Coma or "lost in collimation"

[AstroRookie]

Hello,

after I wrecked my Celestron C8 (see Goodbye Celestron C8 ...), I bought a Orion 8 F/3.9. Because at the moment it is financially the only option, and it looks to me that this is for the time being the best value for money I can get. I also bought the Fairpoint collimeter set and the Baader 2" Coma Corrector MPCC Mark III. Delivery was a bit problematic, as according to the delivery company's tracking data, it was sent to the wrong address. When the box arrived, it looked like it suffered already a bit. First thing I did was reading all sorts of info on collimation, and I "collimated" for the first time a Newtonian (to be honest, I thought collimating a C8 was a pain, but this is nothing compared to what I'm experiencing now). Attached you'll find a test picture I took from M81 and M82. At first I thought the "comet" trails were due to poor collimation, and the following day I started the whole collimation procedure again. I suspect I have now made thing worse by going too far, and also, that these "comet" trails are not collimation errors but coma aberration.  This is the setup:

• Canon eos 500D
• T-ring adapter (the one I used for my C8)
• Baader 2" coma corrector

My questions:

• are the comet trails coma or a collimation problem (sorry if this is a stupid question)
• if it is a coma problem, can it be that I have to use a different T-ring adapter (like this: ultra short adapter)
• or ... and I'm just realizing this while typing, I "inverted" the coma corrector (I suspect it's this, and I know this is utterly stupid, but I'll post it any way,  you never know if somebody may benefit from it)

Kind regards,

AstroRookie

[jonathan]

Have you tried without the coma corrector? 

[michael8554]

I read that the OTA requires a CC for visual and imaging because of Coma.

I don't believe it's possible to accidently mount the MPCC "inverted", the male thread goes into the T-Ring, the other end is female.

Have you removed the thin threaded "Stop Ring" from the T-Ring end of the MPCC, that is for visual use ?

Could be the MPCC is not a good optical match for the scope, perhaps a dedicated Orion CC is required ?

Michael

[AstroRookie]

10 hours ago, michael8554 said:

I read that the OTA requires a CC for visual and imaging because of Coma.

I don't believe it's possible to accidently mount the MPCC "inverted", the male thread goes into the T-Ring, the other end is female.

Have you removed the thin threaded "Stop Ring" from the T-Ring end of the MPCC, that is for visual use ?

Could be the MPCC is not a good optical match for the scope, perhaps a dedicated Orion CC is required ?

Michael

Hello Michael,

thanks for your comment but you underestimate my stupidity 😉 Attached pictures of the t-ring and the baader cc.The first one shows how I assembled it now (I did remove the caps, there are limits ...), but did not have the chance yet to test it. The second one shows how I assembled it wrongly (you see the marks of the screws ). Either way I assemble it, no threads are used - just 3 little screws to keep it in place. Am I using the wrong t-ring?

I did not use the stop ring, and I've found others on astrobin using this combination Orion 8 and baader cc.

Kind regards,

AstroRookie

Edited September 16, 2020 by AstroRookie

[michael8554]

OMG !

The T-Adapter has two parts:

1. The outer part with the 3 tiny circumferential screws.

2. The inner part held by the 3 tiny screws, marked "for CANON (EOS)" in this case, but you buy the complete assembly for your brand of camera, Nikon, Canon etc. 

This part is unique to each camera, but always ( ? ) has a Female M42 x 0.75mm thread on one side 

Your MPCC screws into this.

The other side has the camera attachment, in this case a Canon EF-S Bayonet.

 

Edited September 16, 2020 by michael8554