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RASA8 back focus distance (metal spacing) mystery

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#51 bobzeq25

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Posted 10 January 2020 - 07:38 PM

I'm talking now to Starizona.  They have identified yet another "gasket" that I am to remove.  This one is grey and mounted inside the large retaining ring at the base of the threads.  Too fat, they say.  They also say now that the spacer of soft felt-like material in the Celstron M42 adapter should have been left in but they are sending me a "thinner" one.  Gosh, each part is a moon shot.  And, with this gasket removed, the slider drawer is freed.

Just to clarify,  2 "spacing things" had been manufactured too thick.  The grey, felt-like material glued into the Celestron M42 adapter, and the gasket embedded at the base of the large retaining ring, needed to be replaced with thinner substitutes.

But I am going ask again about the clear window.  They have told me twice now to leave it in.

 

Now on to the next problems which will no doubt surface tomorrow...I have no guarantees the camera will now correctly focus.

 

Later:  I just asked Starizona AGAIN about the clear window, leave it or remove it.  Here is their answer,

deankoenig <dean@starizona.com>
2:58 PM (1 minute ago)
to me

On the 8” RASA you leave the window in on the 11’' RASA you remove it.

The gasket on the retaining ring doesn't change spacing of the camera.  That's set by the base of the adapter against the scope, the retaining ring simply holds things in place.  What it will change is whether the retaining ring conflicts with the drawer.

 

The camera will always focus.  The question is how curved the field will be.

 

i agree about leaving the clear window in with a 1mm thick additional filter.  I'd take it out with a 3mm.  2mm?  Flip a coin.  <grin>

 

This is not an easy scope to get working well.  I've spent hours on it, and $250 for PreciseParts adapters.


Edited by bobzeq25, 10 January 2020 - 07:40 PM.

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#52 arrowspace90

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Posted 10 January 2020 - 07:49 PM

The gasket on the retaining ring doesn't change spacing of the camera.  That's set by the base of the adapter against the scope, the retaining ring simply holds things in place.  What it will change is whether the retaining ring conflicts with the drawer.

 

The camera will always focus.  The question is how curved the field will be.

 

i agree about leaving the clear window in with a 1mm thick additional filter.  I'd take it out with a 3mm.  2mm?  Flip a coin.  <grin>

 

This is not an easy scope to get working well.  I've spent hours on it, and $250 for PreciseParts adapters.

Please keep telling me what you know.  I have fun being frustrated by all of it.



#53 bobzeq25

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Posted 10 January 2020 - 09:08 PM

Please keep telling me what you know.  I have fun being frustrated by all of it.

Suggestion.  Forget filters for the moment (the Starizona flter drawer may be a good spacer) and get things working first without them.  Get a baseline for when you add filters.


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#54 RogeZ

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Posted 12 January 2020 - 01:04 PM

Bob:

 

PM sent with link. Please post your results.



#55 bobzeq25

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Posted 12 January 2020 - 09:03 PM

Here they are.  Methodology.

 

Star aligned everything to the 12nm frame.  That allowed me to use Dynamic crop to select exactly the same pixels on all 3 frames.

 

I cropped an identical small section of nebula that way on all 3, used PI Statistics to determine the brightness in ADU.  Did the same thing with a small section of background only a very small distance away.  The difference in ADU represents the contrast available from each.

 

Results, everything is in 14 bits.  12nm had a nebula level of 3030 ADU, background of 2615, contrast was 415 ADU.  6nm; 1900, 1565, 335.  3nm 1035, 825, 210.

 

12nm is "best".  Note that unstretched contrast is not the entire story, stars look better at small bandwidths, compete with the nebula less.  Processing provides opportunities to increase contrast, 3nm has the best _relative_ contrast.

 

As usual, there are tradeoffs.  But I think more people would be happy with 6 or 12, instead of the very expensive 3.

 

IMPORTANT.  All this applies at F2.  At slower F ratios, I'd expect 3 to win.


Edited by bobzeq25, 12 January 2020 - 09:05 PM.

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#56 mxcoppell

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Posted 15 January 2020 - 07:48 AM

Here they are.  Methodology.

 

Star aligned everything to the 12nm frame.  That allowed me to use Dynamic crop to select exactly the same pixels on all 3 frames.

 

I cropped an identical small section of nebula that way on all 3, used PI Statistics to determine the brightness in ADU.  Did the same thing with a small section of background only a very small distance away.  The difference in ADU represents the contrast available from each.

