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help with ronchi 8in f2.6 pyrex mirror

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#301 macleod

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Posted 20 August 2019 - 04:49 PM

MKV- thank you, thats exactly what I am building. Using parts from previous builds, spare cell /focuser/secondary etc, as per your DPAC ???? website. Macleod.



#302 macleod

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Posted 22 August 2019 - 10:18 PM

Ok so I have built a full OTA for my little mirror, and sat the scope mirror up on an adjustable platform close   over a dish of water - will change to oil this is temporary.So aligned secondary with my laser to point at center of primary.Can,t get enough reflection off uncoated primary to align  back to center of focuser. By luck the focuser hole 50mm diam aligns with my Ronchi set up, only its too far away, about 600mm from tube, but I,ll sort that. 

Out of curiosity I shone my Howie Glatter red Laser thru Ronchi screen onto secondary mirror and onto center of primary. Looked thru Ronchi and saw 4x  or so horizontal parallel  lines with a series of equally spaced dots on the lines. What is this please ? Is it just the secondary mirror ?

The lines are at rt angles to Ronchi lines.Now to get water/ oil bath reflection  to return to me, do I adjust the tilt of the whole scope on its platform ? Going to need a bit of fidling and a much brighter LED , maybe white. Macleod



#303 macleod

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Posted 23 August 2019 - 12:07 AM

Forgot to mention in #302 - mirror 8in  f2.65 Ronchi 133lpi red laser. Any comments ?



#304 MKV

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Posted 23 August 2019 - 01:01 AM

Stewart,

 

For 0 degree incidence angle you have 4% reflection off the primary, and about 2% of the 4% of the water flat, and 4% of that total. That leaves you with very little to work with. Liquid flats are not best suited for testing mirrors still being figured because either the flat or the test optics should be aluminized. 

 

For that you'll need a glass flat. In your case, you may want to try some of those super-super bright LEDs. You can increase water reflectance somewhat with dark food water coloring. 

 

Don't do anything to you OTA. Its optics should have been aligned on a star. Place the OTA face down on the adjustable platform and move the platform adjustment screws until you see the reflection through the focuser. A leveling bubble helps. 

 

Mladen



#305 macleod

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Posted 23 August 2019 - 06:14 AM

Thank you Mladen. What were the laser lines I was seeing.



#306 MKV

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Posted 23 August 2019 - 07:08 AM

Diffraction. This is probably what you observed.

 

https://encrypted-tb...sIjnYt9uRFhEjpk

 

Laser beams are collimated (parallel). You need a diverging lens in front of the laser or to remove the laser's collimting lens to use the laser as a point source. 

 

Anyway, it's not a good idea to use a laser beam with your Ronchi grating.

 

Mladen

 

_____

PS

 

this is a ronchigram of a sphere usng a lensless laser as a light source.

 

SAM_4422_LR.JPG

 

gets grainer as you approach the ROC

 

SAM_4425_LR.JPG

 

Grainy but it may give you enough light for an uncoated oil flat null test.

 

Use the camera screen to view rather than direct visualization.

 

What's the wattage and amp rating of your laser?


Edited by MKV, 23 August 2019 - 07:31 AM.


#307 Pinbout

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Posted 23 August 2019 - 07:43 AM

collimating for dpac isn't as critical as collimating your scope for viewing

 

all you need is to see the full primary in the 2ndry mirror and then the led in the center of the primary in the 2ndry.

 

cause you can place the led/grating almost anywhere if you see the above condition... another reason to remove the focuser. aligning with lasers isn't necessarily going get you there.

 

just saying



#308 macleod

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Posted 23 August 2019 - 04:15 PM

The big problem is the uncoated mirror - I can see and align on the central dot on the BACKSIDE of the mirror very clearly. In a laser test on the surface at low incidence angle the light goes thru to the back, NO Surface Reflection at all.Yes I can see all primary in secondary.I will get a superbright LED - so far I,m only playing with it, and haven,t got scope mirror parallel to surface of water. I,ll play with the set up over weekend.Tkx for help, Macleod.



#309 MKV

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Posted 23 August 2019 - 05:26 PM

The big problem is the uncoated mirror - I can see and align on the central dot on the BACKSIDE of the mirror very clearly. In a laser test on the surface at low incidence angle the light goes thru to the back, NO Surface Reflection at all.Yes I can see all primary in secondary.I will get a superbright LED - so far I,m only playing with it, and haven,t got scope mirror parallel to surface of water. I,ll play with the set up over weekend.Tkx for help, Macleod.

Vignetting can be a problem, so I agree that it is advantageous to remove the focuser, but a standard 2-inch focuser shouldn't be too much of a restriction. The problem is how far from the secondary you can observe. And that, in an OTA, is dictated by how big the primary mirror is and how fast it is. 

 

The reason why I like to use the focuser in AC is because they have fine controls, but I suppose one can improvise something else to achieve similar precision.



