Filter Affect on Flattener Spacing

[glend (Glen)]

Just setting up my 152mm refractor to work with the Hotech Flatterner and am finding the various historical threads about spacing a bit confusing. The flattener has a requirement for 55mm of spacing to the sensor, and that's pretty straight forward, however research tells me that I need to follow the 1/3 rule - meaning I need to add spacing to compensate for the affect of the filter in the imaging path. My 2mm filters, would need an additional 0.66mm of spacing added to the path in front of the filter. I can make up spacing that is close to that (0.33 close) and could use some thin Delrim spacer washers to get it spot on, but how much leeway do i have, is 0.33mm close enough, how exact does the spacing need to be?
Secondly, what allowance, if any, needs to be made for the cover glass window in the end of the camera (an ASI1600)? The window seals the sensor chamber, but I do not know how thick it is (probably about 1mm). Does that need to be seen as a filter and affecting the spacing?

[luka]

Glen, do you have the Hotech SCA flattener?
If yes, I asked them a while ago about how accurate the 55mm spacing to sensor has to be and got this response:

The tolerance is +/- 0.5mm. This varies depending on the focal ratio of your scope but keep the range within 1mm will be good. Thank you.

Not sure how much it helps but at least it is a guide how close you need to get. I am guessing that the tolerance is smaller for faster scopes.


By the way, you can calculate the shift using
d = t * (1-1/n)
where t is the thickness of your filter (2mm) and n is the refractive index of your filter. n=1.5 may be good starting guess (glass has refractive index of 1.5-1.6).
This will give you a shift of 0.67mm. Remarkably close to the 1/3 rule

You do have to increase the shift again because of the sealing glass. Assuming 1mm thickness and the same refractive index for the filter and the glass (to make the calculations simpler), you get the total shift of:
d = (2mm + 1mm) * (1-1/1.5) = 1mm. Double from the manufacturer specified +/- 0.5mm.

I did similar calculations when I removed the AA and UV-IR filters from my DSLR - the spacing was shortened by 0.53mm if anyone is interested.

[glend (Glen)]

Thanks Luka, yes it is the Hotech SCA flattener for fast refractors. Their advice is appreciated, and i do not think they put that in the instruction sheet that came with the flattener. I will make allowances for both filter and chamber cover.

[ericwbenson (Eric)]

FWIW on my website I have the derivation of the focus shift induced by an optical window in a converging beam. See here:

http://www.faintgalaxy.com/focusshift.htm

Regards,
EB

[Atmos (Colin)]

My understanding is that with most correctors keeping within 1mm is quite acceptable. I have found that reducer correctors appear to be a lot more demanding at times but this could be down to lens (corrector) design.

Just as a further tid bit, my previous imaging setup was off by what I estimated ~1.5mm and although it did have an effect, on a KAF-8300 it was still very much acceptable.