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RASA8 back focus distance (metal spacing) mystery

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#1 mxcoppell

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Posted 10 December 2019 - 03:25 PM

This post is for discussing optimal back focus distance (metal spacing) for RASA8 when filters with different thicknesses are used.

 

The quality of images made by Celestron RASA8 is very sensitive to the back focus distance (metal spacing). I still don't have clear understanding how it works and what's the right back focus distance in different scenarios.

 

Let's start from the built-in glass window, by my measurement:

  • 46mm in diameter
  • 2.28mm ~ 2.33mm in thickness

But I truly don't know if it's counted in the 29mm back focus distance described in the user manual (the reason I don't want to use 25mm on top of the camera adapter - the padding at the bottom of the camera adapter is soft, could be suppressed when tightening the clamping ring, not accurate at all. 29mm is from the corrector module metal surface to camera sensor).

 

Still, the 29mm everybody uses is different from Celestron published "Recommendation for Customized Adapter (RASA8")" guide. (https://celestron-si...dapter_Rev1.pdf). In this guide, from the corrector module metal surface to camera sensor should be 28.73mm. This is something that I'd like to use as more accurate measure.

 

In the following views we can see the 28.73mm is based on the scenario that the clear glass window is installed.

 

post-280998-0-51009200-1575936557.jpeg

 

post-280998-0-55966400-1575936589.jpeg

 

post-280998-0-50736900-1575936623_thumb.

 

Not sure about the optical back focus distance added by this clear window. Assume it's still the simple 1/3 rule, it adds about 0.8mm.

 

Now, with 3mm filter installed and clear window removed, the back focus distance increases 0.2mm comparing to the scenario only the glass window is installed.

 

I'd like to hear from more people about how to design the spacing. Let me share my own first test images in my first-light session (ABE and stretch in PixInsight only): https://www.astrobin.com/9h9seb/

 

The stars looks okay at the corner. But HFR values are around 1.9 - 2.0. (Perfect V curves in SGP with Celestron AF Motor).

 

This was done by the following setup:

 

- Glass window removed.

- Camera adapter 3.5mm (padding ripped)

- Starizona Filter Slider (18.70mm)

- Astronomik 12nm Ha 2" (1mm thick)

- ASI1600mm-Pro (6.5mm)

 

I measured the total thickness of the Starizona filter slider installed together with M42 camera adapter (again, padding ripped), it's about 22.20mm. On the scope side of the Starizona filter slider, there is a 1mm thick M42-M48 ring, but the 1mm was consumed by the "sunk" circle of M42 camera adapter.

 

post-280998-0-45873000-1575938432.jpeg

 

Putting the numbers of the metal pieces together, WITHOUT the glass window:

 

3.5 + 18.7 + 6.5 = 28.7mm

 

Let's assume the clear glass is removed, and use my "lucky number" here that 28.7mm of metal spacing worked with 1mm thick filter, for 3mm thick filter the metal spacing should be increased 0.67mm to 29.37mm (Astronomik 12nm Ha is only 1mm thick). ;-)

 


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#2 mxcoppell

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Posted 10 December 2019 - 03:28 PM

But there is also a possibility that my back focus is off in my test. I was never able to get the HFR down to 1.6-1.7 range with ASI1600mm. In Chuck's review video, he got it. That made me wonder as well if the back focus distance could be optimized more.

 

Here is another calculation:

 

Defined by the adapter design document, with glass window installed (let's say 2.3mm thick) - The optimal metal spacing is 28.73mm.

 

So, with 3mm thick filter installed, the optimal metal spacing should be increased by (3mm - 2.3mm) / 3, and the optimal metal spacing in this case should be 28.963mm.

 

--

 

With the same theory to revisit my first-light test:

 

Using 1mm thick Astronomik 12nm Ha filter instead of the 2.3mm thick glass window - the optimal metal spacing should be 28.73mm + (1.0mm - 2.3mm)/3 = 28.3mm. Mine was 28.7mm, it could be the 0.4mm longer than expected spacing led to the larger HFR in my testing.



#3 mxcoppell

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Posted 10 December 2019 - 03:31 PM

I submitted a technical support ticket to Celestron:

Hello,

I am expecting a more technical/precise answer for the optimal back focus distance with 
my RASA8. I am in the process of ordering an customized adapter to work with my OSC camera 
and the cost is high. I definitely need to make it work or it's a waste of a lot of money.

