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EdgeHD 9.25 & Reducer - Stars are elongated on the edges

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#1 dcm_guitar

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Posted 21 September 2021 - 11:28 PM

I have a EdgeHD 9.25.  When I first got the OTA there were no reducers available.  As a result, I've been imaging at the native f/10.  I've been impressed with getting, nice round stars all the way to the edge of the image.

 

I recently found a new 0.7x reducer.  After adding the reducer, the stars on the edge of the images are elongated.  It looks like the back focus is too short and needs to be increased.

 

I have an Optec 2" TCF-Lynx I use for focusing.  The mirrors are locked and I do not use the focuser on the OTA.  I'm using the exact same focuser, OAG, EFW and camera as I did previously.  The only difference is the reducer.  Given that when the focuser moves it's changing the back focus distance I'm having a tough time concluding that the back focus distance is incorrect since the back focus distance changes every time I run an autofocus run in NINA.

 

Thoughts?  Clearly I'm missing something.  The stars to the edge of image are round without the reducer and very elongated with the reducer.

 

 



#2 AaronH

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Posted 21 September 2021 - 11:57 PM

You need to get the correct distance between the reducer and the camera (generally referred to as the backfocus).

 

This is usually specified for the reducer/flattener. For example, it may be 55mm. This means the distance between the back of the reducer and the camera needs to be 55mm. This can be taken up by a filter wheel, an OAG, extension tubes, whatever you like.

 

Adjusting the focus by altering the distance between the reducer and the OTA won't make a difference. You need to ensure the camera sensor is at the correct distance behind the reducer.


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#3 dcm_guitar

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Posted 22 September 2021 - 12:45 AM

I have the correct back focus.  For the EdgeHD 9.25 it's 146mm.  The documentation states that the reducer does not change the back focus requirement; it's still 146mm.

 

I have imaged with this OTA for months at native f/10.  Everything is fine and stars are round all the way to the edge of the image.  It's the reducer that's the difference.

 

I have re-collimated the OTA since adding the reducer, and am using the same image train with the correct back focus.  The issue is that it LOOKS like the back focus may be too short.  However, the way the TCF-Lynx focuser works on the EdgeHD OTA, it changes the back focus every time it re-focuses.  The whole image train is moved forward or backward from a fixed point (in the case, the end of the reducer).  It looks like a back focus issue, but I think it's spmething else.



#4 Tapio

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Posted 22 September 2021 - 01:18 AM

What camera are you using ? If you are using large format - APS-C or full format then you need to be more careful with back focus.

 

Here's once again a good reference image of what to do  with back focus.

Attached Thumbnails

  • backfocus.jpg

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#5 AaronH

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Posted 22 September 2021 - 01:43 AM

I'll freely admit that I'm no EdgeHD/SCT expert... but is it at all possible to adjust the backfocus between the reducer and the camera sensor using your external focuser, while also adjusting the overall focus using the OTA focus knob? Once this is dialed in, you should be able to re-lock your mirrors.

 

Everything points to the backfocus between the reducer and camera being off, and to get this dialed in right, you really need to be able to adjust both the distance between the reducer and camera and the OTA focuser independently.


Edited by AaronH, 22 September 2021 - 01:52 AM.


#6 rkinnett

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Posted 22 September 2021 - 03:29 AM

I have the correct back focus.  For the EdgeHD 9.25 it's 146mm.  The documentation states that the reducer does not change the back focus requirement; it's still 146mm.

The reducer for the Edge 925 has the same numerical backfocus specification as the OTA, but they're different requirements.  When you shoot at prime focus, your sensor needs to be ~146mm behind the shoulder of the OTA visual back.  When you use the reducer, the OTA backfocus is entirely irrelevant and instead your sensor needs to be 146mm behind the shoulder of the reducer.  Put your focuser between the visual back and the reducer, that way changing focus does not affect the backfocus from your reducer to your sensor.

 

EDIT:  sorry, I see that putting the 2" focuser between the 3.25" visual back and reducer is not an option.  That reducer and focuser are incompatible - you really don't want your focuser to change the distance between the reducer and camera.


Edited by rkinnett, 22 September 2021 - 03:40 AM.


#7 Oort Cloud

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Posted 22 September 2021 - 07:49 AM

The EdgeHD scopes require a reducer that does not flatten, since the scopes are inherently aplanatic. It sounds like you are using a reducer meant for normal SCTs, which is trying to flatten an already flat image, and as such, is introducing field curvature. AFAIK, the only reducer available that works for the EdgeHD is the one from Celestron. (I do not own one of these, but have researched it quite a bit because I plan on getting one as soon as my finances will allow it, and wanted to know what my options were).

Edited by Oort Cloud, 22 September 2021 - 07:50 AM.


#8 RogeZ

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Posted 22 September 2021 - 07:52 AM

I'll freely admit that I'm no EdgeHD/SCT expert... but is it at all possible to adjust the backfocus between the reducer and the camera sensor using your external focuser, while also adjusting the overall focus using the OTA focus knob? Once this is dialed in, you should be able to re-lock your mirrors.

 

Everything points to the backfocus between the reducer and camera being off, and to get this dialed in right, you really need to be able to adjust both the distance between the reducer and camera and the OTA focuser independently.

Think of the external focuser as a variable spacer, vary the external focuser to obtain the right backfocus and then focus with the knob and repeat. 



#9 jonnybravo0311

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Posted 22 September 2021 - 08:49 AM

In order for a reducer to work properly, it must be _after_ the focuser, such that the distance between the reducer and camera sensor remains constant. So, it would look something like OTA -> focuser -> reducer -> spacers if necessary -> sensor.

