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Coma Corrector focus on 8" Newtonian

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#1 Cooperbrad

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Posted 16 September 2020 - 11:36 PM

I get great pictures of the inside of my telescope.....The image shows the secondary mirror (taking up about 40% of the picture) and the spider vangs going to the edge of the inner tube of the scope.

 

I have a Skywatcher 8" Quattro f/4 Newtonian and use a Canon Rebel T1i DSLR. I have taken 100's of photos since purchasing the scope a few months back. So I made the plunge and bought the Skywatcher Coma Corrector specifically made for this series of telescope. The coma corrector says it requires 55mm of back focus so I set up as follows:

 

T-adapter on one end of the coma corrector, inserted into my focuser and placed flat against the focuser tube edge. Other end of the coma corrector I have a 10mm spacer, attached with a Canon adapter to my Canon T1i.  Data I found says the Canon has a 44mm (or 44.5mm) sensor to flange distance, so add these together and it is almost equal to the 55mm back focus I read in the Coma corrector spec's. I can mover the focuser all in, mid-way, all out and I basically get the same picture described above.

 

Without the coma corrector, my normal set up is a T-adapter connected to the Canon adapter, connected to the camera, no spacers.  So in my mind in this case the 'backfocus' it is just the flange to sensor distance of 44mm which is built into the camera. The focuser is generally half-way of its total range of movement when I take all of my exposures.

 

Any help is much appreciated. I don't have any more spacers at this point to try a larger back focus value so I thought to ask first.

 

Thanks



#2 Jim Waters

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Posted 17 September 2020 - 12:14 AM

The Sky-Watcher Quattro Coma Corrector is suppose to be very good and requires 55mm of back focus to the sensor.

 

https://www.highpoin...wtonians-s20204


Edited by Jim Waters, 17 September 2020 - 12:14 AM.


#3 Cooperbrad

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Posted 17 September 2020 - 02:27 AM

Yes, I think I have 55mm if I am doing this correctly as I described....so I'm not sure what I am doing wrong?



#4 Avgvstvs

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Posted 17 September 2020 - 03:06 AM

You are not alone, I have had assorted coma correctors and I find they are all a pain to set up



#5 Alen K

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Posted 17 September 2020 - 08:43 AM

Exactly which coma corrector is it? It might not make a difference, actually, since most of the coma correctors I have seen, and certainly the two I know of from SkyWatcher (this one and this one), drop into the focus tube like an eyepiece. So when you say you are using a T-adapter on the end, that doesn't sound right. But perhaps I am misunderstanding you. Can you provide a picture of your setup? 


Edited by Alen K, 17 September 2020 - 08:49 AM.


#6 kathyastro

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Posted 17 September 2020 - 09:40 AM

All DSLRs, when they have their matching T-ring (what you are calling the Canon adapter) are 55mm deep.  That is why coma correctors commonly are designed for the 55mm distance.  You don't need to worry about that distance: it is already 55mm.  You should not use any other spacers between the coma corrector and the camera's T-ring. 


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#7 Alen K

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Posted 17 September 2020 - 10:09 AM

All DSLRs, when they have their matching T-ring (what you are calling the Canon adapter) are 55mm deep.  That is why coma correctors commonly are designed for the 55mm distance.  You don't need to worry about that distance: it is already 55mm.  You should not use any other spacers between the coma corrector and the camera's T-ring. 

Good point! I missed that. The 10mm spacer must go. However, it doesn't explain his problem of being majorly out of focus.



#8 Cooperbrad

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Posted 17 September 2020 - 12:38 PM

 

Here are my pictures. It is the Sky-Watcher Quattro Coma Corrector as referenced above in the link.

From left to right is the Adapter that goes into the focus tube, then the coma corrector, then the 10mm spacer, then the Canon adapter that goes to my camera.
Also included picture I got when connected to my scope.....you can see the secondary and vangs.

 

Appreciate any help

coma corrector
Coma 03
Coma 02

Edited by Cooperbrad, 17 September 2020 - 12:40 PM.


#9 Alen K

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Posted 17 September 2020 - 12:46 PM

Yeah, that's what I thought. Omit the T-ring on the end of the corrector and insert the corrector into the focuser directly instead. That should correct the focus problem. Omit the 10mm spacer on the camera end, as Kathy said, or else your stars won't be good away from center. 



#10 Cooperbrad

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Posted 17 September 2020 - 12:50 PM

OK, so how far do I insert the coma corrector. I'm pretty certain it will stick out a fair amount into the OTA itself...that will be OK? Just seems odds to my newbie self.



#11 kathyastro

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Posted 17 September 2020 - 02:15 PM

OK, so how far do I insert the coma corrector. I'm pretty certain it will stick out a fair amount into the OTA itself...that will be OK? Just seems odds to my newbie self.

Just play with it until you find a spot where it is easy to reach focus.  Slide it in and out with the focuser at its midpoint. until the view is sharp.  Then mark the spot with a parfocal ring, or just a bit of tape.



#12 Cooperbrad

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Posted 17 September 2020 - 06:24 PM

Excellent and thanks all!




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