Astro Image with tear drop stars around out side of image

[Hagar333 (Peter)]

Hi all,
I have been trying to fine tune my PHD guided Skywatcher 200mm f4 Astrograph with baader mark III coma corrector to get round stars but I still have small fat tear drop stars in corners of image. My question is this the best I can expect from my Newtonian and the baader corrector or do I have to fine tune the distance of this lens to my DSLR Nikon D3 full frame sensor with spacers?
I have intentionally loaded a bigger image any help appreciated
regards Peter

http://digitalmacro.com/Files/lagoon1.jpg

[Atmos (Colin)]

It still looks like coma so my only guess (being a refractor guy) would be to play around with spacing.

[glend (Glen)]

The Baader MPCC is designed for Canon sensor spacing, which i believe is 55mm from the shoulder of the MPCC housing (not the threads). It works perfectly on my Canon without any spacing changes required. It screws into a Canon bayonet t-thread adaptor which is about 9mm thick an that is part of the 55mm. What sort of bayonet is used with your camera and is it of similiar thickness?

[billdan (Bill)]

Hi Peter,
Welcome to Ice in Space, are you confident you have the correct 55mm spacing between the MPCC and the CCD chip?

If so then the next step is to check collimation, coma correctors are a progressive corrector. They do no correction at the centre of the frame and gradually increase correction until max correction at the edge of the frame.

So the CC expects perfect round stars at the centre where no correction is applied, but if collimation is out then the round stars may be off centre. This causes the CC to over correct one half of the frame and under correct the other half of the frame.

So if you have some good collimation tools I would get that right first and you should be OK.

Cheers
Bill

[Atmos (Colin)]

Nikon and Canon DSLR cameras have different back focus requirements. If the D3 is directly connected without any account for Canon spacing, it may be a few mm too close or too far.

It looks more like coma than it does tilt.

[Hagar333 (Peter)]

Thanks Colin I will check this out over the weekend and do some measuring.

[Hagar333 (Peter)]

Originally Posted by billdan

Hi Peter,
Welcome to Ice in Space, are you confident you have the correct 55mm spacing between the MPCC and the CCD chip?

If so then the next step is to check collimation, coma correctors are a progressive corrector. They do no correction at the centre of the frame and gradually increase correction until max correction at the edge of the frame.

So the CC expects perfect round stars at the centre where no correction is applied, but if collimation is out then the round stars may be off centre. This causes the CC to over correct one half of the frame and under correct the other half of the frame.

So if you have some good collimation tools I would get that right first and you should be OK.

Cheers
Bill

Hi Bill,
Thanks for the welcome
No I am not confindent that I have 55mm spacing but after posting a message here I am much wiser on how to fix this !

Regarding the collimation I am a bit green on this but I will revist this and re read all comments here as well and start on the easy end first.

Cheers Peter

[Hagar333 (Peter)]

Originally Posted by glend

The Baader MPCC is designed for Canon sensor spacing, which i believe is 55mm from the shoulder of the MPCC housing (not the threads). It works perfectly on my Canon without any spacing changes required. It screws into a Canon bayonet t-thread adaptor which is about 9mm thick an that is part of the 55mm. What sort of bayonet is used with your camera and is it of similiar thickness?

Hi Glen,
thanks for your reply
I use a Nikon bayonet type Adaptor ( and about 9mm thick) and Baader MPCC straight on top of t ring adaptor.
No issues focusing but I will Double check every thing and do some more testing thanks for the Specs to compare.

Peter

[Atmos (Colin)]

I know there is a difference between the Nikon and Canon flange distances.

From a very quick search I came up with the Nikon F Mount being 46.5mm and the Canon EF Mount of 44mm.

Take those with a grain of salt.