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8" f6 Mark II - addicted to mirror making

ATM Beginner Mirror Making Optics
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#26 Knight Sky

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Posted 12 April 2022 - 03:46 AM

I did the stroke mentioned in #25.

 

Since the center mound was looking unplesant to my eyes, I did what I perhaps shouldn't have - applied a 2 stroke ToT 1/3 D. I have not been able to evaluate the mirror but will do so after this post.

 

Ronchis after stroke #25:

 

IMG 20220412 131038 2
 
IMG 20220412 130934 2
 
IMG 20220412 131115 2
 
 
Ronchis after 1/3D stroke:
 
IMG 20220412 131200 2

IMG 20220412 131257 2
 
IMG 20220412 131333 2
 
1/3D may have reduced correction on outer zones a bit even though it seems to have smoothened the center a bit.
 
EDIT: Ronchis were taken soon after the sessions and most likely the mirror may not have reached the state of equilibrium. As usual I will take the readings later in the evening/night.

Edited by Knight Sky, 12 April 2022 - 03:52 AM.

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#27 Knight Sky

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Posted 12 April 2022 - 04:06 AM

Seems like zones 4 and 5 are slightly overcorrected after this session. Need to wait for the ke readings to assess the mirror.



#28 dogbiscuit

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Posted 12 April 2022 - 04:25 AM

I was just getting ready to send you the stroke that together with the accented pressure on zone 4 would have gotten you to 1/20 wave PV. smile.gif 

 

The TOT accented pressure strokes are a bit complex and really not so likely you would do them right the first time.

 

the Ronchi looks as though you might be able to do fairly normal MOT parabolizing from what you have now.  See what the test readings show.


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#29 Knight Sky

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Posted 12 April 2022 - 04:35 AM

I was just getting ready to send you the stroke that together with the accented pressure on zone 4 would have gotten you to 1/20 wave PV. smile.gif

 

The TOT accented pressure strokes are a bit complex and really not so likely you would do them right the first time.

 

the Ronchi looks as though you might be able to do fairly normal MOT parabolizing from what you have now.  See what the test readings show.

Seems like I may have added a little more pressure than I should have as moving and gripping the tool seemed a little tricky.

 

I will be posting the ke readings as soon as I can. Ronchi suggests I may have fallen back a little (quite a bit).

 

EDIT: Have we forgotten about the edge? How can the edge be improved from here on? It seems to me that we are pretty close to making it decent.


Edited by Knight Sky, 12 April 2022 - 04:38 AM.


#30 dogbiscuit

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Posted 12 April 2022 - 05:25 AM

I don't think 4 and 5 are overcorrected.

The edge looks fairly good in the Ronchi images.

Foucault will show the edge better than Ronchi.

I think you did good.

 

The numbers will tell.


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#31 Knight Sky

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Posted 12 April 2022 - 06:30 AM

These are preliminary ke readings across a single dia. Seems like you were right, I didn't do all that bad....z5 is perhaps slightly overcorrected. But all in all not too bad. Need to take confirmatory readings across a few more diameters later on.

 

z1 z2 z3 z4 z5

 

0 0.28 0.79 1.18 1.68
0 0.27 0.7 1.13 1.6
0 0.27 0.73 1.12 1.6

 

avg.

 

0 0.273 0.74 1.143 1.627

 

I haven't run them yet but I think fxp wouldn't be too unhappy with these numbers.



#32 Knight Sky

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Posted 13 April 2022 - 03:23 AM

I was just getting ready to send you the stroke that together with the accented pressure on zone 4 would have gotten you to 1/20 wave PV. smile.gif

 

The TOT accented pressure strokes are a bit complex and really not so likely you would do them right the first time.

 

the Ronchi looks as though you might be able to do fairly normal MOT parabolizing from what you have now.  See what the test readings show.

dogbiscuit, please let me know what strokes you were going to suggest after accented zone 4 pressure stroke. That will add to the learning and could be used in the future.

 

As for the state of the mirror, there seems to be a major ( minor?) setback. As I was about to get comprehensive readings across multiple diameters, I decided to get Foucault for how the edge looks and also check for astig. And to me it seems that a hint of astigmatism is apparent in these Foucault images. I have been meticulous about checking it almost every major session of polishing and or figuring. I am genuinely flummoxed to see it as it wasn't present about 2 days back or so.

