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Mirror, Dielectric, & Prism Performance Comparison

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#151 PowellAstro

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Posted 26 January 2014 - 09:59 PM

Very telling and well done! I am liking the prism a lot and this is showing proof of your viewing reports!

#152 dcoyle

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Posted 26 January 2014 - 11:12 PM

Thanks for going to the trouble of capturing and posting that, Bill.

Dan

#153 george tatsis

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Posted 27 January 2014 - 02:02 AM

These shots are self- explanatory.The only way I guess to describe what one may experience using these diagonals.

Looking forward to the 1.25" group results!

Thanks for your excellent contribution to this relatively underestimated part of the telescopic optical light path! :bow:

George

#154 Sarkikos

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Posted 27 January 2014 - 06:58 AM

Looks like no diagonal won. :thinking:

:grin:
Mike

#155 Asbytec

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Posted 27 January 2014 - 07:09 AM

Bill, thanks for going through the trouble to devise an interesting test. I see a huge difference between straight through and any diagonal, but very little difference between diagonals even after downloading and enhancing the image.

I'm pretty sure what you capture in the test would be manifest in the real world, as well, to some degree due to the behavior of the various coatings and substrates. The indoor "lab" test might be exaggerated, but that's fine. It shows us what we're looking for.

#156 BillP

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Posted 27 January 2014 - 08:20 AM

Looks like no diagonal won. :thinking:


Yes...and by more than just a little! Might need a lounge chair though to observe that way with Juipiter being relatively high for best observing. :lol:

#157 BillP

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Posted 27 January 2014 - 08:22 AM

The indoor "lab" test might be exaggerated, but that's fine. It shows us what we're looking for.


Hey...stop dissing my lab! :4 (kidding of course)

#158 leonard

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Posted 27 January 2014 - 09:06 AM

Hello ,

Great job Bill . :bow:

Even with my ageing eyes,looking at the pic before reading your post I could spot the slight scatter in the pic without the diagonal. No wonder some people like to try viewing without a diagonal at all.
To me its obvious the prism is doing a better job at scatter control .
In an early ad ,Roland at Astro-Physics stated a reflector had (if I remember correctly) about 4 times the scatter compaired to refraction , seems to be close.

Very interesting thread , Leonard

#159 rockethead26

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Posted 27 January 2014 - 10:21 AM

I see a huge difference between straight through and any diagonal, but very little difference between diagonals even after downloading and enhancing the image.


Actually, there's quite a bit of difference, both in the level of edge brightening and in the distance the scatter spreads from the edge.

Your monitor may not be able to show you the differences as well as others can.

#160 Midnight Dan

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Posted 27 January 2014 - 11:37 AM

Just to get a little more analytical, I downloaded a free image analysis program called ImageJ. I drew a long thin rectangle across the middle of you scatter comparison image and plotted the gray value across the rectangle. They are the same test cases as yours from left to right:

No diagonal
Baader 2" Prism
AP MaxBright
AT 2" Dielectric.

-Dan

Attached Files



#161 SteveC

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Posted 27 January 2014 - 11:52 AM

The indoor "lab" test might be exaggerated, but that's fine. It shows us what we're looking for.


Hey...stop dissing my lab! :4 (kidding of course)


Were you wearing a white lab coat? If you're not wearing a lab coat, you might as well be working in your garage.

#162 Midnight Dan

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Posted 27 January 2014 - 12:02 PM

After looking at the plots above from ImageJ, I realized it's pretty hard to compare them. So I saved the data, brought it into Excel and plotted them on the same graph. Better!

Note: Vertical axis is brightness of the image, horizontal axis is the location of the pixels from left to right on the image.

-Dan

Attached Files



#163 pga7602

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Posted 27 January 2014 - 12:20 PM

Been experimenting with multiple methods to show via a picture, instead of just describing, scatter differences between diagonals. It has not been easy and spent all weekend conducting various experiments to determine which shows differences most discretely. Indoor star test inadequate since you get back reflection off the surface where the hole is drilled, Illuminating a flat reflective surface that is offset from a dark background also did not work well due to inability to get a bright enough reflection. No method where I did not incorporate the telescope worked at all (i.e., direct imaging using diagonal only of very bright targets). Finally, what seems to demonstrate scatter differences best in a lab setting is to photograph a clear christmas light bulb. The bulb is offset from a dark black bacground a few feet so no light from the bulb exposes the background. Camera settings were all fixed - focus, shutter, f-stop, ISO, white balance all fixed. The TSA-102 with the 5mm Pentax 5XO were used and afocally photographed the light bulb with a Fuji X-E1 camera. The telescope was focused for each shot on the filament of the bulb. Precise focus is easy with the Fuji X-E1 camera since the Electronic View Finder has it's own zoom function to facilitate precise focus. Plus it automatically adjusts its brightness to the camera settings, so a quick flip of the shutter to 1/4000th sec lets me see the filament not over exposed. Then backed it off to the test shutter speed (1/8 sec, 6400 ISO, f/1.4, WB=2700K).

