Coma corrector back focus distance

[x6gas]

I need some advice on the distance between by new coma corrector and imaging chip.

I'm very pleased that Santa dropped off a Baader RCC-I coma corrector at Christmas and I am looking at the back focus distance to the image sensor on my system.

The image train is:

RCC-I / Atik OAG / Atik EFW2 / Atik 383L+

and the contributions to back focus I have are:

OAG=24mm; EFW2=21.8mm; 383L=17mm = 62.8mm

Now I plan on using the M54 2" adapter that ships with the OAG and has an M48 female thread that will accept the male M48 on the RCC-I. As luck would have it, this adapter adds 30mm to the image train which becomes

RCCI / M54 2" adapter / OAG / EFW2 / 383L = 30+24+21.8+17 = 92.8mm

Now the optimal chip distance from the M48 thread on the RCC is stated as 94.5mm and the alpineastro site states that the "distance needs to be held to within +/-3mm for best image quality". I am obviously within this limit, but I could do better by inserting a spacer.

My question is, how critical is this distance? Should being 1.7mm out be close enough or would I notice a difference by getting closer to the optimal distance?

The set up will be used on my 10" f/4.5 newt and my 6" f/5.

Thanks in advance for any help!

[steppenwolf]

I have no direct experience of the Baader RCC-I, only the Baader MPCC but I would be surprised if 1.7mm would be an issue. However, an easy fix would be to use a 1.2mm Delrin 'T' spacer ring between the Atik 383L+ and the Atik EFW2 thus closing the disparity down to just 0.5mm. I know that a 1.0 and a 0.9 would get you closer but I'd prefer not to 'stack' these for just a 0.3mm gain but I have no doubt that it would fine too.

[x6gas]

Many thanks for the reassurance Steve. I think I will try to get the distance down a tad more...

The connection between the 383L+ and EFW2 is a circular dovetail (at around 54mm) but I can squeeze a little out of the M48 thread on the RCC-I. The T2 Delrin spacers will be too small diameter, obviously, but I'm sure I can source something...

Thanks again and Merry Christmas.

[steppenwolf]

The connection between the 383L+ and EFW2 is a circular dovetail

Is there not a male 'T' thread on the camera side of the circular dovetail on to which you could fit one of the Delrin spacers? There is on my EFW2.

[x6gas]

Is there not a male 'T' thread on the camera side of the circular dovetail on to which you could fit one of the Delrin spacers? There is on my EFW2.

Ah, I am sure you are right Steve. Got mine second hand and the dovetail is welded to the t-thread so I've never had them apart...

Thanks again, Ian