measurements (that's another discussion). Then we cut the zones out with a razor blade, and create a cardboard backer that it fits into, then it hangs over the mirror. I'll find a photo of a mask on a mirror and send that next. CZ

I bought a 10" quartz blank back in January 2008 with the intent of grinding a mirror and building a telescope. These blanks were all over EBay back then, I think they came out of the semiconductor industry. I ground and polished to the point of testing but found I didn't have a good place to set up for testing and that I really didn't understand how to test. It got put back in the shipping box. I am now retired and have a decent shop to work in. I looked at the blank a couple years ago and thought about working on it again but stuff got in the way. I did strip the old pitch off the tool expecting that it was contaminated from storage and purchase new pitch and polish. My wife has recently been shooting pictures of northern lights and comets and it would be nice to have something better than a camera on a tripod with a long lens. I got out the mirror and set up a Ronchi tester. My lathe and mill worked out to the right distance and I can measure distance traveled with the DRO on the mill. My goal isn't Hubble level, just something nice enough for backyard observing. The mirror is 10" diameter, about .75" thick, ROC is about 113.5", grating is 85 lpi. I am not exactly sure what I am seeing. Today I played with camera exposure and shot these sequences. The step interval on the photos is .1000" as measured by the DRO. The links are to Google Photo albums. Outside ROC by .1" steps - https://photos.app.goo.gl/seaech3JQtyvyANw9 Inside ROC by .1" steps - https://photos.app.goo.gl/TijAS84aW4SuPhmdA I need to build a table again (my last one was EMT conduit and worked well) and repour the pitch. Then understand where I am at and finish the mirror. Any input will be appreciated. I am about 40 miles south of Rochester NY if there is anyone nearby that can take a look at it. Dave

Hello I got a scratched GSO mirror 12"x1.5" (300x38mm). Probably BK7 and I stated to regrind it. For fun I did a stress test and now I'm as stressed as the glass. Should I anneal ? If so, I found a local glass sculptor/jewerly maker that could anneal it in his kiln. However, what firing schedule should I ask for ? I'm in Europe BTW. I already asked on reddit, webastro, and cloudynights. Here is my last chance. I fell like either people don't look at strain OR they fine anneal everything. I did not find many people annealing blanks and even less technical information as professional who anneal tend to have their own kiln. If the firing schedule is a secret you need to keep guarded for professional reasons I understand, just tell me so.

I was offered a chance to buy a Starmaster 7" Oak Classic for a very reasonable price. So I was excited when I met the lady to pick it up. I about fell over though when tipped the tube down and saw what was written on the mirror.