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18" Cassegrain project


Sfarndell
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It seems to be mirror grinding season again! 4 years ago I started an 18x1" mirror to see if I could do it. Bear in mind while reading that this is my 1st and only mirror making project and there are some real knee slapper...learning experiences.

in 2009 I got it to flash-polish stage  which showed a good spherical shape then the project was shelved it when my daughter was born...fast forward 4 years.

After clearing off the cobwebs, I remade my pitch lap and proceeded to try and polish out the mirror properly. I picked up a number of deep scratches which after much head-scratching and process re-visiting ended up being that I had accidentally put aluminum oxide (+-800 - 1000 grit) in the polishing mix instead of cerium oxide (there were two little white double-bagged packets in the CeO hard container and I picked the wrong one). DOH!

Since I now had to make a tool anyway (tossed the old tool when we moved house 4 years ago) and the focal ratio was 4.4 and I really wanted a faster mirror for a cassegrain, I had to bite the bullet and regrind from 80 grit. I made a tool out of 18" cutouts from some old kitchen cupboard doors which I glued together then varnished to seal. Mistake #1 was to not use screws. I then cast a curve using cement (face down on a covered mirror) and glued and sealed the tiles with fiberglass resin.

Grinding was straight-forward, but time consuming and was done exclusively MOT. 3 hours on 80 grit got a good contact with the tool, and then going through the grits took 60-90 minutes. my sequence was 80, 120,240,320,400,600 then 1000 grit....then 400, 600, 800 polish. I worked every night for a week and spent 2-3 hours on it on the 1st weekend so got to the 1000 grit quite quickly. At the 1st round of 600 grit I went to an experienced mirror maker at my club to help core out most of the centre hole as my drill press didn't have a long enough arm to do the job. Too much talking and not enough concentrating (Mistake #2) and he accidentally didn't stop 2 mm from the face, but cut through on +-25%. This led to a number of issues on polishing....more to follow.

I filled where we cored with cement and plugged each surface (back and front) with a thick layer of fibre-glass epoxy. As the centre got deeper, this was process was repeated a few times before polishing. at 1000 grit I picked up some deep scratches near the edge from clumping - there was no indication that the scratched originated at the core, but it is possible. I then went on holiday for a few weeks and it was a month before I started work again. By this time, it was clear that I hadn't sealed the tool adequately and it had warped enough that there was no longer good contact with the mirror (aaarrrrggghhh)

fortunately 2 hours at 400 grit was enough to get good contact again.  To prevent any additional warping from impacting my mirror,  I resealed the tool and ground to 800 in 2 sessions - about 5 hours total.

I decided to polish from 800 grit using my re-made 12" lap and some zirconium oxide provided by a colleague at the mirror-making class. That stuff is aggressive and works fast!! in 90 minutes I had a glossy surface (not polished out though), but there was a deep channel at 70-80% of the mirror which I think was from pressing too hard and flexing the mirror over the edge of the tool. The edge looked and centre looked good so  I focused my polishing stroke CoC with 1/4 (2" overhang")  to deepen the centre to the depth of the channel. The zirconium oxide works fast and after 5 hours total polishing time (not prep), I considered the mirror to be fully polished out. The time seems fast, but my lap had good contact, Zirconium Oxide is very aggressive and I used a lot of pressure while polishing to try get to depth of that channel. The surface was quite smooth, but not as smooth as the CeO which I am using to figure with. I know that a lot of amateurs don't fully polish out on their 1st mirror, but I'm not too concerned as to my  eye the mirror is fully polished and the additional few hours to get rid of the zonal issue I have will likely take care of that. I will get it tested by the experts at the class once the zonal issues are fixed though just to be 100% sure!!!

The groove from the coring has made polishing occasionally difficult and "grabby" when pitch gets caught in the groove (I press hard when polishing), but it's an easy task to clean out and there are currently no visible scratches in the mirror.

There is a small turned up edge and a hill from about the 50% zone with some parabolization in the 50-95% zone. Attached are progression pics of my polishing. I think it looks promising - there is no noticeable astigmatism when the mirror is properly settled on the (somewhat rough-and-ready) 22 deg whiffletree stand. 

The next step is move to use an 8" lap to attack the hill and then the 12" to blend with the rest of the mirror. I'll address the Turned up edge when the zonal issue is done. ANY help with figuring technique is most welcome!!

After 5 hours of polishing by my reckoning...

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6 hours polishing...

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7 hours...

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8 hours...(3 hours figuring to me :).

