53 replies to this topic

[scooke]

With the help of the members of this forum, including special thanks to Peleuba and Moshen for giving generously of their time and wisdom, I conducted my first DPAC test tonight.  It uses an 8" flat and at 1.5 volts, I think the led is too bright but it worked and I couldn't be more excited.  Attached is my first image, handheld with my iphone.  It is of my AT92 using a 133lpi ronchi grating.  What do you all find is the best voltage for the led and the best way to photograph the image?

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[peleuba]

With the help of the members of this forum, including special thanks to Peleuba and Moshen for giving generously of their time and wisdom, I conducted my first DPAC test tonight.  It uses an 8" flat and at 1.5 volts, I think the led is too bright but it worked and I couldn't be more excited.  Attached is my first image, handheld with my iphone.  It is of my AT92 using a 133lpi ronchi grating.  What do you all find is the best voltage for the led and the best way to photograph the image?

 

Glad to see you were able to get the test going.  Looks to be typical high quality from AT/Astronomics - very little to no appreciable spherical aberration.  

 

3 Volt LED - powered by two AAA batteries. You should make yourself a permanent power source.  I will attach a photo of mine.  It has an on/off switch, and a potentiometer to adjust the brightness of the LED

 

For the iPhone, I use an app called RAW+.  Can be obtained from the AppStore.  It will allow you to manually manage the camera settings and work with raw images which are not compressed and will not suffer from compression artifacts.  I also use several digital cameras on a tripod.

 

When saving your images for publication you want the Ronchi bands running vertically.  It makes it way easier to assess them.

 

Your LED looks to have a lot of blue in it.  But this could be the camera.  

 

Good Job.

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Edited by peleuba, 04 February 2020 - 04:19 PM.

[YAOG]

Glad to see you were able to get the test going.  Looks to be typical high quality from CFF - very little to no appreciable spherical aberration.  

 

3 Volt LED - powered by two AAA batteries. You should make yourself a permanent power source.  I will attach a photo of mine.  It has an on/off switch, and a potentiometer to adjust the brightness of the LED

 

For the iPhone, I use an app called RAW+.  Can be obtained from the AppStore.  It will allow you to manually manage the camera settings and work with raw images which are not compressed and will not suffer from compression artifacts.  I also use several digital cameras on a tripod.

 

When saving your images for publication you want the Ronchi bands running vertically.  It makes it way easier to assess them.

 

Your LED looks to have a lot of blue in it.  But this could be the camera.  

 

Good Job.

But I think this is an AT92, Astro Tech/Astronomics scope.

Edited by YAOG, 04 February 2020 - 02:31 PM.

[peleuba]

But I think this is an AT92, Astro Tech/Astronomics scope.

 

Corrected!  Thanks for pointing that out.  I owned a CFF and just recently tested another so had that on my mind.

 

I have tested two of the AT92's and both were excellent.    

Edited by peleuba, 04 February 2020 - 04:21 PM.

[Tyson M]

The AT92 was one of my fav scopes I have ever used.

Only sold it to (eventually)acquire another premium scope with similar properties/functionality.

Under the stars it was incredible, and the optical quality was some of the best Ive used.

[scooke]

Thanks Paul.  I did try the RAW+ app tonight and tested my SW150ED that gives amazing images.  It's DPAC test is not as good as the AT92.  The AT92 has an interferogram that shows .983 strehl and I don't know what the 150 is but clearly less.  Star testing using the Suiter book made me pretty certain the 150 is about 1/6th wave.  This scope was purchased from Chas.  We have both used 100x per inch without image breakdown so I know it's really good.  I guess this just shows how sensitive the DPAC test really is. . I'm not sure why orientation is still wrong.

[scooke]

Just for fun, I also tried my diagonals.  I saw zero difference in the test whether using my AP Maxbright, Baader prism, TV Everbright, or APM prism. The AT 99% mirror diagonal showed just the tiniest degradation.  At first I didn't even see it, it was so small.

[Jeff B]

Thanks Paul.  I did try the RAW+ app tonight and tested my SW150ED that gives amazing images.  It's DPAC test is not as good as the AT92.  The AT92 has an interferogram that shows .983 strehl and I don't know what the 150 is but clearly less.  Star testing using the Suiter book made me pretty certain the 150 is about 1/6th wave.  This scope was purchased from Chas.  We have both used 100x per inch without image breakdown so I know it's really good.  I guess this just shows how sensitive the DPAC test really is.IMG_0089.JPG . I'm not sure why orientation is still wrong.

Is that the SW 150ED in the picture?  If so, it's a nice lens and which side of focus was that image taken?

 

Jeff

[scooke]

Yes, just outside of focus.

[peleuba]

Thanks Paul.  I did try the RAW+ app tonight and tested my SW150ED that gives amazing images.  It's DPAC test is not as good as the AT92.  The AT92 has an interferogram that shows .983 strehl and I don't know what the 150 is but clearly less.  Star testing using the Suiter book made me pretty certain the 150 is about 1/6th wave.  This scope was purchased from Chas.  We have both used 100x per inch without image breakdown so I know it's really good.  I guess this just shows how sensitive the DPAC test really is.IMG_0089.JPG . I'm not sure why orientation is still wrong.