#310 macleod

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Posted 23 August 2019 - 06:38 PM

MKV - I can get to within 160mm of secondary in my set up. Just bought a Cree LED  24600mcd - about 3Lumins - narrow 15deg emission white. So I,ll see how that goes when I get it set up.



#311 MKV

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Posted 23 August 2019 - 07:25 PM

MKV - I can get to within 160mm of secondary in my set up. Just bought a Cree LED  24600mcd - about 3Lumins - narrow 15deg emission white. So I,ll see how that goes when I get it set up.

Great, that's plenty. Make sure the source is diffused (a of Scotch invisible tape, to maximize transmission, will do if placed a short distance ahead of the LED



#312 macleod

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Posted 23 August 2019 - 10:51 PM

Frustrating to say the least ! Tried alignment.Can see the odd reflection off water slopping in secondary , but so far no lines  visually.I can get to 160mm from secondary, very darkened room, but all I see is right thru to back of the mirror, mounting pads cell markings etc. Leveled with spirit level both tube and platform but not accurate enough obviously.Think it might be a waste of time uncoated, may have to do Bath. Still have NOT heard from Edmunds re lens, hopeless outfit, sent 4x e-mails.Probably don,t want to trade outside of USA, like McMaster Carr.



#313 macleod

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Posted 23 August 2019 - 11:43 PM

Update  - success ! At least I got a small image of what looked like full mirror, with pretty straight lines. Will have to now change to oil because of shimmering. But at least its given me hope that it will work. Question - should the whole big view of mirror as seen in secondary be covered in lines, or only a small image ? How large should image be ? Macleod


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#314 MKV

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Posted 24 August 2019 - 02:31 AM

Question - should the whole big view of mirror as seen in secondary be covered in lines, or only a small image ? How large should image be ? 

Only the mirror should show the Ronchi bands. The bands should increase or decrease in numbers and width as you rack in and out of focus. If correctly aligned you should see the whole mirror. However, if you're still figuring a mirror and checking progress then we cut corners as long as we can see most of the mirror. :o) 

 

Here's an example of a 6-in f/8 paraboloid tested in the vertical configuration as you'd use for oil testing. The flat wasn't aluminized (but this defnitely convinced me that is should be!).     

 

6inf8 vert AC_1.jpg

 

and here it's in the negative whch better shows the test results

 

6inf8 vert AC.jpg


Edited by MKV, 24 August 2019 - 02:32 AM.


#315 macleod

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Posted 24 August 2019 - 03:01 AM

Thank you Mladen.I assume that by tipping the platform I will see whole mirror - I still have a bit of experimenting to do. However the disc with straight lines enlarged and reduced as I moved to and from focus , and amazingly looked straight. However I will get oil tomorrow and eliminate ripple. This is an encouraging start - I am only a raw beginner at this, but already I see how it works.

 One big problem is the secondary mirror for a 8in f2.65 is 75mm MA , and you really only have the outer edges of the mirror working, and the shadow looks enormous ! I suppose that makes figuring easier in as much as a "hole" or " hump" at the center doesn,t matter.



#316 MKV

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Posted 24 August 2019 - 06:16 AM

Yes, Stewart, by tipping the platform you'll see the return beam. It should look no different than the "regular" Ronchi test except the bands won't be of the same shape, and you'll see that huge "Cyclops Eye" in the middle -- and it should be pretty well centered, just like in the image above. 

 

Mladen



#317 macleod

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Posted 24 August 2019 - 06:56 PM

Tomorrow ( Monday here ) I,ll give oil a try.Painted inside of tray flat black to help reflections.


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#318 MKV

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Posted 24 August 2019 - 07:02 PM

Tomorrow ( Monday here ) I,ll give oil a try.Painted inside of tray flat black to help reflections.

waytogo.gif



#319 Jeff B

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Posted 25 August 2019 - 08:08 AM

Macleod, DPAC is a great tool!

 

You can't read surface accuracy, but at a glance you can see what's going on..under/over correction, edge issues, zones, astigmatism, smoothness.  The eye is also very sensitive to the straightness of lines.

 

A very nice and powerful tool to have.

 

However, not to divert this thread, but, for those who know, I have some questions as the bulk of my DPAC experience is with refractors and Cats.

 

Macleaod's mirror is very fast and I was wondering how that may affect his setup tolerances as he will be looking a bit off-axis.  For example, a bit of "key stoning" of the lines can happen if things are not properly lined up.

 

How would coma show up or do you even get it in DPAC?

 

I've been really enjoying this thread macleod.

 

Jeff


Edited by Jeff B, 25 August 2019 - 08:10 AM.


