29 replies to this topic

[chantepierre]

Hello dear ATMers,

 

I'm building a new wider-field scope, a 6" f/2.8.

 

I was on the way to finish the primary - just reached the L/4 envelope according to my Bath, but the surface was a bit zoney.

 

 

I tried a few local actions on that exterior bump, given that I'll use a 59mm secondary... I could live with the central bump.

 

Local actions not bringing me very far, I opted to smooth it TOT with my full size tool, aiming to de-correct it to -0.7 or so, to re-start correction from a smoother point.

 

And doing so, I made a deep, long, circular scratch on the optical face.

So, back to fine grinding, and I'll document my process for the second attempt here !

 

Clear skies.

[Pinbout]

you could talk it out with a full size lap, using the edge of the lap, scrubbing over the kink

 

https://www.youtube....h?v=tWEkTHAuc70

 

you can do the same this for the center, just never letting the edge over past the center...

 

https://www.youtube....h?v=Vr5c3NL9yR4

[duck]

when I see a plot like that, which has many slopes, I think about figuring for a ROC change.  If that figure was plotted against a r^2 curve instead of the straight line, what gains could be made in the figure.

 

I would have used a MOT stroke attacking the hill.  Then I would've switched to a TOT relatively neutral stroke to lengthen the ROC.  Both of those maybe two minutes total each.

 

But you suffered the common (at least to me) calamity of getting so close, then going hugely backward because of some penalty.

 

I grieve for you.

[starspangled]

when I see a plot like that, which has many slopes, I think about figuring for a ROC change.  If that figure was plotted against a r^2 curve instead of the straight line, what gains could be made in the figure.

 

 

He can do exactly that just by playing with / changing the number  of the reference ROC the software null uses,  in the top left box.

 

In this case all he needed to do was rub down the raised ring at r60 by 1/16th wave on the surface , and call it done . The 50% diameter secondary would have more than covered the central hill. Such a shame he was so close .
 

[Dale Eason]

He can do exactly that just by playing with / changing the number  of the reference ROC the software null uses,  in the top left box.

 

In this case all he needed to do was rub down the raised ring at r60 by 1/16th wave on the surface , and call it done . The 50% diameter secondary would have more than covered the central hill. Such a shame he was so close .
 

There is a more preferred way.  There is a defocus control in the lower right.  You selected that and then adjust the amount of defocus which is really changing the desired ROC.    It of course will raise  or lower the center or the edge depending on settings.  The RMS error will go up and the strehl will go lower of course.  But then you can polish the new high area.  However I expect that since that ring is not at the edge doing that exercise will not help.  Except if you can polish the wide area of the edge away to the right profile.

[chantepierre]

Hi,
I often play with defocus as it allows to select more optimistic strategies, but in that case, as Dale said, it did not give me an alternative path.

I had trouble bringing that hill down with a small polisher, mainly because of temperature I think. The whole thing was figured at 10ºC, and I think that was way too low for short polishing spells, even with gugolz 54.

Hence most of the way to the correction was done on a full size lap that just gave better contact than the 30% lap I used to get some correction going.

But temperature is now rising a bit and I suspect I'll have a better time for my second attempt.

[chantepierre]

The mirror is fine ground again ! No going back now.

I started with 9µ alox. It took 40 minutes to wipe the scratch away.

I then did some 5µ and some 3µ.

 

I always avoided 3µ because I've read that the risk of making a scratch was high. I did not get a scratch and the mirror has a nice pre-sheen to it.

 

Temperatures have stopped rising and it's 9°C in the workshop. I will wait before getting back at polishing.

[chantepierre]

Polishing started again, with temperatures on the high side !

 

A few first impressions : I fine-ground the surface MOT only after my failure, now the polish starts with the edge.

 

A first flash polish of 15 minutes showed a classic oblate, but not too pronounced.

 

The next 30 minutes show that the edge straightens / the oblate crest is pushed towards the outside, and the central 80% are fairly spherical.

