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How DPAC your refractor

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#201 Jeff Morgan

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Posted 22 November 2019 - 03:39 PM

Jeff, you may find that going to the red and blue LEDs may really saturate the images and you will need to adjust your camera setting for each color, particularly in red  Also, I use red and blue filters as well as good quality neutral density filters.  DPAC will also tell you immediately if you have a bum filter too.  Typically you can set the color temperature as well, for example cool is more towards the blue and warm towards the red.  In fact, you may find yourself "going to the opposite ends of the spectrum " to tame the red and blue....warm for blue and cool for the red.  My phone camera has an image stabilizer function which helps me a lot.

 

Hey but mess around .  With experimentation you will find the right focus and image settings for each color for your camera.....just write them down! grin.gif

 

And above all else, have fun!

 

Jeff

 

On the color filters ...

 

I was looking at "common" color filters at Agena Astro, and they look more like long-pass (where transmission charts are available). That is to say, the Green cuts off blue but has high transmission into the red.

 

Sure, they are not going to be sharp like a line filter ... but it makes me wonder if common color filters are more "permissive" than the LED's to be filtered. Am I looking in the wrong place? 



#202 Wildetelescope

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Posted 22 November 2019 - 04:31 PM

Who bought the 6" Flat off of ebay for $79 ?.  Hope to see some test results soon.

Just curious, why won't the coatings matter?

 

JMD



#203 Jeff B

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Posted 22 November 2019 - 04:41 PM

Jeff, you are correct as some filters are more of the "long-pass" variety, especially yellow.  However, the Baader green and  is a pass band filter and has worked well for me.  I found my Green LED has some yellow content to it and using the green pass-band filter cleaned the color up a bit and I got a little sharper images.  Is there still some yellow in there?  Yes, but it's way down in level relative to the peak in green.

 

But the yellow filters I have are indeed long-pass and let the red through as did the yellow LED so I found I could stack the green filter with the yellow which then really knocked down the orange/red content to better isolate the yellow  The camera liked that too despite the dimmer image.

 

Jeff



#204 peleuba

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Posted 23 November 2019 - 04:48 PM

Just curious, why won't the coatings matter?

 

 

The flat does not need to be coated in order for it to be useful in DPAC.  It still works as the highly polished surface of the flat will reflect the light back through the OTA under test just fine.  The coated flat will be brighter, but uncoated works too.


Edited by peleuba, 23 November 2019 - 04:48 PM.


#205 fate187

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Posted 10 December 2019 - 01:27 AM

Ok so I went and got me one of those glass solar filter thingies to try as a flat.  Pretty cheap.

 

I put one of my scopes in the rig and focused to three lines in green.  I then removed the regular flat and put the glass solar filter in place and aligned it with the scope.

 

OMG!!   scared.gif   Shield your eyes!!.   Eyecrazy.gif

 

Tell the children not to look as they will have nightmares and need therapy (at least in California)!!  bigshock.gif

 

People turned to stone. what.gif vomit.gif twitch.gif step.gif

 

Yeah, it was that bad.

I received my Ronchi gratings last week. I already had an 8" mirror from a lab supplier. In the last evenings I was trying to figure out the configuration. I ended up with my cell phone shining through several layers of sheets of paper (as a diffusor and reducing the very bright LED), the light shining through the grating into the scope and than observing the reflection through the grating with my eye. Without any eyepiece. Everything hand held :D.

 

The "lab" mirror I purchased is crap. There is no pattern near focus that looks remotely like what I see here or anywhere else. The pattern was all over the place. But after reading Jeff B's comment I used a Baader energy filter (ERF). Those are interference type filters, so I presumed they should have a pretty flat surface. And it did work indeed! On top of that I got the light from LED filtered. Transmittance is between 610 to 690 nm according to Baader, so everything else (blue/green) will be reflected back and will be visible.

 

Using this ERF I was able to see a pattern similar to those posted. I tested my Equinox 120, LZOS123, LZOS 130 and CFF185. The CFF was way to big to adequately put my much smaller 110mm ERF in front of it. For the other scopes I was able to place it right it front of the lens assembly, fixing it with a book :D. The was some convex curvature inside of focus increasing when moving towards the focal point. I was able to see the "null" at focus (do I get the terms right?). Outside focus the curvature was concave.