 

I am trying to think about all the reasons that could cause astigmatism and I am listing those here:

 

1. My technique - I was not able to do long Ws of #16 properly and strayed slightly off the diameter (at some minor angle). My technique could be questioned again for the strokes of #25, that I wasn't stroking exactly perpendicular to the diameter. If I think back I think #25 went more smoothly and confidently than #16 though. But it is possible that the mirror moved between strokes in #25 since the force was off the CG of the mirror.

 

2. Being a thin mirror some of this is a result of tester stand. I see the shape of the mirror change once I put it on the mirror stand and watch it for a minute or two. The dark shadows tend to move more towards the bottom ever so slowly. These don't seem to be thermal currents, that are a little more wayward.

 

3. If at all mirror is changing shape on stand, there is a likelihood that I have "imprinted" some of the deformation permanently on the mirror as I have picked the mirror straight from the stand and started working on figuring/polishing while the mirror is still "deformed". So the question is, how long does it take for a deformed mirror of this thickness (200mm, standing 15-16mm at the center) to come back to original shape once laid down flat?

 

4. Being a thin mirror, at times I have kept it for too long in my hands before working and hence the temperature distribution is uneven.

 

As of now these are all the potential causes that I can think of. I am sure I am missing some of the more obvious ones. Edge of the mirror is also not that good, so not quite sure what course of action I should take going forward. Had there been no other issues a larger bevel could have done the job for a TDE. In my mind 1/2 - 1/2 Zs should improve astigmatism but will totally ruin the figure. Edge could be worked on by pressing a star and working 1/4-1/6Ds, that will also reduce overcorrection in z5. I am sure there are better ways, so please suggest.

 

I do have the scope ready so star test is something that could be done handily.

 

Foucault images are at mentioned rotations of the mirror from a fixed marker position.

 

IMG_20220413_131523.jpg

 

90°

IMG_20220413_131644.jpg

 

45°

IMG_20220413_131738.jpg

 

135°

IMG_20220413_132044.jpg

 

Seems like these thin mirrors are a little too much for a newbie like me. frown.gif Just to mention that the astigmatism is not that clearly apparent in Ronchi, specially that characteristic 'S' deformation. But I will double check it to be sure.



#33 Knight Sky

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Posted 14 April 2022 - 03:25 AM

I did run Foucault Unmasked on roughly 16 images across 2 orientations of the mirror (0° and 45° from my marker position), and do not see any major astigmatism, my ability to discern it could be questioned. My understanding is that If there's any first order astigmatism, it should have a hint in one of these two orientations (0° and 45°). Basically astigmatism should reveal itself at within one of the 90° orientations (eg. 0°, 15°, 30°, 45°, ..., 75°) of the mirror.

 

What I see as astigmatism in the images above could be a localized effect in one of the zones (3 or 4) that I wasn't able to execute properly in last 2-3 days. I also took Ronchi and moved in and out of the RoC and didn't see any 'S' warping of the single band. There still might be some minor astigmatism that evades my abilities to discern.

 

TDE looks bad (close to 5 - 6mm or so) and I have decided to work on it. Had it been about 2mm ±, I would have happily chamfered it off given the figure I have on the mirror; by lowering the center a little more, I perhaps could have reached 1/16 waves or so. It will most definitely ruin the whole figure and I might have to go back to sphere. I'll try and see if I can handle TDE without disturbing the figure, seems like an impossibility.

Attached Thumbnails

  • 2022-04-14_RonchiAstigTest.png


#34 Knight Sky

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Posted 15 April 2022 - 02:51 AM

I performed star test last night and it looked pretty good to me astigmatism-wise. I could not identify astigmatism at about 260X. But my ability was partially marred by the fact that collimation was off as I could distinctly see the stretched our wireless signal sign at very close to focus. The issue could also have been exacerbated by the barlow I am using (GSO 3 elem 2.5X). The same problem (collimation issues) are marring my ability to judge astigmatism on my previous (first) mirror. I also had the issue with gluing the mirror and that caused a lot of astigmatism that wasn't as apparent when the mirror was not glued. So, I also need to learn how to hold mirror to the cell and whether to glue or not to glue, and if so how to glue.