In Celestron document: 
https://celestron-site-support-files.s3.amazonaws.com/support_files/RASA8_customized_camera_adapter_Rev1.pdf

The diagram shows optimal metal spacing distance (back focus distance) with the glass window 
installed is 28.73mm.

Question #1: What's the official thickness of the glass window? 
             I measured it's between 2.28mm ~ 2.33mm. But an official number would help.

Question #2: With the optical window removed, what's the optimal metal spacing 
             distance (back focus distance)? It can't be the same 28.73mm.

Question #3: With the optical window removed, and using an 2mm UV/IR-Cut filter on the light path, 
             what's the optimal metal spacing distance (back focus distance)?

Question #4: With the optical window removed, and using an 3mm narrowband filter on the light path, 
             what's the optimal metal spacing distance (back focus distance)?

Question #5: With the optical window removed, and using an 1mm UV/IR-Cut filter on the light path, 
             what's the optimal metal spacing distance (back focus distance)?

Thanks and looking forward to your official answers!
-Min from Dallas

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#4 bobzeq25

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Posted 10 December 2019 - 05:47 PM

But there is also a possibility that my back focus is off in my test. I was never able to get the HFR down to 1.6-1.7 range with ASI1600mm. In Chuck's review video, he got it. That made me wonder as well if the back focus distance could be optimized more.

 

Here is another calculation:

 

Defined by the adapter design document, with glass window installed (let's say 2.3mm thick) - The optimal metal spacing is 28.73mm.

 

So, with 3mm thick filter installed, the optimal metal spacing should be increased by (3mm - 2.3mm) / 3, and the optimal metal spacing in this case should be 28.963mm.

 

--

 

With the same theory to revisit my first-light test:

 

Using 1mm thick Astronomik 12nm Ha filter instead of the 2.3mm thick glass window - the optimal metal spacing should be 28.73mm + (1.0mm - 2.3mm)/3 = 28.3mm. Mine was 28.7mm, it could be the 0.4mm longer than expected spacing led to the larger HFR in my testing.

0.4mm spacing change would only make a minimal difference in FWHM.  It might be worthwhile to try to get it right, but generally anywhere less than 1mm off is good.

 

You simply can't compare your FWHM to somebody elses.  Anyone elses.  Seeing drives the train.  My FWHMs vary from 1.7 (best ever) to over 4 (where I quit imaging).   They can change by 1 in 5 minutes.  If they're under 3, I'm happy.

 

In a recent imaging session I started out a bit under 3, and went to over 4.  So I quit.


Edited by bobzeq25, 10 December 2019 - 05:51 PM.


#5 mxcoppell

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Posted 10 December 2019 - 06:20 PM

0.4mm spacing change would only make a minimal difference in FWHM.  It might be worthwhile to try to get it right, but generally anywhere less than 1mm off is good.

 

You simply can't compare your FWHM to somebody elses.  Anyone elses.  Seeing drives the train.  My FWHMs vary from 1.7 (best ever) to over 4 (where I quit imaging).   They can change by 1 in 5 minutes.  If they're under 3, I'm happy.

 

In a recent imaging session I started out a bit under 3, and went to over 4.  So I quit.

Bob, 

 

Yes. there are other variables that are not fully under control - for example, the camera back focus. QHY168c specification says 18+/-0.5mm, oh well...

 

But I do want to get as close as possible. You are right, the comparison doesn't make full sense but still could be used as a reference. For example, I believe the contrast from Astronomik 12nm can't compare with Baader F2 Highspeed. My data seems pretty washed. 

 

Do you mean HFR or FWHM? It's extremely good data for FWHM at 1.7! Mine is around 5.0-5.4 in FWHM (arcsec) or 2.1-2.3 in FWHM (pixel) for the RASA8/ASI1600mm. 

 

Still, I want to understand better of the RASA8; especially how the best metal spacing is calculated. I will use RASA8 with APSC OSC as well and that will be even more sensitive to it. 

 

Thanks!


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#6 mxcoppell

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Posted 12 December 2019 - 05:03 PM

Got the first response from Celestron:

 

In order to best address your question, we are reaching out to our Product Development team for the answer. This may take up to 3 business days, but rest assured that your inquiry is very important to us and we will respond with an answer as soon as we are able. Please let us know if you have any questions in the meantime and thank you for your patience.



