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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 



#60 mxcoppell

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Posted 19 January 2020 - 02:52 PM

Here is an initial report with QHY168c (APSC) and PreciseParts M54 camera adapter. The one I ordered is with 9.6mm back focus spacing. Baader Moon & Skyglow UV/IR-Cut filter (M48) was screwed right in front of the RASA8 optical window (with RASA8's filter adapter ring for installing the original glass window). 

 

WechatIMG132.jpeg

 

- With the above setup, there are about 5% dark edges along the short sides. Should be easy to handle, either flat them out or cut them out. 

- Center star no issue. Stars at the corner show very slightly elongated forms. I think there are still spaces for optimizing the spacing. 

 

All the pictures shown below are imaged from my inner city sky (SQM 17.96, Class 9 with all neighbor's garage lights on. :-O ), 40 x 60s subs, Gain 1 / Offset 35, QHY168c. Test target is M44. Minimum processing.

 

WechatIMG131.jpeg

 

I have 17% curvature. 

 

WechatIMG129.jpeg

 

WechatIMG128.jpeg

 

Question about this shape of curvature - Is this too much spacing or I need to add more spacing? If it's the first cause, I can't do anything but live with it. For the latter, I will perform more test. 

 

I will attach the corner crops in next post.

 

-Min


Edited by mxcoppell, 19 January 2020 - 03:25 PM.


#61 mxcoppell

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Posted 19 January 2020 - 02:56 PM

Corner crops of the M44 processing result. 

 

WechatIMG45500.jpeg

 

WechatIMG45499.jpeg

 

WechatIMG45497.jpeg

 

WechatIMG45495.jpeg

 

 


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#62 mxcoppell

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Posted 19 January 2020 - 03:00 PM

I've also did some test on bright stars - The following test performed on the same environment and setup for Capella (Mag 0.02, 6th bright stars in my sky). The halo is very big. I am not sure about the cause. Any suggestions are welcome.

 

WechatIMG45490.jpeg



#63 mxcoppell

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Posted 19 January 2020 - 11:09 PM

Well, I kinda found the answer to my own question:

 

WechatIMG133.jpeg

 

I think the spacing is fine. But I might have some slight tilt somewhere (I know it's not the camera). Hate to spend more time on it. I will live with it.


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#64 bobzeq25

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Posted 20 January 2020 - 12:13 AM

Corner crops of the M44 processing result. 

 

attachicon.gifWechatIMG45500.jpeg

 

attachicon.gifWechatIMG45499.jpeg

 

attachicon.gifWechatIMG45497.jpeg

 

attachicon.gifWechatIMG45495.jpeg

I'm with post #63.

 

My current plan is to live with results like that until the Baader tilter is available.  They're "good enough" for me, and I'm not sure if my residual issues are tilt, spacing, or collimation.  So, I'm not going to mess with it until I've ruled out tilt.

 

I like the images I'm making, and I've always advised people not to focus on analytical numbers when they like their images.  Taking my own advice.  <smile>

 

I will say I think PreciseParts is the way to go for camera adapter/spacer (in one piece).  For now, anyway.  <smile>


Edited by bobzeq25, 20 January 2020 - 12:18 AM.

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#65 mxcoppell

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Posted 20 January 2020 - 12:49 AM

I'm with post #63.

 

My current plan is to live with results like that until the Baader tilter is available.  They're "good enough" for me, and I'm not sure if my residual issues are tilt, spacing, or collimation.  So, I'm not going to mess with it until I've ruled out tilt.

 

I like the images I'm making, and I've always advised people not to focus on analytical numbers when they like their images.  Taking my own advice.  <smile>

 

I will say I think PreciseParts is the way to go for camera adapter/spacer (in one piece).  For now, anyway.  <smile>

Same here! No more messing up! Need to collect some data with this tiny beast! LOL


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#66 mxcoppell

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Posted 20 January 2020 - 01:11 PM

One more test I am going to do - I stacked the default 2mm thick window glass and 1mm thick Baader Moon & Skyglow filter together to gain more metal spacing. 



#67 bobzeq25

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Posted 20 January 2020 - 03:21 PM

One more test I am going to do - I stacked the default 2mm thick window glass and 1mm thick Baader Moon & Skyglow filter together to gain more metal spacing. 

That's what I do.  Note that I believe it "gains" you 1/3mm.


