Coma and Barlows

[acarleton]

I am thinking of getting a Barlow (http://www.bintel.com.au/Eyepieces-and-Barlows/Barlow-lenses/TeleVue-2x-BIG-Barlow--2--/1208/productview.aspx) to reduce the FOV and gain better detail on some of the larger galaxies such as M83. i am not sure how i would connect this to my Canon 600d, but i will talk to the guys at Bintel for that. my main question is, what will happen with coma ... my newt is a F4 10" and when left uncorrected it has some significant coma. will a barlow eliminate coma (effectively making it F8) or will i still need to connect my coma corrector, however that would work.

[rustigsmed]

Hi Aidan,

I think you would need this http://www.bintel.com.au/Eyepieces-and-Barlows/TeleVue-/Barlows-and-Powermates/TeleVue-Power-Mate--br-T-Ring-Adapter-2x/884/productview.aspx [/URL]
but always best to check with the guys selling the gear. adaptors / connectors etc drive me crazy. it may need this [URL]http://www.bintel.com.au/Eyepieces-and-Barlows/TeleVue-/Barlows-and-Powermates/TeleVue-Powermate-br--2x--2--/881/productview.aspx (http://www.bintel.com.au/Eyepieces-and-Barlows/TeleVue-/Barlows-and-Powermates/TeleVue-Power-Mate-br-T-Ring-Adapter/883/productview.aspx) rather than the one you chose, but again, best to ask them at Bintel.

As for coma, it will essentially be gone at F8 or near enough to it.

Tracking may become more of a problem with double the focal length and that is something to consider.

cheers,

rusty

[acarleton]

Thanks for that, yeah, i will have a talk with the Bintel guys today.
tracking is definitely the issue, i tried using PHD yesterday (through an orion 50mm guidescope) for the first time, with some success, but it will need to be perfect.

[astro744]

See http://stargazerslounge.com/topic/164967-can-barlowing-reduce-coma/

Also there is a coma correcting Barlow at http://www.apm-telescopes.co.uk/apm-coma-barlow.html

also http://www.astronomyalive.com.au/apm-germany/apm-coma-correcting-1-1-4-ed-glass-barlow-2-7x.html

and http://www.astronomyalive.com.au/apm-germany/apm-screw-on-element-coma-correcting-1-25-inch-ed-glass-barlow-2-7x.html

and some images at http://www.astrobin.com/gear/1451/barlow-coma-2x-apo-apl/

Note the Paracorr will eliminate coma and give you a 1.15x increase in focal length and ratio.

[Merlin66]

The original Klee barlows were "optimised" to reduce coma in fast reflectors.
I have both the x2.2 and x2.8 versions - from the ol' days.....

[acarleton]

So in case any one is interested for posterity's sake, i have talked with the guys from Bintel and they have informed me that putting a barlow or a powemate in a F4 scope does not make it act as a F8 scope. the coma is an artifact of the mirror and magnification will only zoom in on the issue, not correct it. therefore if you want to have a flat FOV you need to attach the barlow/powermate to a coma corrector. this is what i will be doing, hoping for some good results.

[Merlin66]

This aspect of "barlows" is well discussed and explained in" Telescope, Eyepieces, Astrographs" by Hallock-Smith, Ceragioli and Berry, p 373.
(That's why I mentioned the Klee design)

[rustigsmed]

Yep sorry Aidan I was a bit slack in my explanation. It will change the field of view, and as you know the coma is worse toward the edges of the fov than the centre. so not as much zoom in on the issue but 'crop' the fov toward the centre where its not as bad. will it suffice? hard to know, depending on your optics and how well you have centred the subject. the pictures I have seen where people have photographed with a barlow are usually planetary nebs which I suppose have again been cropped in processing as they are generally small. it would be interesting to test out on a star field.


edit - I know with an f5 it would be ok - f4 I couldn't say, it is exponential ..