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back focus problems pics attached

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11 replies to this topic

#1 wheelers4life

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Posted 14 November 2019 - 10:27 PM

ok. I have a celestron edge hd with .7x focal reducer for the edge hd. I have a zwo asi 294mc pro. I started out using the reducer, t ring (49.5mm , 11mm, 16.5mm,11mm spacers and the camera has 6.5mm to the sensor. acording to my calculation this is 104.5,, back focus, but my stars were way crazy looking. I ended up with the 11mm and the t ring this should calulate to 67mm of back focus witch is the picture below. This seems to give me the best looking image and plate solved worked in sharpcap.  what do you think of the stars. or am I calculating this wrong. The pic was taken with sharp cap. at 4.2 second exposure, gain at 350, bining is 3, camera cooled to -15, total exposure is 63 seconds with 30 dark frames. I just do not understand why at the proper back focus the stars were oblong. I attached a picture of my collimation. I think it is pretty good it took me like 15min to get this far after installing bobs knobs.

Any help would be great

Attached Thumbnails

  • dumbell1114.jpg
  • collimin.jpg


#2 wheelers4life

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Posted 15 November 2019 - 12:44 AM

Now this is orion this time I added some spacers to equal 114mm and seems to have decent stars. am I getting better or worse. It is hard for me to tell. 

this was shot at 

1400ms 90 frames stacked in sharpcap

30 dark frames

bining 3

 

Attached Thumbnails

  • orion1114.jpg


#3 jrcrilly

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Posted 15 November 2019 - 03:48 AM

To evaluate these things you need to determine FWHM and curvature. CCDInspector will be your friend.



#4 nic35

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Posted 15 November 2019 - 08:37 AM

what mount ? Why are you doing bin 3? In some color spaces (16 raw for one) it does nothing except make the file smaller and could lead to blurring, as it averages intensity over a 3x3 pixel matrix.

J

#5 Rickster

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Posted 15 November 2019 - 10:13 AM

The Orion shot looks good to me. The four brightest trapezium stars are well defined.  No doubt you will be able to make incremental small improvements over time, as we all do.  I think you have the basics down and are entering the polishing stage.



#6 wheelers4life

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Posted 15 November 2019 - 11:44 AM

I was using bin 3 because it allowed me to see more stars. Thanks for the help

#7 wheelers4life

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Posted 15 November 2019 - 11:45 AM

My oag will be delivered today. So I will add that and finish polishing

#8 wheelers4life

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Posted 15 November 2019 - 01:21 PM

I do not see me spending tha much money on the ccd inspector, I will have to use my eyes for this, I will add and take away spacers untill I like the image. I do not even know what FWHM is. but thank you for your help. This time a year money is getting tight.

Thanks



#9 Clouzot

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Posted 15 November 2019 - 03:26 PM

You could also use ASTAP, which includes a version of CCD Inspector, and is free

 

In the M27 it shows a fairly large amount of tilt.


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#10 wheelers4life

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Posted 15 November 2019 - 04:25 PM

I will give that a try tonight. What is tilt is that from adjusting the screws to much to one side or is this the primary mirror.

Thanks



#11 Clouzot

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Posted 15 November 2019 - 04:59 PM

Probably the primary after refocusing. You should always focus and collimate by focusing in or out, but not both

#12 nic35

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Posted 15 November 2019 - 05:16 PM

I was using bin 3 because it allowed me to see more stars. Thanks for the help

So, you must have been using an 8 bit color space.  As I understand it, when using 16 bit color space the pixels are averaged, while when using the 8 bit space, they are  summed. 

 

Increasing the gain will have the same effect as summing in low read noise cameras like the 294.

 

j




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