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Tilt or Backfocus

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#1 flyingg103

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Posted 09 November 2019 - 03:37 PM

I have an ASI1600mm Pro with ed80 scope that appears to have curvature but not sure if this is tilt or backfocus. I measured the backfocus and i have 56MM  from the back of the flattener to the sensor. Not sure if the 56 is possibly too far or not enough distance to the sensor.

Attached Thumbnails

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  • top left.JPG
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  • bottom left.JPG

Edited by flyingg103, 09 November 2019 - 03:46 PM.


#2 flyingg103

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Posted 09 November 2019 - 03:46 PM

could not attach all images at the same time. Here is the zoomed shot of the bottom right of the image, and the image train for the scope.

 

Measurement are:

 

OAG-           15.9mm

Spacer -       11.4mm

T2-T2 adap- 2.2mm

FW-              20mm

camera sensor from front of camera 6.5mm

 

total of 56mm backfocus 

Attached Thumbnails

  • bottom right.JPG
  • image train.JPG


#3 PirateMike

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Posted 09 November 2019 - 04:02 PM

I would recommend that you spend a night to figure out this issue.

 

I would take some short exposures of a "star filled" area of the sky and process them so that the background is a very dark gray, almost black. No need to stack, just single frames.

 

Now look at the stars and see if they produce a pattern. If there is a circular pattern then the issue is the spacing.

 

 

Spacing.jpg

 

 

If all the stars are pointing in the same direction then the problem is most likely tilt.

 

 

Miguel   8-)

 

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#4 bugbit

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Posted 10 November 2019 - 02:54 AM

I ran your pic via ccdi and the tilt numbers were 0 while your curvature was high so Back focus is the issue imho.


Edited by bugbit, 10 November 2019 - 03:06 AM.

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#5 flyingg103

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Posted 10 November 2019 - 09:36 AM

I ran your pic via ccdi and the tilt numbers were 0 while your curvature was high so Back focus is the issue imho.

 

I would recommend that you spend a night to figure out this issue.

 

I would take some short exposures of a "star filled" area of the sky and process them so that the background is a very dark gray, almost black. No need to stack, just single frames.

 

Now look at the stars and see if they produce a pattern. If there is a circular pattern then the issue is the spacing.

 

 

attachicon.gif Spacing.jpg

 

 

If all the stars are pointing in the same direction then the problem is most likely tilt.

 

 

Miguel   8-)

 

.

Thank you, let me give this a try and see what the images show. 


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#6 PirateMike

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Posted 10 November 2019 - 09:42 AM

I ran your pic via ccdi and the tilt numbers were 0 while your curvature was high so Back focus is the issue imho.

Good move. bow.gif

 

 

Miguel   8-)

 

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#7 flyingg103

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Posted 10 November 2019 - 09:43 AM

I ran your pic via ccdi and the tilt numbers were 0 while your curvature was high so Back focus is the issue imho.

Thank you for validating this, based on the graph that PirateMike sent i think the BF is too close as the images get tend to show stars elongated pointing to the corners of the image. What is typically the amount increments you should try with BF?


Edited by flyingg103, 10 November 2019 - 09:44 AM.

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#8 james7ca

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Posted 10 November 2019 - 01:16 PM

You may also need to add about 1mm to the required spacing to account for your filters. Thus, if the recommended backfocus is 55mm you'd want something close to 55mm + 1mm or 56mm. You probably also can't depend upon the camera manufacturer's stated back focus. In fact, I think they only claim an accuracy of +/- 1mm, so just with these two factors you may have up to +/- 2mm of error. Lastly, have you checked your spacing using a micrometer? Sometimes just adding up the manufacturer's claimed thicknesses doesn't produce an accurate result.

 

Best case would be that you are still too short since you can easily add spacers in 1mm increments. In fact, Baader makes some Delrin spacers that go down to about 0.6mm.


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#9 PirateMike

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Posted 10 November 2019 - 04:21 PM

Setting the flattener can always be a tricky thing. It's kind of a "trial and error" type of situation. You may have to make adjustments a few or more times, that's why I suggest that you plan a night for the process... it could take some time.

 

 

Best case would be that you are still too short since you can easily add spacers in 1mm increments. In fact, Baader makes some Delrin spacers that go down to about 0.6mm.

Those Delrin spacers work very well but they are only available in 42mm, I wish they came in 48mm too. But they are very affordable.

 

And if you're really into spending money, TS sells spacers made of Alunimum or Steel.

 

 

Miguel   8-)

 

.

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#10 flyingg103

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Posted 11 November 2019 - 02:50 PM

Setting the flattener can always be a tricky thing. It's kind of a "trial and error" type of situation. You may have to make adjustments a few or more times, that's why I suggest that you plan a night for the process... it could take some time.

 

 

Those Delrin spacers work very well but they are only available in 42mm, I wish they came in 48mm too. But they are very affordable.

 

And if you're really into spending money, TS sells spacers made of Alunimum or Steel.

 

 

Miguel   8-)

 

.

.