 

If your setup is OTA -> reducer -> focuser, that will never work for precisely the reasons you're stating: the distance from the reducer to the sensor is constantly changing.


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#10 RogeZ

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Posted 22 September 2021 - 09:43 AM

In order for a reducer to work properly, it must be _after_ the focuser, such that the distance between the reducer and camera sensor remains constant. So, it would look something like OTA -> focuser -> reducer -> spacers if necessary -> sensor.

 

If your setup is OTA -> reducer -> focuser, that will never work for precisely the reasons you're stating: the distance from the reducer to the sensor is constantly changing.

This is incorrect for EdgeHD. 



#11 jonnybravo0311

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Posted 22 September 2021 - 09:45 AM

This is incorrect for EdgeHD. 

Explain, please.

 

If you've got a focuser on the back (not using the knob to adjust the mirror), which is what the OP indicated, and you set it up as OTA -> reducer -> focuser, as you rack that focuser in and out, you are most assuredly changing the distance between the reducer and the sensor.



#12 RogeZ

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Posted 22 September 2021 - 10:54 AM

You are correct but remember there is a backfocus tolerance. Therefore, you are using the focuser as a variable spacer, setting to 146 total for your whole image train, then focusing with the knob to get both focus and the right spacing.
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#13 jonnybravo0311

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Posted 22 September 2021 - 11:05 AM

You are correct but remember there is a backfocus tolerance. Therefore, you are using the focuser as a variable spacer, setting to 146 total for your whole image train, then focusing with the knob to get both focus and the right spacing.

Sure, but that's not what OP's use case is. Quoted from the first post:

 

I have an Optec 2" TCF-Lynx I use for focusing.  The mirrors are locked and I do not use the focuser on the OTA.

Thus, the problem.

 

I definitely hear what you're saying and agree with you that you could do it as you suggest by using the Optec focuser as a variable spacer, then using the mirror to achieve focus.


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#14 dcm_guitar

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Posted 22 September 2021 - 03:56 PM

I just got off the phone with Jim at Optec (maser of my focuser).  Based on all Celestron documentation what works at f/10 should also work at f/7.  However, my experience has been different.

 

BUT.....

 

Jim did make a very good suggestion.  I can set up the Optec so that it is further back (increase back focus).  I'll lose a little bit of available focus range, but will effectively increase the back focus.  

 

So, that's the plan for tonight.



#15 RogeZ

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Posted 22 September 2021 - 04:08 PM

I just got off the phone with Jim at Optec (maser of my focuser).  Based on all Celestron documentation what works at f/10 should also work at f/7.  However, my experience has been different.

 

BUT.....

 

Jim did make a very good suggestion.  I can set up the Optec so that it is further back (increase back focus).  I'll lose a little bit of available focus range, but will effectively increase the back focus.  

 

So, that's the plan for tonight.

On a properly designed image train this is not an issue as you are running your focuser at 50% or so drawtube length or so. Also, the travel distance for focusing is small. 



#16 dcm_guitar

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Posted 29 September 2021 - 05:49 PM

Update:

 

After clouds had passed (it's California.  We have clouds?!?) I was able to run some tests.  I believe I have discovered the issue and the solution.  Here's the basic issue:

 

I definitely hear what you're saying and agree with you that you could do it as you suggest by using the Optec focuser as a variable spacer, then using the mirror to achieve focus.

I THOUGHT the Optec focuser was built to the same back space dimensions as the Celestron T-adaptor.  This allows the user to use the Optec focuser rather than the T-adaptor for the correct spacing when mounting the imaging train.

 

BUT......

 

The focuser is designed to provide 3.5" of back focus from front of the focuser to the mid-point of the focuser position.  Converting to millimeters, this is 89mm of back focus at the mid-point of the focuser.

 

The EdgeHD 9.25 requires 146mm of back focus.  55mm of this are taken up by my camera, EFW, OAG.  This leaves 91mm of back focus required of the focuser.  

 

You can see that the focuser at the mid-point of it's travel is 2mm too close which was causing my issues.  

 

The solution is to force the focuser back 2mm from it's mid-point and achieve focus at this point.  The focuser defaults to the mid-point every time you turn it on.  The focuser has 7,000 steps, so it defaults to position 3,500 every time it turns on.  I need to determine what position the focuser needs to be at to achieve the correct back space.

 

The focuser has 15.2mm of total travel broken into 7,000 steps.  This means there are 460.53 steps per millimeter on the focuser.  If I need the focuser to move back 2mm, I need to position the focuser to 460.53x2=921.06 steps further back from the mid-point of 3,500.  So, I need to have the focuser at position 4,421 for all of my imaging.

 

I tried this out last night and it was, of course, a BIG help.  The downside is that I believe I have now identified some tilt.  With this corrected back focus the corners of my image has nice stars EXCEPT for the top left corner.  Sigh......

 

The net result is that I need to add another process to my set up routine where I make sure the focuser is at position 4,421.  Then, I can unlock the mirrors, adjust the OTA mirror focuser until I get it in focus, and then lock down the mirrors.

 

I hope this helps anyone else using this setup.   


Edited by dcm_guitar, 29 September 2021 - 06:02 PM.


#17 RogeZ

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Posted 29 September 2021 - 09:18 PM

But you dont have to do that….

You can leave the focuser at 4421 and focus happily with the Optec while the mirror is forever locked in place. The EdgeHD backfocus has a tolerance….


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