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long slow newtonian + optical flat from scratch

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34 replies to this topic

#26 tim53

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Posted 19 August 2019 - 12:32 PM

Far better to use vegetable oil for your flat.  



#27 Pinbout

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Posted 19 August 2019 - 12:39 PM

 

-rest the scope vertically and upside down over a pan of mercury

karo syrup works too, the least likely to show vibrations...

 

just thin enough to self level.

 

it tasted better too. grin.gif


Edited by Pinbout, 19 August 2019 - 12:40 PM.

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#28 dan_h

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Posted 19 August 2019 - 01:33 PM

I do have a massive amount of mercury in the lab...maybe

-build the cope

-rest the scope vertically and upside down over a pan of mercury

-put an led and knife edge in the focuser.

-run DPAC test

-apologise to wife and parents for exposing us to mercury

 

In any case, this is all just rambling until I start the actual grinding in the next few days.

Aside from the obvious health concerns, mercury doesn't work well for this application as it makes the setup really sensitive to vibrations.  There are far better liquids to choose from.

 

dan



#29 Ed Jones

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Posted 19 August 2019 - 04:04 PM

3 uncoated reflections will need a very bright light source and difficult to align due to the very faint image.


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#30 tim53

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Posted 19 August 2019 - 04:41 PM

3 uncoated reflections will need a very bright light source and difficult to align due to the very faint image.

I had pretty good success lining things up using a Lasermax holographic laser.  I had little trouble seeing the reflections, though I was testing coated optics (with large areas of missing coatings).  Once I was collimated, it wasn't too hard to find the ronchigram.  Newtonians need a lot of vertical room to test with an oil flat, though.  So I was glad to find a 12" optical flat.  

 

-Tim.



#31 BGRE

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Posted 19 August 2019 - 05:41 PM

Aside from the obvious health concerns, mercury doesn't work well for this application as it makes the setup really sensitive to vibrations.  There are far better liquids to choose from.

 

dan

The usual solution is to use a thin layer of mercury in a silver dish (with which the mercury forms a thin amalgam layer making a thin layer of liquid mercury possible).

mercury flats were used extensively at the PTB for absolute calibration of horizontal transmission flats.



#32 ckh

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Posted 24 August 2019 - 08:51 AM

You can cover the mercury with oil or some other viscous transparent liquid to suppress the ripples.



#33 GarethBarry

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Posted 30 August 2019 - 07:02 AM

OK so an update;

The sagitta of primary mirror has been hogged out using a diamond coated angle grinder blade. At this depth it should be around f7.5 ish or so. I (or rather, we) have begun hogging out the curve of the mirror with a hole in it. I intend to use this for a conjugate/Waineo/Maksutov type null-this null intrigues me and I think I would like to try it.  In any case, it will give some experience in dealing with a mirror with a hole in it, as I would like to one day make a Dall-Kirkham. If I fail, I will go back to Foucault+Ronchi+star testing.I have a lot more confidence in my ability to read the zones with the Foucault test now that I have some practice under my belt.

I also rebuilt my Foucault tester. My original was the Stellafane one which had a bit too much slop in it for my liking, probably due to my poor execution of the design. Essentially I used two roller-bearing sliding drawers with a lead screw for translation along the x-axis. I then used two hinges, separated by some distance, to make platform that rotates the knife edge. It seems much more solid now.

 

So I guess this title should rather read -"long slow newtonian+perforated spherical mirror". I decided to go with a perforated spherical mirror because I think that will be easier for me to make and test than a flat. My understanding is that the distances can be set in the Waineo null test to give pretty high sensitivity.

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Edited by GarethBarry, 30 August 2019 - 07:10 AM.


#34 GarethBarry

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Posted 09 October 2019 - 05:33 AM

Time for an update.

So the unperforated mirror has been ground to 800 grit-I think I might try and go as far as 1200 grit this time in order to hopefully save some time in pitch polishing.

 

I need to start thinking about mounting this thing. So the Headmaster of the school I am doing this for is keen to gradually build this up till we can do some astrophotography. So my question is, is this a decent astophotography setup?

-10 inch f8 newtonian on a dobsonian mount, with push-to, sitting on a equatorial platform for tracking

-colour CCD camera like the one that orion sells (starshoot G3)

This is under suburban/city skies.

 

 

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Edited by GarethBarry, 09 October 2019 - 06:27 AM.


#35 GarethBarry

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Posted 16 October 2019 - 06:31 AM

So I poured a pitch lap today and thought I would experiment. I bought a rubber car mat, trimmed to match the diameter of the mirror, placed the mat over the mirror, made a dam wall out of masking tape, then poured the pitch over the mat. When cooled down somewhat, I placed the tool over the pitch. Let cool completely, remove rubber mat and here we are. I have to say this is the least painful process I have used so far and has made the prettiest pitch lap.

Anyway, on to polishing!

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