I was able to add just over 1mm to the image train and will see how this performs next less cloudy night. I think the Delrin spacers is a good option, so will order and try the new BF distance to see how this performs


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#11 flyingg103

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Posted 11 November 2019 - 02:53 PM

You may also need to add about 1mm to the required spacing to account for your filters. Thus, if the recommended backfocus is 55mm you'd want something close to 55mm + 1mm or 56mm. You probably also can't depend upon the camera manufacturer's stated back focus. In fact, I think they only claim an accuracy of +/- 1mm, so just with these two factors you may have up to +/- 2mm of error. Lastly, have you checked your spacing using a micrometer? Sometimes just adding up the manufacturer's claimed thicknesses doesn't produce an accurate result.

 

Best case would be that you are still too short since you can easily add spacers in 1mm increments. In fact, Baader makes some Delrin spacers that go down to about 0.6mm.

I was using a micrometer to validate the backfocus which is 56mm currently. Since these images were taken i have added just over 1mm and will see how this performs. Will update once I can get back out which may be Tuesday night


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#12 PirateMike

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Posted 11 November 2019 - 03:31 PM

I was using a micrometer to validate the backfocus which is 56mm currently. Since these images were taken i have added just over 1mm and will see how this performs. Will update once I can get back out which may be Tuesday night

It could be a little bit of work, but keep going until you get it as good as you can. I have heard of people even taking milk jugs and cutting them in to spacers. You may not to want to go that drastic.

 

Rember...

 

Short exposures (20 sec or less), this will reduce the effects of guiding and show you how your optics are performing.

 

Single subs no stacking, stacking can make your stars appear rounder even with a non-flat imaging train.

 

Somewhat light sky, will show you more of the star than your usual final images.

 

 

 

Miguel   8-)


Edited by PirateMike, 11 November 2019 - 03:56 PM.

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#13 flyingg103

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Posted 14 November 2019 - 10:33 PM

I was using a micrometer to validate the backfocus which is 56mm currently. Since these images were taken i have added just over 1mm and will see how this performs. Will update once I can get back out which may be Tuesday night

 

 

I was able to run some more tests tonight with up to 1.4mm additional spacing which takes my measured backfocus to 57.4mm. I can say the stars in corners look MUCH better but still some elongation in stars. Still waiting on the delrin spacers to come in which would allow me to lengthen the BF further. One question for those who know what is a good recommendation on getting spacers in custom lengths. The current spacer is between the filter wheel and OAG is 11 mm ( measured) and i would imaging i would like to get a 13mm to replace that one. I can use the delrin spacers to make micro adjustments if needed but think a solid spacer is preferred over 2-3MM of spacers

Attached Thumbnails

  • image train.JPG

Edited by flyingg103, 14 November 2019 - 10:36 PM.

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#14 PirateMike

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Posted 15 November 2019 - 10:13 AM

One question for those who know what is a good recommendation on getting spacers in custom lengths.

Precise Parts makes custom spacers. They are a little costly (well... they are custom) but they make quality parts. 

 

I would suggest that you order any parts from them after you have the spacing perfect, and remember to order the length of the spacers, not the total back focus distance.

https://www.precisep...in/adapter.html

 

 

As I mentioned above, TS makes metal spacers. They make 1.0mm, 0.5mm and 0.3mm

 

Those Delrin spacers are pretty good and probably all you will need for 42mm diameter spacers.

 

Miguel   8-)

 

.



#15 j.mobile

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Posted 15 November 2019 - 12:38 PM

Agena has aluminum spacers in various sizes:

https://agenaastro.c...12mm-extension/
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#16 elmiko

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Posted 15 November 2019 - 04:08 PM

Agena Astro just got a 8 mm from them. Works fine and less expensive than most.



#17 flyingg103

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Posted 17 November 2019 - 01:54 PM

I finally had a chance to get back out and try some different backfocus settings and but still not able to get start to improve their shapes.

 

I had tried to 57.4mm backfocus but still see the same pattern the bottom left hand side of the stars are elongated pointing to the bottom left corner which would indicate not enough backfocus. I tried 59mm and still had the same issue. I was did not have enough threads in the flattener to add more delrin spacers so tried something drastic which was to add a 5mm spacer to try 60mm in total backfocus which did not produce much change.

 

I then went back to my original backfocus 56mm and took some shots to get myself back to a starting place again.I downloaded the trial version of CCD inspector and ran pics through the curvature and 3d plot analysis.

 

Looks like the direction of curvature is in the direction where i have the elongation of stars in the bottom left hand side of the image( of course with a completely untrained eye using CCDI ).

 

Any thoughts on the amount of curvature - indicated as 18.2% and tilt of 11% roughly in the direction of elongated star? Is this a small amount or is this a substantial value

 

I will try to run some more tests again tonight with different BF amounts to see what happens.

Attached Thumbnails

  • 56mm curvature.JPG
  • 56mm 3d side.JPG
  • 56mm 3d.JPG
  • bottom left hand crop.JPG

Edited by flyingg103, 17 November 2019 - 02:00 PM.



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