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#68 mxcoppell

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Posted 20 January 2020 - 10:40 PM

Rushed to perform another test. The added 2mm clear glass is supposed to increase the required backfocus distance by 0.67mm. I didn't get to see how big the difference it is - I rotated the camera by 90% and one corner has horrible tilt - While the opposite corner is perfectly in focus with sharp stars, the problem corner shows donuts. 

 

Then I rotate the camera back, the tilt situation improved a lot. But can't get to the stage that I tested 2 days ago. 

 

This really concerns me. Because I have no clue what caused the tilt. I need to look into it and try to understand first.  


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#69 arrowspace90

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Posted 21 January 2020 - 11:45 AM

Rushed to perform another test. The added 2mm clear glass is supposed to increase the required backfocus distance by 0.67mm. I didn't get to see how big the difference it is - I rotated the camera by 90% and one corner has horrible tilt - While the opposite corner is perfectly in focus with sharp stars, the problem corner shows donuts. 

 

Then I rotate the camera back, the tilt situation improved a lot. But can't get to the stage that I tested 2 days ago. 

 

This really concerns me. Because I have no clue what caused the tilt. I need to look into it and try to understand first.  

I have a nearby Rasa 8 that I am trying to set up.  So your problems are a concern.  



#70 RogeZ

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Posted 22 January 2020 - 06:06 PM

Well, I kinda found the answer to my own question:

 

attachicon.gifWechatIMG133.jpeg

 

I think the spacing is fine. But I might have some slight tilt somewhere (I know it's not the camera). Hate to spend more time on it. I will live with it.

Min: Where did you get this image? Its gold....

 

BTW: My stars are still "too far away" with an Astronomiks 6nm O-III filter and the following:

 

4mm Artesky flange plate thickness
7mm Artesky flange neck height
0.3mm +0.5mm Baader precise M42 spacers
9mm Astromania spacer
6.5mm ZWO (plus or minus 0.5mm)

This equals 27.3mm exactly and it is still way wrong....confused1.gif



#71 mxcoppell

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Posted 22 January 2020 - 07:49 PM

@Roger,

 

I found it somewhere here: https://stargazerslounge.com/

In one of its discussions. 

 

Any test picture to show? And your imaging train connections?


Edited by mxcoppell, 22 January 2020 - 07:50 PM.


#72 bortle2

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Posted 22 January 2020 - 09:37 PM

Min: Where did you get this image? Its gold....

 

@Roger,

 

I found it somewhere here: https://stargazerslounge.com/

In one of its discussions.

Hint (for Roger), just in case:

 

If you want to find threads where that image was used/discussed, then

  • in Chrome, right-click on the image and select "Search Google for image",
     
  • Chrome will identify that image as a "mesh", and will search for similarly-looking "meshes" returning a pile of... irrelevant images,
     
  • in the Chrome search field, replace word "mesh" with "site:stargazerslounge.com" (but keep image reference, represented in the search field as tiny thumbnail) and search again,
     
  • you'll get about seven links to threads (at stargazerslounge.com) containing this image.

P.S. A RASA has never been on my list of future purchases, but I keep reading all updates to this thread: every other message fills me with fascination and horror...


Edited by bortle2, 22 January 2020 - 10:39 PM.

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#73 bobzeq25

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Posted 22 January 2020 - 11:29 PM

Rushed to perform another test. The added 2mm clear glass is supposed to increase the required backfocus distance by 0.67mm. I didn't get to see how big the difference it is - I rotated the camera by 90% and one corner has horrible tilt - While the opposite corner is perfectly in focus with sharp stars, the problem corner shows donuts. 

 

Then I rotate the camera back, the tilt situation improved a lot. But can't get to the stage that I tested 2 days ago. 

 

This really concerns me. Because I have no clue what caused the tilt. I need to look into it and try to understand first.  

Do you have any spacers between the PP adapter and the camera?



#74 mxcoppell

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Posted 22 January 2020 - 11:43 PM

Do you have any spacers between the PP adapter and the camera?

No... The PP adapter has male M54 thread and attach directly to the camera female M54 thread. I will post a picture later.



#75 RogeZ

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Posted 23 January 2020 - 06:23 PM

Check the corners on this bad frame!

 

get.jpg?insecure

 

My stars are still "too far away" with an Astronomiks 6nm O-III filter and the following:

 

4mm Artesky flange plate thickness
7mm Artesky flange neck height
0.3mm +0.5mm Baader precise M42 spacers
9mm Astromania spacer
6.5mm ZWO (plus or minus 0.5mm ?)

 

This equals 27.3mm exactly and it is still too far....Next step is 27.00mm...

 

Thanks Bortle2 for the search method.




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