 

I can only work 30 minutes at a time because of 22°C ambiant, but it seems to work okay.

 

Technique is full size lap, 1/3TOT + 1/3 MOT + 1/2 MOT + 1/3 MOT with a bit of lateral motion + 1/3 TOT with a bit of lateral motion.

 

It seems to spherize correctly.

 

The milky surface also clears quickly, I guess because of the coarsest grit being 9µ alox, and going to 3µ

Edited by chantepierre, 08 August 2024 - 06:37 AM.

[Tony_Gondola]

I'm curious, what are you going to use for a comma corrector at the F/ratio?

[ccaissie]

Polishing started again, with temperatures on the high side !

 

A few first impressions : I fine-ground the surface MOT only after my failure, now the polish starts with the edge.

 

A first flash polish of 15 minutes showed a classic oblate, but not too pronounced.

 

The next 30 minutes show that the edge straightens / the oblate crest is pushed towards the outside, and the central 80% are fairly spherical.

 

I can only work 30 minutes at a time because of 22°C ambiant, but it seems to work okay.

 

Technique is full size lap, 1/3TOT + 1/3 MOT + 1/2 MOT + 1/3 MOT with a bit of lateral motion + 1/3 TOT with a bit of lateral motion.

 

It seems to spherize correctly.

 

The milky surface also clears quickly, I guess because of the coarsest grit being 9µ alox, and going to 3µ

In ATMT it is discussed to use fine grit on a pitch lap to get very fast semipolish and/or a real deep asphere step.   Then brush some hot pitch over the surface and continue with cerium/whatever.

 

I repolished a 12" f/3 because of a scratch.  Interesting to see that I had really polished so much glass in figuring....when grinding, only the edge was in real contact with the old tool.  

 

 

https://www.bbastrod...parabStrategies

Edited by ccaissie, 08 August 2024 - 06:54 PM.

[chantepierre]

I'm curious, what are you going to use for a comma corrector at the F/ratio?

I plan to only use that scope at 33x with a 13mm eyepiece (3.3° TFOV), as a rich field "finder scope" piggybacked on the 16.5" I'm building. I will first see if the 4-element GSO corrector provides enough coma correction for this specific use case (even if the strehl tanks over the field, at 33x, it's okay).

If it does not, an explore scientific HRCC would be okay for this low magnification work. I'm planning to make it a bit of a one trick pony.

Edited by chantepierre, 09 August 2024 - 05:03 AM.

[chantepierre]

In ATMT it is discussed to use fine grit on a pitch lap to get very fast semipolish and/or a real deep asphere step.   Then brush some hot pitch over the surface and continue with cerium/whatever.

 

I repolished a 12" f/3 because of a scratch.  Interesting to see that I had really polished so much glass in figuring....when grinding, only the edge was in real contact with the old tool.  

 

 

https://www.bbastrod...parabStrategies

This is really interesting. "Master Optical Techniques" from A.S. De Vany also has chapters on very fast aspheres, with semi-rigid fine grinding tools to start aspherization at the end of fine grinding.

[ccaissie]

From ATMT Vol 1, pp. 14,15

Attached Thumbnails

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[chantepierre]

I have my starting sphere, finished initial polishing using ball milled rouge. I like the feeling and texture a lot ! It also seems to produce a nice polish.

I'm starting to aspherize with a 40% tool, mix of COC and offset strokes. The temperatures are high so channels close very quickly.

I hope this won't be a problem.

[chantepierre]

My 40% tool suffers from the high temperatures, I "fixed" it by copying Ed Jones and stuck a patch of LP66 on it with acetone. No more binding, I'm still using rouge, quite thick. I'm starting to put in some correction and will post test results once I reach approx. 50%.

[ccaissie]

The softer pitch was a good idea as long as you don't plow the edge.  Not sure about the pads...I'm sure you can parabolize, but expect to finish with soft pitch again.  I love the washed or milled compounds.