 

Results, everything is in 14 bits.  12nm had a nebula level of 3030 ADU, background of 2615, contrast was 415 ADU.  6nm; 1900, 1565, 335.  3nm 1035, 825, 210.

 

12nm is "best".  Note that unstretched contrast is not the entire story, stars look better at small bandwidths, compete with the nebula less.  Processing provides opportunities to increase contrast, 3nm has the best _relative_ contrast.

 

As usual, there are tradeoffs.  But I think more people would be happy with 6 or 12, instead of the very expensive 3.

 

IMPORTANT.  All this applies at F2.  At slower F ratios, I'd expect 3 to win.

This is great test. My test (visually) is on the same side of Bob's result. I am settling on 12nm now. No more Chroma 5nm filter fantasies.



#57 arrowspace90

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Posted 15 January 2020 - 10:13 AM

Here they are.  Methodology.

 

Star aligned everything to the 12nm frame.  That allowed me to use Dynamic crop to select exactly the same pixels on all 3 frames.

 

I cropped an identical small section of nebula that way on all 3, used PI Statistics to determine the brightness in ADU.  Did the same thing with a small section of background only a very small distance away.  The difference in ADU represents the contrast available from each.

 

Results, everything is in 14 bits.  12nm had a nebula level of 3030 ADU, background of 2615, contrast was 415 ADU.  6nm; 1900, 1565, 335.  3nm 1035, 825, 210.

 

12nm is "best".  Note that unstretched contrast is not the entire story, stars look better at small bandwidths, compete with the nebula less.  Processing provides opportunities to increase contrast, 3nm has the best _relative_ contrast.

 

As usual, there are tradeoffs.  But I think more people would be happy with 6 or 12, instead of the very expensive 3.

 

IMPORTANT.  All this applies at F2.  At slower F ratios, I'd expect 3 to win.

Was that in english?

Ha, it merely illustrates how far behind the curve some of us are.  I have little idea what most of that educated post meant.



#58 bobzeq25

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Posted 16 January 2020 - 09:51 PM

Was that in english?

Ha, it merely illustrates how far behind the curve some of us are.  I have little idea what most of that educated post meant.

OK, I'll take that as a challenge.  <smile>

 

Light hitting the pixels of our sensors creates a digital signal.  The more light, the more signal.

 

The measurement we read from the sensor as a result of that light is frequently stated in units of ADU.  Analog to Digital Units.  A detail is you must state the bit depth so that other people know what kind of ADUs you're talking about.  A dim target might have a signal of a few ADU, a bright star, thousands of ADUs.  The maximum level possible depends on how many bits you're talking about.

 

I used PixInsight to measure the average ADU generated by a small piece of the background sky, and a small piece of nebula.  The greater the difference in ADU, the more difference in how the sensor, behind various filters, saw the difference between the dim nebula and the background sky.

 

A detail was that PI has tools (StarAlign, DynamicCrop) which allowed me to select _exactly the same small piece of background and nebula from three different frames taken at different times.  One was tilted slightly with regard to the other, the PI tools compensated.  Tools like this are one reason why advanced imagers prefer PI.  But you have to know how to use them.  <smile>


Edited by bobzeq25, 16 January 2020 - 09:53 PM.


#59 arrowspace90

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Posted 17 January 2020 - 07:49 AM

OK, I'll take that as a challenge.  <smile>

 

Light hitting the pixels of our sensors creates a digital signal.  The more light, the more signal.

 

The measurement we read from the sensor as a result of that light is frequently stated in units of ADU.  Analog to Digital Units.  A detail is you must state the bit depth so that other people know what kind of ADUs you're talking about.  A dim target might have a signal of a few ADU, a bright star, thousands of ADUs.  The maximum level possible depends on how many bits you're talking about.

 

I used PixInsight to measure the average ADU generated by a small piece of the background sky, and a small piece of nebula.  The greater the difference in ADU, the more difference in how the sensor, behind various filters, saw the difference between the dim nebula and the background sky.

 

A detail was that PI has tools (StarAlign, DynamicCrop) which allowed me to select _exactly the same small piece of background and nebula from three different frames taken at different times.  One was tilted slightly with regard to the other, the PI tools compensated.  Tools like this are one reason why advanced imagers prefer PI.  But you have to know how to use them.  <smile>

 

Ah. Pixinsite.  Territory I’m not equipped for yet.  I won’t be trying to grasp PI as a beginner 




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