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Help with Coma Corrector Backspacing

Equipment Reflectors DSLR Coma
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EvanSpiesTheHeavens avatar
EvanSpiesTheHeavens avatar
Hello,

     I recently finally purchased a coma corrector for my 6" f/5 Newtonian and I am having some serious difficulty getting the backspacing correct.
Everywhere seems to say 55mm is the most common, but I have seen in a couple places that 75mm might actually be the number needed for the corrector I got? Idk. A big part of my problem is I do not know how deep the sensor of my DSLR is. It is a Canon Rebel T6 and I cannot find the information anywhere. If anyone knows what it is or where I can find it, I would appreciate that. 

     I made an attempt to find the correct spacing by taking images with a range of different spacing and then trying to figure out which one looks the closest, but for some reason I just could not tell much from that. Also, another oddity I discovered. The coma pattern seems to be off-centered. The radiant point from which the stars elongate seems to be on one side of the frame. I do not think I have noticed that before, so I do not know if it is caused by a flaw in the corrector or if my secondary mirror is off or what else it could be. If anyone knows what is up with that, please let me know.

     I would appreciate any help or suggestions. I knew this would be a big pain and it is why I held off for several months before finally deciding to get a coma corrector. It does not do me any good if it is making the coma worse instead of better.

Thank you,

EvanSpiesTheHeavens
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Christian Großmann avatar
Christian Großmann avatar
The backspacing of the Canon EF mount is 44mm. This is the distance from the mount ring where the lenses are usually attached to the sensor, which all Canon cameras with EF mount have in common. Typical T2 camera adapters for the EF mount have 11mm so with a corrector with 55mm backspacing you shouldn't need additional rings. Just screw everything together and you are ready to go.

If your corrector needs 75mm, you will need an additional distance ring with 20mm. Thats hard to say, because you did not specify what corrector you use exactly. But with a Newton corrector, 55mm should be very commen.

It also seems, that your collimation is quite off. If everything is shifted, I would guess the position of the secondary is off. With my cheapish Newtons, I never was able to get everything exactly centered. It seems that the mounting hole for the focuser is not in the optimal spot. So I have to live with some offset. However, It should be at least near the center of the sensor.

Also check, if your focuser is attached well to the tube of the scope. I had some loose screws on one of my Newts after purchasing it. This created a bit of tilt which may also cause off centered images and more.

CS
Christian
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andrea tasselli avatar
andrea tasselli avatar
Hello,

     I recently finally purchased a coma corrector for my 6" f/5 Newtonian and I am having some serious difficulty getting the backspacing correct.
Everywhere seems to say 55mm is the most common, but I have seen in a couple places that 75mm might actually be the number needed for the corrector I got? Idk. A big part of my problem is I do not know how deep the sensor of my DSLR is. It is a Canon Rebel T6 and I cannot find the information anywhere. If anyone knows what it is or where I can find it, I would appreciate that. 

     I made an attempt to find the correct spacing by taking images with a range of different spacing and then trying to figure out which one looks the closest, but for some reason I just could not tell much from that. Also, another oddity I discovered. The coma pattern seems to be off-centered. The radiant point from which the stars elongate seems to be on one side of the frame. I do not think I have noticed that before, so I do not know if it is caused by a flaw in the corrector or if my secondary mirror is off or what else it could be. If anyone knows what is up with that, please let me know.

     I would appreciate any help or suggestions. I knew this would be a big pain and it is why I held off for several months before finally deciding to get a coma corrector. It does not do me any good if it is making the coma worse instead of better.

Thank you,

EvanSpiesTheHeavens

You failed to mention which CC you've got. And as others have stated, the backspacing of any EF Canon is 44 mm and a T2 adapter is 11mm +/- 0.5mm. As for the off-centered nature of the coma it is most likely due to collimation rather than anything else.
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EvanSpiesTheHeavens avatar
EvanSpiesTheHeavens avatar
Christian Großmann:
The backspacing of the Canon EF mount is 44mm. This is the distance from the mount ring where the lenses are usually attached to the sensor, which all Canon cameras with EF mount have in common. Typical T2 camera adapters for the EF mount have 11mm so with a corrector with 55mm backspacing you shouldn't need additional rings. Just screw everything together and you are ready to go.

If your corrector needs 75mm, you will need an additional distance ring with 20mm. Thats hard to say, because you did not specify what corrector you use exactly. But with a Newton corrector, 55mm should be very commen.

It also seems, that your collimation is quite off. If everything is shifted, I would guess the position of the secondary is off. With my cheapish Newtons, I never was able to get everything exactly centered. It seems that the mounting hole for the focuser is not in the optimal spot. So I have to live with some offset. However, It should be at least near the center of the sensor.

Also check, if your focuser is attached well to the tube of the scope. I had some loose screws on one of my Newts after purchasing it. This created a bit of tilt which may also cause off centered images and more.

CS
Christian

andrea tasselli:
You failed to mention which CC you've got. And as others have stated, the backspacing of any EF Canon is 44 mm and a T2 adapter is 11mm +/- 0.5mm. As for the off-centered nature of the coma it is most likely due to collimation rather than anything else.

The CC is the Apertura one. I think it is basically the same as the GSO one. I know its is kinda cheapy but it's what I could afford.

--Evan
Christian Großmann avatar
Christian Großmann avatar
The manual of this corrector specifies the back focus distance to 55mm.

https://www.highpointscientific.com/amfile/file/download/file/1827/product/17627/

This is the only one I could find with a quick search. But more than 55mm for a Newton coma corrector would stress the focuser. So I bet yours is 55mm, too. So with a standard T-Adapter you should be close enough. I would assume, all other strange things come from collimation issues.

I hope you will get your stuff working fast.

CS
Christian

Edit: Oh sorry! I found this page and it has indeed 75mm.

https://www.teleskop-express.de/en/astrophotography-and-photography-15/imaging-correctors-for-telescopes-138/gso-2-inch-coma-corrector-for-newtonian-telescopes-for-observation-and-astrophotography-17606
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