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Looking for advice on flattening image

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#1 Capella19

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Posted 12 July 2020 - 08:23 PM

Hi, just shot my first image with the Explore Scientific 127mm CF combined with a Canon EOS Ra.  Took the image with a Starfield 1.0x Field Flattener as well as an Optolong L-Enhance filler.  I haven't had this problem in the past with my previous scope so asking for advice on what to do with the stars looking way off along the edges.  My setup to attach the camera to the scope is kind of a mess as well so I am wondering if that has anything to do with it?  

 

35 Light frames (5 minutes each)

60 Darks

Some bias

Some flats

Image is just from the DSS save, untouched. (I convert the CR3s to TIFF before stacking)

 

1.jpg

 

 

My camera adapter goes from the focuser: Filter>  Starfield 1.0x > 30mm Extension > M48 to M42 adapter > Canon Ra T thread

 

DSC_0052.JPG


Edited by Capella19, 12 July 2020 - 08:30 PM.


#2 imtl

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Posted 12 July 2020 - 08:40 PM

you need to increase the spacing. You see that the elongation is radially out.


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#3 james7ca

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Posted 12 July 2020 - 08:54 PM

Yes, the spacing looks to be incorrect. What is the backfocus requirement for the flattener (it's usually something like 55mm)?

 

If you are using a 30mm extension tube and then add in the M48-to-M42 adapter and then use a standard t-thread adapter on the Canon your distance must be at least 30mm + ? (M48 to M42) + 55mm ≈ 85mm.

 

I guess this depends upon the thickness of your Canon Ra T-thread adapter. However, most T-thread adapters add enough spacing to bring the system to 55mm and that's what I've shown in the above calculation (although I guess your Canon adapter could be different).


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#4 Capella19

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Posted 12 July 2020 - 09:07 PM

Yes, the spacing looks to be incorrect. What is the backfocus requirement for the flattener (it's usually something like 55mm)?

 

If you are using a 30mm extension tube and then add in the M48-to-M42 adapter and then use a standard t-thread adapter on the Canon your distance must be at least 30mm + ? (M48 to M42) + 55mm ≈ 85mm.

 

I guess this depends upon the thickness of your Canon Ra T-thread adapter. However, most T-thread adapters add enough spacing to bring the system to 55mm and that's what I've shown in the above calculation (although I guess your Canon adapter could be different).

Wasn't aware that spacing could do this.  I just measured it to be 73mm from the end of the field flattener to the end of the t-thread (I can only find thick t threads for the mirrorless system).  Starfield's website states that it uses the standard 55mm of back focus.  Would you say it has to do with it being about 20mm too long?  I could always get a shorter extension than the 30mm extension. 


Edited by Capella19, 12 July 2020 - 09:11 PM.


#5 james7ca

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Posted 12 July 2020 - 09:18 PM

If the flattener has standard T-threads then you should be able to screw it directly into the Canon Ra T-thread adapter, that should give you a spacing of 55mm. I suspect that the T-thread adapter is adding about 35mm to the spacing, so 35mm + 20mm (lens flange to sensor spacing for Canon RF lens mount) gives 55mm.



#6 imtl

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Posted 12 July 2020 - 09:19 PM

That is weird. Usually you get an image like that with too short spacing. But I guess that with the spacing so far off it might actually be some second order aberration.

 

In any case, you should just connect your flattener to the Tring. Its probably a 35mm width. Plus the 20mm of your camera edge to sensor that will be your 55mm.


Edited by imtl, 12 July 2020 - 09:21 PM.


#7 bobzeq25

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Posted 12 July 2020 - 09:23 PM

Wasn't aware that spacing could do this.  I just measured it to be 73mm from the end of the field flattener to the end of the t-thread (I can only find thick t threads for the mirrorless system).  Starfield's website states that it uses the standard 55mm of back focus.  Would you say it has to do with it being about 20mm too long?  I could always get a shorter extension than the 30mm extension. 

The spacing is between the camera end of the flattener (generally either an obvious flange or the last lens) and the camera sensor.  Your spacing (flattener to end of the t-threads is _way too long, you need to add the 55mm from the t-threads to the sensor.  So, it's about 128mm.


Edited by bobzeq25, 12 July 2020 - 09:24 PM.


#8 Capella19

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Posted 12 July 2020 - 09:37 PM

If the flattener has standard T-threads then you should be able to screw it directly into the Canon Ra T-thread adapter, that should give you a spacing of 55mm. I suspect that the T-thread adapter is adding about 35mm to the spacing, so 35mm + 20mm (lens flange to sensor spacing for Canon RF lens mount) gives 55mm.

 

 

That is weird. Usually you get an image like that with too short spacing. But I guess that with the spacing so far off it might actually be some second order aberration.

 

In any case, you should just connect your flattener to the Tring. Its probably a 35mm width. Plus the 20mm of your camera edge to sensor that will be your 55mm.

Thanks for the advice since I had no idea it would cause this.  and yes, the t thread measured at 35mm + the 7ish mm from the m42-m48 adapter.  I remember I tried it without the 30mm spacer but it was just barely out of focus, so I added it because it was the only spacer I had.  Maybe a 5 or 10mm spacer would be better?



#9 imtl

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Posted 12 July 2020 - 09:59 PM

Focusing and field flatness are two different things. You need the correct spacing between the flattener and the sensor. Then you focus with that fixed spacing by changing the distance between the flattener and the lens of the telescope.

 

So fix the spacing to 55mm and then just focus as usual.


Edited by imtl, 12 July 2020 - 10:07 PM.


#10 Capella19

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Posted 12 July 2020 - 10:08 PM

Focusing and field flatness are two different things. You need the correct spacing between the flattener and the sensor. Then you focus with that fixed spacing by changing the distance between the flattener and the lens.

Got it.  So I could try putting the extension ring on the other side of the flattener (between the scope and the flattener) and then thread the camera directly behind the flattener?



#11 imtl

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Posted 12 July 2020 - 10:38 PM

May I ask why do you need the extension at all? Can you fit the flattener into the focuser of the telescope?



#12 Capella19

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Posted 12 July 2020 - 10:47 PM

May I ask why do you need the extension at all? Can you fit the flattener into the focuser of the telescope?

I can fit the flattener, yeah.  The focuser uses a compression ring, so the flattener just slides in. The focuser just doesn't draw out far enough to focus the camera, even if I slide out the flattener a bit.


Edited by Capella19, 12 July 2020 - 10:48 PM.


#13 imtl

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Posted 12 July 2020 - 11:03 PM

Yes I guessed that. Just add the extention before the flattener then (between focuser and flattener just to be clear.)



#14 Capella19

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Posted 12 July 2020 - 11:05 PM

Yes I guessed that. Just add the extention before the flattener then (between focuser and flattener just to be clear.)

Thanks for the help.  If I don't get back on this forum post then all is working well.


Edited by Capella19, 12 July 2020 - 11:06 PM.


#15 imtl

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Posted 12 July 2020 - 11:12 PM

Hope to never see you again lol.gif... in this post  


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