 

So I will need to get a larger flat, as the mirror I purchased does not work, and getting a larger ERF is quite expensive.


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#206 Jeff B

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Posted 10 December 2019 - 03:23 PM

Keep at it!

 

And here is one reason I like DPAC so much.  It's a great, easy and quick diagnostic tool.

 

Have a look:

 

https://www.cloudyni...g-dpac-results/

 

Jeff



#207 Jeff Morgan

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Posted 13 February 2020 - 01:22 PM

A thought just occurred to me, I wanted to run it by the group for a Sanity Check.

 

Once you DPAC a scope and it is a "known quantity", could you then put a star diagonal in place to verify the diagonal?

 

And if so, could a prism diagonal be tested using different colors?



#208 Jeff B

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Posted 13 February 2020 - 03:20 PM

A thought just occurred to me, I wanted to run it by the group for a Sanity Check.

 

Once you DPAC a scope and it is a "known quantity", could you then put a star diagonal in place to verify the diagonal?

 

And if so, could a prism diagonal be tested using different colors?

With the high quality Baader, Lumicon, and AP diagonals I have, I see scant differences in SA correction for the different colors, even with the Baader Zeiss spec prism diagonal working at F7.  Now, the individual colors do not tell me anything about the relative focus positions of each of the colors with the prism diagonal (there are no differences with the mirror diagonals).  However, when I use my white light LED (which is really 3 LEDS, R, G, B in one), I do see extra blue and red fringing on the edges of the bars over that when using the mirrored diagonals, even working at F10 (though that extra fringing is more pronounced at F7).

 

BTW, DPAC testing is a great way to screen filters and diagonals too.  I've run across bum samples of both with the defects readily seen in DPAC.  

 

The other Jeff



#209 peleuba

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Posted 14 February 2020 - 09:10 AM

A thought just occurred to me, I wanted to run it by the group for a Sanity Check.

 

Once you DPAC a scope and it is a "known quantity", could you then put a star diagonal in place to verify the diagonal?

 

And if so, could a prism diagonal be tested using different colors?

 

 

A mirror diagonal will not add any sort of correction to the image.  It can add astigmatism, but that's it.  For me, the star test is a better method of determining astigmatism and its axis.  But double pass can catch it, too.  I just prefer the star test.

 

A prism diagonal is an entirely different animal and does add correction.  If the correction is opposite sign of what is in the objective, it will improve the image.  If not, it won't.  You probably can detect the diff in correction using DPAC.  However, I think it would be subtle.

 

I don't own any prism diagonals - am not a fan - so have never tested with one.

 

Jeff B has more experience with this and Binoviewers and their related plumbing and its affect on the image seen in DPAC.


Edited by peleuba, 14 February 2020 - 09:11 AM.


#210 evan9162

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Posted 16 February 2020 - 12:16 AM

Celestron C80ED (133lpi screen)

 

Inside:

IMG_9473.JPG

 

Outside:

IMG_9474.JPG


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#211 peleuba

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Posted 16 February 2020 - 08:04 PM

Celestron C80ED (133lpi screen)

 

 

 

 

 

Evan - you should start your own thread on this telescope as it looks like a good one.  

 

I have tested several ED80's and all have been very good.

 

I'd be interested in seeing your test setup.



#212 evan9162

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Posted 16 February 2020 - 08:37 PM

My setup is a work in progress.  I don't actually have a real setup so to speak.

 

I put my flats on the floor under my scope, and point them down while attached to one of my mounts.  I use a laser collimator to do rough alignment.

 

I'm still working on my Ronchi screen holder.  I turned one out of wood as a quick prototype using 3 separate LEDs for R,G, and B.  I've fiddled with it a ton to tweak LED and viewing positions.  

I really need a single RGB LED instead of 3 separate ones, and I need to machine a new holder out of metal.  Plus, a flat holder and V-blocks for the scope.  

 

 

I agree that my C80ED is excellent.  I use it for DSO imaging, and it's really surprising how much detail it captures.  It's even a decent little planetary scope.  I doubt I'll ever let this one go


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