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22"!! Big gulp....preferably enhanced by whisky while you grind away :). That really is HUGE. Manual or machine grind? If manual, your shoulders and arms will showing very improved definition by the end of it :D.

Just spotted a typo ...  I have a small turned down - not up -  edge (only really noticeable in the ronchi outside focus to the untrained eye) about 3-4 mm wide. I'm hoping that it will be fixed with the hill zone issue, but if it's only reduced then I'll take a short-cut and just bevel the edge more (currently <1mm bevel). Watch this space...I think that if a mirror has had an interesting issue, it was this one so the fix will be informative. Never give up, never surrender!!

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After another 60 mins with the 8" tool and a further 10 minutes with the 12" to smooth things out the hill is looking a lot better. Still a lot more work to do here. The TDE hasn't changed too much so should still be manageable once the centre is better. I also seem to have removed some correction at about the 60% zone. Any input from the pros? Glasspusher? Neil?

I also unfortunately picked up a horrible 4cm long scratch from the small tool when it was first used which will not likely be polished out. Sigh. Still, if that's all the mirror has then it won't be too bad. Herewith a pic of the current status. Quality is suffering as it was taken on my phone and enlarged.

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I worked some more on the hill in the centre using the 8" lap COC on a 6" stroke i.e. the stroke ended 2" inside the edge for 40 mins.  The 1st pic was just after the 8" lap work and the centre hill looks mostly removed.

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This was followed by another 40 mins of smoothing with the 12" lap with a 3" overhang and a 2" offset (chordal stroke, not a "W"). The results were promising and the hill was mostly removed. The edge seemed to look worse though. I then continued with a chordal stroke for 30 mins then a W stroke for another 30 mins, using a 2" overhang and the results are in the pic below: the central hill is returning and the edge looks bad and rolled. I press my lap overnight using the weight of the mirror, so it is in good contact. There does seem to be some roughness as the lap passes over grooves where the coring came through the face of the mirror, but it is minor and likely the reason for the slight roughness.

Figuring +-5.5 hours - inside focus

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Figuring +-5.5 hours - outside focus

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Mirror making gurus please chip in and correct my logic if it is off: My plan is to address the edge while monitoring the centre so I intend to use a "W" stroke with 1-2" max side swing and a negative 1/2" overhang (i.e. stop short of the edge) for an hour first to deepen the mirror uniformly to just inside where the rolloff starts so that it forms a hill, then shave that off by eventually extending the strokes to a 1" overhang to take the curvature over the edge...

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I worked some more on the hill in the centre using the 8" lap COC on a 6" stroke i.e. the stroke ended 2" inside the edge for 40 mins.  The 1st pic was just after the 8" lap work and the centre hill looks mostly removed.

post-1508-0-11398400-1387263308_thumb.jp

This was followed by another 40 mins of smoothing with the 12" lap with a 3" overhang and a 2" offset (chordal stroke, not a "W"). The results were promising and the hill was mostly removed. The edge seemed to look worse though. I then continued with a chordal stroke for 30 mins then a W stroke for another 30 mins, using a 2" overhang and the results are in the pic below: the central hill is returning and the edge looks bad and rolled. I press my lap overnight using the weight of the mirror, so it is in good contact. There does seem to be some roughness as the lap passes over grooves where the coring came through the face of the mirror, but it is minor and likely the reason for the slight roughness.

Figuring +-5.5 hours - inside focus

post-1508-0-23022300-1387263575_thumb.jp

Figuring +-5.5 hours - outside focus

post-1508-0-57221400-1387263622_thumb.jp

Mirror making gurus please chip in and correct my logic if it is off: My plan is to address the edge while monitoring the centre so I intend to use a "W" stroke with 1-2" max side swing and a negative 1/2" overhang (i.e. stop short of the edge) for an hour first to deepen the mirror uniformly to just inside where the rolloff starts so that it forms a hill, then shave that off by eventually extending the strokes to a 1" overhang to take the curvature over the edge...

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I would ignore the centre completely and concentrate on the edge. Get that right and the high centre can be dealt with later using a small lap. I would use an off- centre, small W stroke to start and see how that works, moving the offset nearer or further from the centre as testing dictates.

Nigel

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The amount of overhang is not important. The main area to concentrate on is where the centre of the tool is, as that is the area of maximum work, with decreasing effect away from there. A short straight stroke will give the most concentrated effect over a narrow zone and increasing the stroke length and adding a W will broaden the effect and reduce it's localised intensity. The lap size also affects the results with a small lap having a more concentrated effect than a large lap However, short straight strokes will produce depressed zones at the edge of a sub-diameter tool's action unless the pitch facets are feathered towards the edge. With very small tools you can use a "flower" stroke ( little circles centred on the high zone ) to work on very narrow zones but these can result in overwork very easily so should be used sparingly.