#7 RogeZ

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Posted 12 December 2019 - 06:58 PM

Following this thread.

 

The results from the 3nm Chroma are not very encouraging but I need to do more rigorous testing. I get the filter slider this Saturday, we will see...



#8 mxcoppell

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Posted 12 December 2019 - 07:56 PM

Following this thread.

 

The results from the 3nm Chroma are not very encouraging but I need to do more rigorous testing. I get the filter slider this Saturday, we will see...

Rogez, 

 

I will have Chroma 5nm filters next week. Only got the 36mm ones to work with Starizona 36mm-48mm adapters - big enough for ASI1600mm sensor. 

How did you make the back focus distance right without the filter slider? 

 

And, any more images, observations and thoughts to share? 

 

Clear skies!



#9 mxcoppell

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Posted 16 December 2019 - 08:25 PM

Update from Celestron:

 

Hello Min,

 

I wanted to give you an update.

 

Our engineers are modelling this to see what the ideal back focus positions are with these changes. As soon as we receive a response I will let you know.

 

Please note that the tolerance for back focus is ~ +/-0.5mm. Also, the optical window thickness is 2.0mm +/- 0.1mm.

 

Johnny O, Celestron Technical Support

Ticket Details



#10 andysea

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Posted 16 December 2019 - 08:34 PM

I have the sensor at 25mm from the shoulder of the t-treads on the camera adapter and I seem to get good stars to the corners of my ASI1600.

I use the Astronomik 36mm filters which I think are only 1mm  or so in thickness. I also removed the stock optical window.

I can share some raw frames if you like.



#11 mxcoppell

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Posted 16 December 2019 - 08:39 PM

Thanks Andy! I got similar result with 1mm thick Astronomik filter (Ha) - all round stars at the corner. But I am switching to 3mm thick filter, so I just trying to figure this out to see where the precise right value is. 



#12 mxcoppell

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Posted 19 December 2019 - 03:30 PM

Update from Celestron Tech Support:

 

Hello Min,

 

Our Engineers are still discussing details on this. We should have a response for you soon.

 

Johnny O, Celestron Technical Support



#13 RogeZ

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Posted 23 December 2019 - 08:45 AM

Min any updates from Big C?

#14 mxcoppell

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Posted 23 December 2019 - 10:50 AM

No. Still no answer from big C.


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#15 mxcoppell

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Posted 23 December 2019 - 11:05 AM

Just got it!

———

Hello Min,

I apologize for the delay. It took time for our optical engineer to confirm all of the calculations with the various substrates involved in the RASA and filters.


Question #1: The optical window thickness is 2.0mm +/- 0.1mm.

Question #2: With the optical window removed, what's the optimal metal spacing distance (back focus distance)? - 27.270mm

Question #3: With the optical window removed, and using an 2mm UV/IR-Cut filter on the light path, what's the optimal metal spacing distance (back focus distance)? - 27.617mm

Question #4: With the optical window removed, and using an 3mm narrowband filter on the light path, what's the optimal metal spacing distance (back focus distance)? - 27.964mm

Question #5: With the optical window removed, and using an 1mm UV/IR-Cut filter on the light path, what's the optimal metal spacing distance (back focus distance)? - 28.732mm

Johnny O, Celestron Technical Support

Ticket Details
Email ID: MMP-144-27991
Department: Technical Support
Type: Task
Status: Pending Reply
Priority: Normal


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#16 mxcoppell

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Posted 23 December 2019 - 11:31 AM

Guys, I’d like your help on this one again. This is against my previous “corrected” understanding - In Celestron’s answer, the required metal spacing actually decreases when filter is used. 

 

This raises a big concern as well - the Starizona filter slider is way too thick in design. 


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#17 RogeZ

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Posted 23 December 2019 - 11:43 AM

We can always remove the 1mm RASA spacer....

Celestron should publish those numbers...

#18 mxcoppell

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Posted 23 December 2019 - 11:50 AM

RogeZ, I think you are referring to the 1mm M48-M42 adapter/spacer on the Starizona filter slider?

 

No, we can’t. Removing that will make the female thread on filter slider (facing RASA side) 48mm and we don’t have a 48mm camera adapter. 