Below is a pic through 3 of the 2" diagonals. The pic is of the black space just to the right of the right edge of the light bulb. 1st frame is straight through with no diagonal. I think this captures the differences well. None of this tells one how it will perform on real targets as only a field observation will reveal that the best. However, it's nice to see that the scatter levels shown in the pic are ligning up with the field impressions so far. Best if downloaded and viewed in full size btw. In the CN interface can't see any scatter from straight through, but there is some when viewing it after download. Interesting just how much scatter the diagonal adds. Unfortunately my neck will not put up with observing without one :lol:

Was the Tak 1.25 part of this test Bill?

#164 BillP

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Posted 27 January 2014 - 01:57 PM

Was the Tak 1.25 part of this test Bill?


Yes. And still is. Will have scatter pics for all the diagonals in final report. Was just giving a progress report here.

Dan - Can I send you all the pics when I do them to compile a nice graph like this for the article?

SteveC - Thanks so much for the healthy :roflmao:

#165 fjs

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Posted 27 January 2014 - 02:18 PM

Looks like no diagonal won. :thinking:


Yes...and by more than just a little! Might need a lounge chair though to observe that way with Juipiter being relatively high for best observing. :lol:


Great idea! Anybody have build plans for a cantilevered EQ mount I can use with my lounge chair?

#166 Midnight Dan

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Posted 27 January 2014 - 02:46 PM

Dan - Can I send you all the pics when I do them to compile a nice graph like this for the article?


Sure, I'd be happy to.
-Dan

#167 PeterR280

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Posted 27 January 2014 - 03:36 PM

The AP MaxBright has pretty good performance.

#168 Hermie

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Posted 27 January 2014 - 07:10 PM


Bill,

Thank you for undertaking this project! I use an AP maxbright, and have been following your progress with great interest. First you had me contemplating a new prism diagonal, and now I'm thinking I need a new observing chair:

straight through viewing chair

Hermie

#169 herrointment

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Posted 27 January 2014 - 08:56 PM

Tough to fall asleep in that one......

#170 schang

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Posted 27 January 2014 - 09:00 PM

and a torture to my old bones... :grin:

#171 BillP

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Posted 27 January 2014 - 10:10 PM

straight through viewing chair

Hermie


:roflmao:

#172 SteveC

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Posted 27 January 2014 - 10:32 PM

Bill,

Thank you for undertaking this project! I use an AP maxbright, and have been following your progress with great interest. First you had me contemplating a new prism diagonal, and now I'm thinking I need a new observing chair:

straight through viewing chair

Hermie


From what friends tell me, I ought to remember that position fondly.

#173 SteveC

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Posted 27 January 2014 - 10:42 PM

After looking at the plots above from ImageJ, I realized it's pretty hard to compare them. So I saved the data, brought it into Excel and plotted them on the same graph. Better!

Note: Vertical axis is brightness of the image, horizontal axis is the location of the pixels from left to right on the image.

-Dan


Dan, can you plot the pixels from left and right of the center..................I suspect I'm looking for something that resembles a bell curve.

#174 Asbytec

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Posted 28 January 2014 - 05:05 AM

Hey, Bill, I know you're in jest, but just to be clear I was differentiating between "lab" (which I like) and real world tests (which others seem to prefer). I think you did just fine, with or without a lab coat.

Dan, okay, thanks for that analysis. Interesting. I can see the difference looking back at the image, just not as striking as I might have hoped - and not as striking as without. But, hey, every little bit helps.

Hermie, that was humorous and in a sense serious. Science is like that, you test a diagonal and realize the fix is a reclining deck chair. :grin:

#175 The Mighty Mo

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Posted 28 January 2014 - 10:28 AM

This question just hit me this morning, don't know why it took so long...

What happens to the focus position using one of the 2" prisms vs a 2" dielectric? I doubt it stays the same. How far is it shifted and which direction?






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