So you have plenty of variation to play with.

Nigel

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As Nigel points out small tools can result in zones, so take care. With an 18 inch mirror I would use a 75% tool for general polishing. To sort out an edge like yours I would work with the mirror face up and use long strokes through the centre. This will produce a smooth under corrected mirror with a good edge. As Nigel says sorting out the edge is the priority. Once the edge is sorted you can use a 50% tool through the centre with minimal overhang, this will start and deepen the centre. Good luck with the mirror!

John

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  • 9 months later...

It's been a while since my last post, but a lot happened. A tragic accident left the face of the mirror deeply scored and I had to go back to fine grinding and motivation suffered so the project was paused for a few months. In the mean-while I made a spin-polisher, which saved a lot of time and now the mirror is back on track.

The ronchigram below is progress as of last night. the edge is quite good - perhaps a little turned at the last 2 mm, but it could be diffraction. Either way, the current bevel is <1mm so any edge issue will be ground off after figuring. to my eye, the outer 40% is close to being parabolised and the inner 60% needs some deepening (compare with ideal ronchigram - 2nd from right at the bottom).

The pic is rather grainy, but the actual surface of the mirror is smooth with no zones and shows no astigmatism.  Plan is to do narrow "W"  strokes (1-2" swing) with an 8" lap starting at a 25% (2") overhang and reducing that to a -2" overhang in 20 minute sessions to bring correction to the centre followed by a short smoothing session with a 12" lap. 

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  • 4 weeks later...

I did some work on the centre for about 20 mins, but other commitments have prevented much progress since my last post. I am also worried about overworking the mirror without more quantitative analysis. As I am part of a telescope-making club, I took the mirror to class on Saturday for its 1st "professional" Foucault test from guys who've been testing mirrors for 15+ years.

The results were interesting. The centre is still undercorrected, but i was surprised by the level of overcorrection at the 60%-80% zone (see diagram below). Any advice from as to how to approach this? tackle the edge then work the centre, or reduce the centre with a small (6" lap) and then work on the edge?

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Hello Scott

Sorry to hear about the accident you had with this, these things happen.  Glad your motivation has returned to see it through and also nice to see the Ronchi images having just started testing my mirror I have similar images although this is because I'm under polished in the middle.

Keep posting as it's interesting to see how much things change with just a short session 

Damian

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Some people show real dedication ! I am impressed and I will follow this thread closely.

Lol...it will take patience to follow. Work gets done in week-long fits of inspiration followed by long periods of "aaarrrggghhhhh why am I doing this!!!". 

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  • 2 weeks later...

Found some time this weekend to work on the mirror. In the end I decided to correct the centre a bit and catch up with the middle zones before addressing the edge. I did 2x 10 minutes with the 6" lap and a 1" offset followed by 2x 15 minute sessions with the 8" lap (narrow W) and finished off with a 3 minute (single trip around the barrel) with the 14" to smooth things off. I tested after each session, to make sure I wasn't causing major issues.

The centre-deepening technique seems to have added a fair amount of correction, but it wasn't uniform - there is a funny zone betweel 4" and 6" radius. The 70% zone has a big kink which, according to Mel Bartels' matching ronchi calculator, is indicative of an over-corrected/deep 70% zone. This is in line with the readings from the Foucault test and would not have been affected much by the work with the 8" and 6" laps.

To correct the edge, I intend to take the 14" lap and work with a narrow "W" and about a 25% overhang in 10 minute intervals. Hopefully this will push the correction in the outer 2" out over the edge and reduce the high (rolled?) edge. If this works and everything else stays the same, then I will try address the zone at +- 5" and add correction with the 8" lap. I won't work on it until Tuesday in the hope of giving time for more experienced members to comment/ share a better alternative. 

Herewith the ronchigram on decent camera, compared to the ideal. 

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Hi Scott.

I think that you are going in the right direction in using the bigger lap. Personally I would probably use a long and wide W stroke, ignoring the centre and any zones until the edge is O.K. It is very much easier to deal with a high centre than it is to deal with a recalcitrant edge. I might even try using the 14" lap off centre for a while as you are trying to get work into the edge area. Once you have the edge correct it needs to be left alone so smaller laps are used to remove the centre bump and grade the curve up to the corrected edge. I am assuming that all work is being done with the tool on top which is the way I work these larger mirrors.

Nigel

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