Backfocus allowance for filters ?? How critical????

[Skipper Billy]

Having spent ages getting the backfocus distance between my SW FF/FR to Atik 314l+ to the suggested 55mm with a variety of T2 spacers etc I now learn that I have should have made allowances for the filters !!!

Which prompts 2 questions for the experts.

I am using Baader LRGB interference filters - what allowance should I make and do I add it or subtract it from the suggested 55mm ????????

Secondly - how critical is the 55mm is =/- 1mm acceptable is +/- 3mm acceptable - what difference will it make and how will I know / how can I measure / pre-empt it as we have got another 8 weeks of no darkness to play about!!!!!!!!!   

Thanks in advance...............

[Skipper Billy]

PS - the glass in the filters is 2mm thick

[Geordie mc]

I think the figure for Baader filters is -1.7mm ie you take it off the distance but really don't know how much difference it makes. Hopefully Olly or Pers will chip in with more expert advice as I'd be interested to know too.

[wxsatuser]

For filters add the following.

The formula is     t * (n-1)/n

t is filter thickness

n is refractive index about 1.5 normally

I believe....

Astrodon are 3mm thick.......add 1mm

Baader are 2mm with some at 3mm thick.....add 0.7 or 1mm

Astronomik are 1mm thick.....add  0.33mm

[Skipper Billy]

OK - now I am REALLY confused!!!!!!!!

Geordie mc is saying minus 1.7 ie 53.3mm

Wxsatuser is saying add 0.7mm ie 55.7mm

????????????????????

[ollypenrice]

With your chip size you are in tolerant terrain. I wouldn't miss any sleep...*

Olly

* Typing quickly and about to go outside to run the show, so that was a silly remark!  Missing sleep is the whole point, right? D'oh!! 

[wxsatuser]

OK - now I am REALLY confused!!!!!!!!

Geordie mc is saying minus 1.7 ie 53.3mm

Wxsatuser is saying add 0.7mm ie 55.7mm

????????????????????

I believed the filter reduces the distance so you need to bring it back to the original distance by adding the extra.

If that makes sense.

[steppenwolf]

Technically, you need to ADD 0.7mm with your 2.0mm Baader filters but chances are that you won't notice a difference unless you are already right on the edge of the correct spacing.

[lensman57]

Having spent ages getting the backfocus distance between my SW FF/FR to Atik 314l+ to the suggested 55mm with a variety of T2 spacers etc I now learn that I have should have made allowances for the filters !!!

Which prompts 2 questions for the experts.

I am using Baader LRGB interference filters - what allowance should I make and do I add it or subtract it from the suggested 55mm ????????

Secondly - how critical is the 55mm is =/- 1mm acceptable is +/- 3mm acceptable - what difference will it make and how will I know / how can I measure / pre-empt it as we have got another 8 weeks of no darkness to play about!!!!!!!!!   

Thanks in advance...............

Work on adding approximately 1/3 of the glass thickness but a lot all depends on your scope. For normal SW EDs of around F7.5 with 0.85 reduction it really does not matter, when you get down to F5 and faster then you can start to pull your hair out.

A.G

[Adamar98]

I've just had to work this out. I also use the baader filters and I've added 0.7mm to the spacing.