 

Would you mind sending me the serial # for the AT92 in a PM?  

 

Like the star test, DPAC is most useful when comparing two images on either side of focus.   

 

The image is being rotated because of the metadata embedded into it (image) by the iPhone.  You will have to open the pic rotate it, then re-save it.

[peleuba]

Just for fun, I also tried my diagonals.  I saw zero difference in the test whether using my AP Maxbright, Baader prism, TV Everbright, or APM prism. The AT 99% mirror diagonal showed just the tiniest degradation.  At first I didn't even see it, it was so small.

 

 

Prisms will add some color error, but DPAC is insensitive to this unless you are testing in white.  Prisms will change the focus point and usually add some spherical abberation.  A mirror will add scatter and if its poor, will add astigmatism.  No matter what, 45° flat mirror diagonals cannot add to or subtract from the spherical correction of the objective.

 

Much has been made of the differences between mirror diagonals/enhanced/dialectric/prisms etc.  Mirrors add scatter and Prisms add correction and color.  The correction and color of a prism can be beneficial, especially if its opposite sign of what the objective is producing. 

[scooke]

Duh.  That was easy.  I'll do a set tonight.

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[Jeff B]

A nice lens there, not surprised you can get good,  high powers views.  Maybe just a hint of undercorrection but that would be being picky, and some minor keystoning of the lines but that is typically due to small mis-alignments in the DPAC set up.  Edge looks very good but let's see the inside of focus image too to check out what's going on, if anything, in the lower RH band at the edge.  And maybe a very mild center zone, which is where they do least damage, but an in focus view will readily show that if there is one.  Remember, in double pass errors are doubled.  A very nice lens. 

 

I have found adding a green filter helps to sharpen the image and give it a bit more contrast because they are bandpass filters and will further cut out the red and blue content of the LED's light.

 

Good job and welcome to the DPAC club!

 

Jeff

[scooke]

Thanks again for the feedback and help.  Here is the series from the SW150ED:

 

Outside Focus

 

At Focus

 

 

 

 

[scooke]

Inside Focus

[scooke]

Here is my Stellarvue 125 Access:

 

Outside Focus

[scooke]

At focus

[peleuba]

Nice photos.  Some quick thoughts on the Skywatcher...

 

In the null, I see 3 prominent zones: one in the center, very easy to see.    One at ~70% and one near the edge.   All more or less  typical for spherical, machine polished lens.   

spherical correction looks decent.   
 

Regarding the StellarVue - you have a widely turned up/down edge.   It’s more then subtle  but it’s not dramatic.  

Edited by peleuba, 06 February 2020 - 09:32 PM.

[scooke]

Inside Focus

[scooke]

I tried to attach the zygo report but couldn't get the image below 500k and still be readable.  Specs are:

 

PV 0.191 

RMS 0.025

Strehl 0.979

 

It also does an awesome job.

 

 

 

 

[scooke]

This is a lot of fun.  It makes me want to get a bigger flat to test all of my scopes.  Hopefully a really good deal on a 16" will come along...

[YAOG]

That SW150 EVO looks like a keeper. I think that most of them are this good based on my sample of one and the DPAC images I've seen of this one and others. The Stellarvue Access 125mm issues don't surprise me, I think the Access 102s tend to be better. I don't always trust Stellarvue Zygo reports, in recent times they sometimes seem to report being better than the lens is when observed closely. Older Zygo reports from 5-7 years ago seemed to be closer to reality. 

[scooke]

It's amazing how sensitive the test is.  Star testing matches the Zygo on the SV125.  And the Zygo does show a very mild turned up edge.  It sure seems a bit more obvious with my eyes on the test than the picture shows.  However, during observing it behaves as well as any 5" aperture scope.  It supports 487.5x (3-6 zoom Nagler at the 4 setting and 2x barlow) on the moon without image breakdown.  It makes me appreciate the fact that seeing is far more important than the last few digits on the strehl ratio. 

 

It would be interesting to study how much error in this test it takes to really see a difference in viewing objects.

[scooke]

Nice photos.  Some quick thoughts on the Skywatcher...

 

In the null, I see 3 prominent zones: one in the center, very easy to see.    One at ~70% and one near the edge.   All more or less  typical for spherical, machine polished lens.   

spherical correction looks decent.   
 

Regarding the StellarVue - you have a widely turned up/down edge.   It’s more then subtle  but it’s not dramatic.  

Paul,

Thanks for the feedback.  The null is very informative.  I've used Ronchi eyepieces with a star in focus many times and it shows an amazing amount of detail.  Almost as much as the Lyot test.  I love being able to do this without a star.

[scooke]

Of all the star tests I've done, my Celestron 90 fluorite performed the best, followed by the C102 and AP130GTX being a virtual tie.  After that is my 14.5 Zambuto powered Starmaster.  I still have that and would love to see that in DPAC.