Avoidable T2 vignetting with full-frame DSLR

[Legion Of Andromeda]

I have significant vignetting issues with my full frame 6D. I initially assumed it was the front aperature of my Edge 8 hd. But it turns out to be the inner 36mm opening of the male 42mm projection from the rear of the 2" nosepiece. If i replace the nosepiece with a straight 2" tube held in place between the visual back and the T-Ring camera adapter this issue disappears regardless of the backfocus from 50 to 200mm. The 36mm opening is throttling the light and no amount of flat frames can compensate. Now then, my T-Ring adapter consists of a female 42mm (T2) threaded inner ring and a circa 52mm bayonet mounting ring. The inner T2 ring is kept intact by 3x grub screws, if i remove the T2 ring i find that my bayonet ring has a circa 49mm female thread! . i have found that my 48mm filter is too loose and something else i own with a 50mm thread is too big, therefore 49mm seems correct. So my plan is to find a 2" nose piece that is 49mm male at the rear end and 48mm female at the front end for my filters and circa 75mm long. I tried the official celestron adapter today only to find the same small opening at the rear end.

My current setup ; My C8 SCT visual back then my Antares SCT to 2" compression collar, then the aforementioned 40mm long nosepiece and T-Ring which is currently extended by 35mm with a spare 2" cylinder held together with gaffer tape to give me the ideal total back-focus of 135mm. I also have a focal reducer, same issue, but the nosepiece only needs to be 40mm long.

Does anyone know of any adapters that are male 49mm / 2" tube / 48mm female and 40mm long plus a circa 75mm long version?

I'd be amazed if i'm the first person to raise this issue and that there are no simple solutions to this problem. I mean, what is the point of the Full frame sensor if one can only use a a crop-sized area?

TIA.

[johnturley]

Hi there

I've had this same problem with a 'T' thread adaptor, which I think was based on the size of original Pentax srcew fitting lenses., which had a thread size of or around 42mm, the aperture of the Canon bayonet fitting is significantly larger.

You can however get a 48mm to Canon bayonet adaptor, which better matches the Canon bayonet aperture (I have one which fits my ES Coma Corrector which I use on my 14in f5 Newtonian), and should show some improvment.

The problem could be whether there is a 48mm adaptor available which will fit your 8in Edge HD.

John

 

[michael8554]

I too have a M48 T to 2" Adapter on my 6D, with a F6.3 200mm SCT. 

Coma is the main problem now, not vignetting! 

Michael