[chantepierre]

I did three 15mn sessions yesterday with the LP on pitch, surface still seems okay at the foucault, but I have a bit more trouble than usual controlling the shape. I'm sure the underlying tool would benefit to be rechannelled even with the LP on it. We have a week with lower temperatures coming ! If that confirms I'll switch back to straight pitch.

[chantepierre]

Okay, I'm again at the point where I made it a shipwreck last time.

 

I will be wise and not care about the center, because I have just been a fool who wanted to correct a central bump . I corrected it, about five times too much.

 

 

 

The 55mm radius hump was 0.2L higher before my last session where I concentrated on that crest.

I am gonna try to bring it down a bit, and call it done if I reach 0.85+.

 

My process for parabolization was full COC strokes with a 40% tool made of very soft pitch. I needed to reopen channels every 15 minutes. I used LP66 until I reached 50%, then removed it.

 

Now I need to lower that bump..

[chantepierre]

I'm calling it done - sadly my last correction attempt (before reaching .75) made a few sleeks, but not the kind you can feel with your nail. They are here though, and will be visible on the coated mirror - but going back for that reason would be pure ego.

 

I had to do five 5mn sessions on the 50mm radius bump, that reduced in height and moved to the outside, to be located at 60mm radius at the end. I also did a few turns on the edge because it started to get higher as the 50mm bump reduced, and correction dropped to 98%. I also lowered a bit of the crest at the center but that was purely for cosmetics.

 

Still, with all those little sessions on little radiuses with a tiny tool, it's not very smooth. It's okay for its intended use, but I'll keep a bit of frustration on that side.

 

 

[chantepierre]

Here's the first prototype of the scope.

 

A few things will of course change but I worked with what I had on hand :

 

• 3mm carbon rods will be solid 5mm
• Wood slab will be a 360mm aluminum dovetail
• .5mm aluminum slabs in the spider will be 1mm carbon
• Black/red shiny plastic will be matte black

I confirmed focus and image sharpness on a few daytime objects.

 

Cannot wait to silver the mirror and complete it in a few weeks :^)

 

Edited by chantepierre, 18 August 2024 - 10:54 AM.

[chantepierre]

Almost there.. I took heavy inspiration from the legendary Coulter CT-100 for this build, and printed everything in deep blue ASA :

 

 

Still need to adjust the correct length for my eyepieces on the sky and cut the carbon rods. The mirror was silvered last week with the spray process

[ChrisLansdale]

Which focuser model did you use?

[chantepierre]

Hello Chris, I designed it, it's quite basic. There are two pairs of bearings, each 45° away from the vertical, riding on aluminum flats on the tube.

A M5 nylon screw pushes a pusher block with a groove, pushing a smooth rod against an aluminum flat on the bottom of the tube. Friction is adjusted with this screw. The hole in which the smooth rod inserts is slightly oval, allowing it to be pushed without bending.

 

There are a few models based on these principles online, I took inspiration from marcosatm's 2" crayford.

[chantepierre]

Files are now on Printables along with a BOM. You'll find the focuser in them. Don't expect a documented straightforward build though :

 

https://www.printabl...telescope/files

[chantepierre]

A few updates :

• I now have 6 solid carbon rods, might as well not compromise on stiffness. That made me lose 5mm of focal plane out-position because it was unreasonably hard to fully constrain the rods.
• Primary collimation springs are a longer, to move the primary 5mm up. They are 1.5mm thick piano wire springs, 6mm interior diameter. This is super strong.
• "Factory collimated" sticker on the upper cage

I had a quick first light yesterday evening in a 10 minutes clearing (with high altitude clouds). Stars are round and tight and I do not see residual sphericity. The GSO corrector seems to work well, I will need a longer night to find the best backfocus setting.

 

Next steps :

• Light shield
• Quickfinder mounting socket
• 1:10 printed reducer
• Shroud

I hope to finish the 16.5" quickly to be able to mount the 6" !