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Refiguring A 6" F8 achromat?

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#1 saemark30

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Posted 08 July 2020 - 03:27 AM

Can a 6" F8 Synta/Skywatcher objective be refigured easily? Seems it can be  DIY like Clark using oil pan in auto collimator setup. Which surface would be best to do so?



#2 PETER DREW

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Posted 08 July 2020 - 03:39 AM

They can be refigured. "Easily" depends on your experience. One such that I knew of was refigured on r4.You would lose the AR coating.

#3 davidc135

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Posted 08 July 2020 - 07:59 AM

Do you have an achromat that needs refiguring?  David

 

Can a 6" F8 Synta/Skywatcher objective be refigured easily? Seems it can be  DIY like Clark using oil pan in auto collimator setup. Which surface would be best to do so?



#4 ccaissie

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Posted 08 July 2020 - 05:13 PM

Why?   What's wrong with it?   

 

I refigured a 4" f/15 Unitron that had really bad Spherical Abberration.  I worked surface 1, which was uncoated with the help of DavidG.  Finished beautifully, used Ronchi and Foucault and a flat mirror.  Corrected for green light.  The Chromatic error wasn't noticeable.

 

This was after modelling it in OSLO and playing with spacing to see if I could correct it with shims.....couldn't reduce shims any further.  

 

So, again, what's wrong with it?  Diagnosis is at least half the cure.

 

c


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#5 saemark30

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Posted 09 July 2020 - 01:47 AM

Yes David  I have a 6" F8 achromat that has some S.A. I'm sure most of the SW ones need some touchup whether from the machine polishing or poor glass quality.

Which surface would you regrind or refigure?



#6 davidc135

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Posted 09 July 2020 - 03:00 AM

Yes David  I have a 6" F8 achromat that has some S.A. I'm sure most of the SW ones need some touchup whether from the machine polishing or poor glass quality.

Which surface would you regrind or refigure?

It won't need a regrind unless it's seriously bad; under or over-corrected for colour as well as having the SA. An air spaced objective could be re-touched on any of the 4 surfaces but the weakest curve R4 is usual. If the wave-front error is smooth and no sudden, sharp zones it won't matter on which surface the fault lies. (but see below)

 

As youmention you'll need to sort out an auto-collimating set-up before any work on the glass and assess the wave-front. See how practical the oil flat approach is.

 

A good second hand flat if one comes up is optimum.

 

Lastly, another scope can be used to provide collimated light but you have to be sure of it's optical quality as otherwise you'll simply be impressing its figure faults on your achromat although of opposite sign.

 

You'll need to consider the AR coating and whether you remove them chemically in which case you may have to soak the chosen lens for a while and strip both(?) sides. DavidG suggests hot concentrated lemon juice. I don't know if there would be a problem in just polishing away the coating.

 

It's possible that if R2 equals R3 then preserving the AR coating on both sides will help guard against ghosts in which case they are ruled out for refiguring.

 

But the retouching shouldn't be too hard. Tde as usual is tricky if it's there.

 

David


Edited by davidc135, 09 July 2020 - 11:59 AM.


#7 DAVIDG

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Posted 09 July 2020 - 12:16 PM

 It all comes down to having the correct test method to see what is wrong and if your making progress fixing the problem. You need either to setup an oil flat or use a glass optical flat so you can test via double pass autocollimation and also have a three fairly narrow  band filters  that are red, green and blue.  Green is the most important since the lens was most likely design to  have no spherical aberration at this wavelength. So you want to figure the lens until it  nulls out in the green. 

   If the lens show that the figure is smooth but over or undercorrected, that the job is easier to bring it in to full correction. The difficult is when you have zones since they can be on any or all the surfaces. In that case you first need to test the concave surface since that can be directly test via a knife edge and/or Ronchi test. It should be a perfect sphere, if not then the first step to refigure so it is. If the two inner radius are close enough now the spherical concave surface can be used as a  test plate to the check the  R2 which the rear surface of the front element via interference testing to see if it has problems. If it doesn't problems  then that leaves  R1 or R4 so you can try rotate one element and see if the zone rotates with it or put a  finger on a surface  which will heat the glass and cause a physical hill. So if you have a hole on that surface it will test better for a few minutes until the glass cools back off.  So hopefully you can then figure out  what surface as the problem and polish on it to fix the zone(s). 

 

           - Dave 



#8 ccaissie

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Posted 09 July 2020 - 09:00 PM

How do you assess the SA?   The star test will surely show it.    Changing the spacing might be enough...

