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Help perfecting my imaging chain, spacers and field flatness help

astrophotography equipment imaging optics
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#1 c172jeff

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Posted 30 September 2020 - 09:05 PM

Hi,

  I don't believe my imaging setup is all that bad, but I do wish it to be best if I can by making a spacer adjustment

  See this picture I took and plate solved....

 

 

  http://nova.astromet...58576#annotated

 

 I have a 9.25 SCT and a 6.3 focal reducer.  I computed a focal length of  1481. 

 Regarding the above picture data sizes/FOV, and the formula 

 

Focal Length = 57.3 / (Field of View in degrees / sensor size in mm)

 

 found here

 

https://www.astropix...nd-focal-ratio/

 

 

I compute a focal length of 1377 based on my solved image.  I am using a ZWO 1600 MM Pro camera. 

 

  The actual focal length is more than 100mm shorter than what I computed it should be with the reducer.  What can cause this?  Is my spacing correct.  I did match it to what was required by making a custom adapter that puts 105 mm between the field flattener and the image sensor.

 

 What are the implications of not having the exact spacing between the field flattener and the image sensor?  Is there a way that I can plate solve the image and determine if spacing is needed or a reduction must be made? I want the stars to be focused as best they can on the flattest field of view.

 

Thanks,

Jeff

 

 

 

 

 

 



#2 TxStars

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Posted 30 September 2020 - 09:40 PM

I get this when going to your link; images > permission denied

So I can't view the image..

 

With a SCT the focal length can change a lot when you adjust the mirror to focus..(and varies from scope to scope)

Many reducers are designed to have a fixed spacing (for best image) with a certain optical system.

The actual reduction will vary from one unit ..

 

**Scope dependent**

If you have stars that are more or less round across the entire frame then you have the spacing where it should be..

A slight movement longer or shorter ( +/ - 0.001" ) should not change this with systems (slower than F/2)


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#3 c172jeff

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Posted 01 October 2020 - 07:56 AM

Thank you TxStars.  I have attached the image below.  I thought the original was viewable.

 

Jeff

 

fireworks


#4 Stelios

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Posted 01 October 2020 - 07:32 PM

 What are the implications of not having the exact spacing between the field flattener and the image sensor?  Is there a way that I can plate solve the image and determine if spacing is needed or a reduction must be made? I want the stars to be focused as best they can on the flattest field of view.

 

What will happen is that your edge stars will be distorted. See this post

 

It's quite possible that the real spacing is 105mm +/- (a few mm). Don't hesitate to adjust as needed. A variable spacer like the ones from Baader is better than a fixed length custom spacer *if* adjustments are needed. 



#5 TxStars

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Posted 03 October 2020 - 10:22 AM

Looking at the image,, there is some distortion at the outer edges..roughly 80% and further out..

Increasing or decreasing the spacing may or may not help with this.

The only way to know is to take a single frame and examine the stars at different points of adjustment

You may find that there is no way to get the entire field to look the same and will have to compromise as to how much and where the stars start to look off.

Easiest is to let the outer 90% look bad and then crop the image



#6 c172jeff

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Posted 03 October 2020 - 08:25 PM

In my imaging chain, I include a 1mm spacer to get to 105mm.  I think it sounds best to get a variable spacer and vary it after taking test pictures.  If I add a variable spacer, I can take out the 1mm spacer and another spacer other and possibly improve the pics.  I don't mind cropping the edges of the picture.  I just didn't want to leave any FWHM reduction on the table on account of a mm or 2 change.

Thanks all,

Jeff




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