 

Second, I don’t think the 1mm spacer actually contributed 1mm in metal spacing. From my previous post, you can see the total thickness of the filter slider and the camera adapter together actually is only 22.21mm. The 1mm ring got “consumed” by the sunken’ed area of the M42 camera adapter. 



#19 rockstarbill

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Posted 23 December 2019 - 11:50 AM

Interesting response from Celestron for sure..

#20 mxcoppell

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Posted 23 December 2019 - 12:00 PM

Based on Celestron’s numbers, I am now totally lost with my configuration. Way off. 


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#21 mxcoppell

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Posted 23 December 2019 - 01:30 PM

I post more questions to Celestron after reviewing the numbers that are quite confusing.

 

Johnny, looking closely at the results and I got confused.

The metal spacing required for for the following scenarios seem contradicting to each other (all without Celestron optical window glass):

- No filter, 27.270mm
- 1mm thick filter, 28.732mm
- 2mm thick filter, 27.617mm
- 3mm thick filter, 27.964mm

Are we sure about these numbers?

 

Also, I forward these numbers to Starizona to get some suggestions as well.



#22 mxcoppell

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Posted 24 December 2019 - 11:36 AM

Response from Celestron (Thanks to Johnny for the patience, top-notch customer support).

 

Min,

Yes.

Those numbers were modelled with the engineering calculations for the telescope by the optical engineers that designed the RASA 8 and had worked on the RASA 11 and 36. They even went so far as to re-verify the index of refraction of each component in the optical train. We are sure of those numbers.

The fact that they were so careful is why the calculations took so long

Johnny O, Celestron Technical Support



#23 mxcoppell

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Posted 24 December 2019 - 11:39 AM

This doesn’t make sense to me. Then more question asked:

 

Johnny,

 

First thanks to all your effort to get this through the engineering team. I am sure the team got all the expertises to make the calculation. But still. Just by looking at the following 3 scenario:

 

- No filter, 27.270mm
- 1mm thick filter, 28.732mm
- 2mm thick filter, 27.617mm

 

This doesn’t look right. Common understanding is that if a thicker filter is in the light path. More metal spacing is required to compensate. From the number returned by the engineering team, it doesn’t look right. With Celestron optical window removed, from no filter to 1mm thick filter, the metal spacing requirement increased more than 1mm. No glass diffraction factor could contribute more than 1.0 diffraction index.

 

Then, when 2mm thick filter is used, the metal spacing is getting shorter?! This is not right.

 

-Min from Dallas

 

Merry Christmas Folks!


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#24 mxcoppell

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Posted 24 December 2019 - 12:11 PM

Quick response from Celestron, thumbs up!!!

———


Min,

It looks like there was a mistake on how these values were reported.. My most sincerest apologies.



Here are the answers to the questions.



Question #1: The optical window thickness is 2.0mm +/- 0.1mm.

Question #2: With the optical window removed, what's the optimal metal spacing distance (back focus distance)? - 27.270mm

Question #3: With the optical window removed, and using an 2mm UV/IR-Cut filter on the light path, what's the optimal metal spacing distance (back focus distance)? - 27.964mm

Question #4: With the optical window removed, and using an 3mm narrowband filter on the light path, what's the optimal metal spacing distance (back focus distance)? - 28.732mm

Question #5: With the optical window removed, and using an 1mm UV/IR-Cut filter on the light path, what's the optimal metal spacing distance (back focus distance)? - 27.617mm

Johnny O, Celestron Technical Support

Ticket Details
Email ID: MMP-144-27991
Department: Technical Support
Type: Task
Status: Pending Reply
Priority: Normal


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#25 mxcoppell

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Posted 24 December 2019 - 12:39 PM

Give it a simpler form for the optimal metal spacing in different scenarios (Celestron optical window removed for all cases):

 

- No filter: 27.270mm

- 1mm UV/IR-Cut filter: 27.617mm

- 2mm UV/IR-Cut filter: 27.964mm

- 3mm Narrowband filter: 28.732mm

 

And the factory Celestron optical window thickness 2.0+/-0.1mm.

 

This makes more sense! But it looks shorter than published Celestron RASA8 documents. I suggested Celestron/Johnny to publish this as official guidelines to help more users.


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