#320 macleod

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Posted 26 August 2019 - 12:21 AM

JeffB - This test is extremely sensitive alright ! I put thick ( 20W ) oil in the pan, which has a black bottom- NO not the 20,s dance - and I get a great reflection.BUT although this is new oil, I can see floaters in it which are minute and I can,t see by eye ! I can see them in the Ronchi lines as dots with a raised miniscus of oil around them, and they distort the lines locally, terribly. I,ve been too busy to do much today, but the oil pan was CLEAN and dust free, the oil from a new bottle, so I just can,t follow it yet. But the test is doable, obviously.

As you say, I,m very concerned because I,m having great problems trying to collimate my test scope with an uncoated mirror, and it needs to be spot on for a true assessment. I hope its worth all the effort ! I can easily get the Secy pointing at center of mirror with my Howie Glatter Laser, but it won,t return a reflection to the Tublug I use to align the main mirror.The laser goes right thru the mirror - my center spot is on the back and as the incident ray is at 90deg to the mirror, there is NO diffraction in the glass.But getting an uncoated mirror to align with secondary and the 2in hole in the tube is another animal altogether. Any suggestions as to how to set up PRIMARY mirror( uncoated ) will be very much appreciated

For those just joining this thread, I,m doing this fast mirror for the hell of it,to give myself  hopefully a sense of achievement in my 80,s. Sheer madness maybe, but I hope for a second childhood ( teenhood ) maybe !! Macleod.



#321 macleod

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Posted 26 August 2019 - 12:42 AM

Oh I forgot to mention -  how would coating the mirror go with a miscible mixture of glycerine and water ? Maybe I,d get enough reflection for main mirror set up ?

 And I am using a 3in GSO secondary, how much will that show up in the lines if it,s not good ?



#322 MKV

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Posted 26 August 2019 - 02:34 AM

Oh I forgot to mention -  how would coating the mirror go with a miscible mixture of glycerine and water ? Maybe I,d get enough reflection for main mirror set up ?

 And I am using a 3in GSO secondary, how much will that show up in the lines if it,s not good ?

You have three reflections (not counting the diagonal) off of uncoated surfaces. That gives you only 4% or less (still water has almost no reflectance) after each reflection.

 

Now, how are we going to figure out what your secondary will contribute if we have no idea how good/bad it is?!

 

If it's a standard diagonal from a reputable dealer/maker chances are it may be good enough, but there's no guarantee. 

 

If you don't have a small optical flat, of known quality, there's a way to test is with ordinary glass, called the ABC method. You can have three unknown pieces of glass and calculate the flatness of all three of them.

 

See if you can find anything on the Web with three unknowns for flats. The math was described by by various authors (there's one in ATM Book 1 of Ingall's classic series, shown below), and I've been using one by Jeff Baldwin (http://www.jeffbaldwin.org/flats.htm). I believe there may be apps online as well. 

 

abc_flats.jpg

 

Source: Albert G. Ingalls' Amateur Telescope Making  Book One, Scientific American 1978, p. 55

 

Also noteworthy is Ed Jones' water test for flats on Youtube. Very economical and -- it works. :o) 

Look it up. 

Good luck!

 

Mladen


Edited by MKV, 26 August 2019 - 03:08 AM.


#323 MKV

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Posted 26 August 2019 - 03:17 AM

JeffB - This test is extremely sensitive alright ! I put thick ( 20W ) oil in the pan, which has a black bottom- NO not the 20,s dance - and I get a great reflection.BUT although this is new oil, I can see floaters in it which are minute and I can,t see by eye ! I can see them in the Ronchi lines

That's why they have oil filters and funnels for them. :o) You're supposed to filter the oil! If it's not clean enough to pour straight into the engine, it's not going to be  clean enough for optics.

 

PS

 

Make sure the bottom of the pan is clean and particle-free. Rinse it and let it dry. The pour the oil starting at the center and slooowly and at a height no more than an inch or so from the bottom of the pan The oil doesn't have to be deep -- 1/4 inch is fine. If you don't pour slowly you'll create air bubbles and will have to scoop them out manually with a spoon. 


Edited by MKV, 26 August 2019 - 03:23 AM.


#324 dave brock

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Posted 26 August 2019 - 05:03 AM


Now, how are we going to figure out what your secondary will contribute if we have no idea how good/bad it is?!


Can't you do a few tests and rotate the primary between? What doesn't rotate is on the secondary.
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#325 BGRE

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Posted 26 August 2019 - 05:31 AM

Oh I forgot to mention -  how would coating the mirror go with a miscible mixture of glycerine and water ? Maybe I,d get enough reflection for main mirror set up ?

 And I am using a 3in GSO secondary, how much will that show up in the lines if it,s not good ?

Using a water glycerine mixture is probably a bad idea. Glycerine is hygroscopic and the composition of the mixture will change over time. This will likely lead to surface shape instabilities accompanied by changing refractive index and refractive index gradients.  

Such instabilities have been observed in other mixtures.

 

Try a Rayleigh "water" test (using such a mixture) of a small flat (or near flat) first to check if such instabilities can be observed. 




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