 

I found the original thread with all the discussions, so maybe reviewing it will be illuminating and helpful.

 

https://www.cloudyni...a-4-fraunhofer/



#9 davidc135

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Posted 10 July 2020 - 02:49 AM

I see there are ex soviet 5'' flats on ebay for around $100. A bit more awkward than using full diameter but I bet not as awkward as using oil. You could always sell it if a bigger one comes up, which it will. There's no substitute for having a testing setup into which you can quickly place and test an optic after each few minutes of work.

 

As ccaissie says perhaps altering the spacing maybe all that's needed.

 

But if figuring work is needed it wouldn't be a matter of altering radii, rather tweaking zones or a conic. And for that, stable indoor testing is invaluable.

 

David



#10 MKV

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Posted 10 July 2020 - 09:20 AM

I see there are ex soviet 5'' flats on ebay for around $100. A bit more awkward than using full diameter but I bet not as awkward as using oil. 

Using subdiameter flats in AC seup is misleading. There's nothing awkward about using an oil flat. Below is an example of a (rather bad) Surplus Shed 153 mm f/8 objective in its original cell. The setup is simple, whether you're testing the lens in a cell or the entire OTA on a mount. The results are perfectly comparable to those of an optical flat. The only thing to look out for is dust in oil. This can fixed with a spoon, just scooping it up, and keeping the oil flat covered with a suitable cover until use.

 

In the example below, a heavy duty cast iron pan was used on top of a bag of cat food to dampen vibrations (a sandbag will do even better). A green LED light source was used behind a Ronchi screen. The light source  is placed at the objective's infinity focus. A weak laser diode helps align the oil flat. Adjustments are made until the laser beam retraces itself upon reflection from oil. This is easy using a cat food bag, a bean pillow, or an adjustable platform with leveling screws. 

 

Mladen

 

oil flat DPAC rig .jpg oil flat_1.jpg oil vs glass DPAC.jpg

 

And here's a test setup for an OTA 

 

refractor_OTA.jpg


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#11 saemark30

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Posted 11 July 2020 - 12:31 PM

MRV how are you holding the lens cell above the oil pan? Does the distance matter of lens/oil ?



#12 Jeff B

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Posted 11 July 2020 - 01:28 PM

I have found it easier to do good DPAC with the entire OTA, if I can.  I usually just butt the cell's face right up against the flat or even let it sit on the flat.

 

But how about the actual polishing techniques? 

 

Jeff

 

 

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#13 BGRE

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Posted 11 July 2020 - 09:29 PM

MRV how are you holding the lens cell above the oil pan? Does the distance matter of lens/oil ?


The oil pan sits on a stiff circular plate sitting on the "sand" an annular disk with a central hole larger than the telescope aperture sits above the oil pan.
The objective in its cell sits over the hole.
3 adjustable legs with spherical tips to set the distance between the objective and the oil pan and are used for aligning the objective or OTA with the surface of the oil.
A relatively large radius tip is desirable particularly when the oil pan base board isn't hardened (at least at the point of contact)
Coachscrews are suitable with a large radius of curvature head. Some of these have smooth heads, any raised lettering needs to be removed with a file or equivalent.
Coachbolt.jpg

If a large disk like knob (preferably with a knurled rim ) is fitted to the other end and locked in place fine adjustment of tilt is easily achieved.
Using a stainless coachscrew with a brass (or bronze) nut works well.
A stainless nut also works especially if lubricated (Apiezon grease preferred to avoid migration).
Fine adjustment screws (80 tpi, 100 tpi etc) are often available but these tend to have a tip with a small radius of curvature requiring a hardened seat to avoid indenting the disk on which the oilpan sits.
The length of these may be too short without having them sit on pillars fixed to the oilpan baseplate.
In which case going full Maxwell and having a V-groove (axis pointing to the centre of the baseplate) in the tops of the pillars is advisable for stability.


Spacing between the oil surface and the objective isn't critical as long as it isn't excessive. In practice achieving a suitable clearance btween the top of the oil pan and the underside of the annular disk limits the minimum separation.
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#14 MKV

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Posted 12 July 2020 - 10:02 AM

MRV how are you holding the lens cell above the oil pan? Does the distance matter of lens/oil ?

BGRE answered both questions already. I just want to add that in this case I used suitable (machinist's) shims under the lens cell until the light signal retraced itself to the source.


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