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Comet-like Trail on Menkib

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#1 Domer

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Posted 26 February 2021 - 12:21 AM

I was experimenting with the use of the Optolong L-eXtreme filter in the heavily light polluted area of San Diego with the California Nebula as my target.  After focusing and getting the auto-guiding fine-tuned to less than 1 arc-sec, I took some test shots.  There's a star near the nebula called Menkib and it is showing up with a short comet-like trail in my images.  Nearly all the other stars are all crisp and round but if you zoom in on some of the brighter stars in the upper-right corner, you can see very faint trails.  All the trails appear to point toward the center of the image.  What could be causing this?

 

Equipment:

SkyWatcher Esprit 100

Starizona APEX focal reducer (0.65)

Optolong L-eXtreme dual band filter

ZWO ASI2600MC-Pro imaging camera

ZWO ASI290MM guide camera on an OAG

Pegasus FocusCube 2 auto-focuser

 

The back focus for the Starizona focal reducer is 58 mm and the user manual conveniently has instructions for my exact setup, which I followed precisely.  The only difference is that I have the filter installed which is only 1.85 mm thick which should only add 1.85/3 = 0.6mm to the back focus.  So it doesn't seem like back focus is the problem, but maybe I'm missing something.

 

The attached image is a single 60-sec exposure - no calibration, no stacking, just debayered.

Attached Thumbnails

  • Back Focus.JPG
  • Comet Menkib (small).jpg

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#2 Jim R

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Posted 26 February 2021 - 01:29 AM

Could it be an internal reflection flare from the filter?  I had something similar once from L Enhance filter.



#3 Domer

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Posted 26 February 2021 - 11:11 AM

Could it be an internal reflection flare from the filter?  I had something similar once from L Enhance filter.

How would I find out?

 

After digging into this a bit further, I'm starting to think it really is a back focus issue.  According to the attached illustration, the symptoms are consistent with the back focus being too short (sensor too close).  I just don't understand how since I followed the instructions in the APEX manual exactly and even double-checked it with calipers.  I guess it wouldn't hurt to add small increments of spacers and see if the problem goes away. 

Attached Thumbnails

  • Back Focus Errors.jpg


#4 Tapio

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Posted 26 February 2021 - 11:19 AM

I'm not sure if this is a back focus issue.

It looks like that bright stars is the only one with issue and it's pretty near center.

 

But I'm not sure what else could it be - frost or something else in optics surface ?



#5 Domer

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Posted 26 February 2021 - 11:35 AM

I'm not sure if this is a back focus issue.

It looks like that bright stars is the only one with issue and it's pretty near center.

 

But I'm not sure what else could it be - frost or something else in optics surface ?

If you zoom in on a couple of the bright stars in the upper-right corner, you'll see the same phenomenon, albeit a lot fainter.  All the trails are fan-shaped and point toward the center of the image.

 

And someone pointed out to me that in the Apex FR user guide, there is an illustration indicating that these fan-shaped tails are a symptom of the back focus being too long.  That seems to conflict with the illustration I posted earlier, but it's easier to understand how the back focus can bee too long (maybe some spacers aren't cranked down all the way) than too short.  So I'll play with it some more tonight.

Attached Thumbnails

  • Zoomed-in Trails.jpg
  • APEX.JPG


#6 Domer

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Posted 27 February 2021 - 12:29 AM

I experimented with 5 other back focus lengths tonight. What I found was that the trails on the bright stars diminished by *reducing* the back focus, the opposite of what the APEX manual suggests.  At 51mm, the trails were gone but all the other stars started to elongate as shown in that image I posted above from OPT, indicating that the back focus is too short (which it is). Also, at 51mm of back focus, I can no longer get my guide scope to focus - the camera is all the way into the OAG and the focuser is at the minimum setting, but the starts are still out of focus.

 

I uploaded 6 images here if anyone wants to see what I'm talking about. The file names indicate what the back focus is as well as the widths of each component used to achieve that back focus.  If you download them and flip through them quickly on your PC, you'll see how the bright stars improve with the shorter back focus distances while the dimmer stars simultaneously get worse.

 

So now I'm at a loss as to what to do.



#7 Domer

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Posted 28 February 2021 - 11:21 AM

I'm wondering if this is related to the Optolong L-eXtreme filter I'm using.  I'm going to remove it and (weather permitting) will try again tonight and see what happens.  I'm going to be very disappointed if the filter is causing this.



#8 Domer

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Posted 28 February 2021 - 10:23 PM

I removed the filter and the problem is gone.  I'm going to start a new thread about this issue and the L-eXtreme filter.




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