 

Seeing the results of the star test last night, I have decided to take a little time to think over whether to work on the mirror or not. I started this mirror with the aim of an astigmatism free 1/16 ± wave mirror. If there's any astigmatism in the mirror, my judgement is that it is very little. I would safely be able to lower the center a little more but that doesn't seem like its going to change mirror profile by much. The main concern is the edge, which I am afraid to work given that the temperatures are soaring to 42-43°C already and minimum temperatures are not lower than 26-27°C. I am afraid that the pitch will be too soft to improve any edge and would in fact exacerbate it.

 

Another thought I have is to make another pitch lap that works in ~27-32°C. But I am not sure if that is even possible with the Rosin I have, and even if I made a new harder pitch lap it might just scratch the mirror and bring me back to where I am hesitant to go.

 

If chamfering off 3mm or so off the mirror brings it to nice diffraction ring then I think I am willing to do it. I think 5 mm or so of bevel means ±10% loss of aperture. But I am willing to take it.


Edited by Knight Sky, 15 April 2022 - 03:08 AM.


#35 davidc135

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Posted 15 April 2022 - 06:31 AM

The Ronchi grating can be moved from a few bands inside focus to a few outside and the orientation watched for any 'clocking', which may be more sensitive than looking for any S suggestion. But it's probably fine.

 

David


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#36 dogbiscuit

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Posted 15 April 2022 - 11:23 AM

There is some S like shapes in you Ronchi images in post 26 and also  in 45°and 120° images of post 33.  The Foucault images show some irregularities.

 

I think you would have less trouble if you do no more accented pressure work, and maybe no more TOT.

 

My recommendation is MOT work only, go back to a sphere and parabolize again.

Start taking measurements before getting so close to 100% correction.

Consider that the outer zones usually don't take correction as quickly as the center.

If you take the center down too far before getting correction in the outer zones you will have trouble correcting the outer zones without making a hyperbola.

 

The edge... If you can't make a good edge when spherical there is little chance you will do so while parabolizing.


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#37 Knight Sky

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Posted 15 April 2022 - 05:04 PM

The Ronchi grating can be moved from a few bands inside focus to a few outside and the orientation watched for any 'clocking', which may be more sensitive than looking for any S suggestion. But it's probably fine.

 

David

David I did check it and it seems almost fine...but dogbiscuit is right, I looked closely and it is, perhaps, very marginal.

 

There is some S like shapes in you Ronchi images in post 26 and also  in 45°and 120° images of post 33.  The Foucault images show some irregularities.

 

I think you would have less trouble if you do no more accented pressure work, and maybe no more TOT.

 

My recommendation is MOT work only, go back to a sphere and parabolize again.

Start taking measurements before getting so close to 100% correction.

Consider that the outer zones usually don't take correction as quickly as the center.

If you take the center down too far before getting correction in the outer zones you will have trouble correcting the outer zones without making a hyperbola.

 

The edge... If you can't make a good edge when spherical there is little chance you will do so while parabolizing.

Spot on. I did check it as well and seems like it's caused by my imprecise execution of the #25 you mentioned. I had surmised it that I perhaps went slightly off the perpendicular which seems to have caused that mild S bend. I should have been a lot more careful with that stroke. It seems to me that the astig damage was not too bad had we had a chance to smoothen the curve; am I right in thinking this way? Edge seems like a bigger problem. I have already started going back to sphere and finished with the first session.

 

I think doing most work MoT once sphere seems like a better plan...but task is to first getting back to sphere. I am not quite sure if there's a time of the day where I can find suiting temperature to work with the current lap. Should I remake a harder lap now? I think if I have to make a new lap, this is perhaps the best time to make it as the temperatures are only going to soar high from now on. Alternative is to move the whole setup to a room with AC but that is going to open a whole new level of can of worms, least of which is a possible exposure to dust.  undecided.gif


Edited by Knight Sky, 15 April 2022 - 05:09 PM.

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