View Full Version here: : Calculating/maintaining back focus

22-09-2017, 10:53 AM

I have been putting together a new imaging rig and have now got everything I need except an adapter to connect the ccd to the ota.

After some serious umming and arring I went the mono path for better or worst.

So I have an SW Esprit 150mm which comes with a dedicated flatener and needs 100mm back focus to the ccd. I purchased the Moravian G3 16200 mono with external filter wheel with a set of Astronomik 6nm filers (L,R,G,B,Ha, OIII and SII) and the M68 OAG.

So from the flatener I have 4mm from the glass to the edge of the housing and 61.5mm from the OTA side of the OAG to the sensor(according to Moravian website) Then there is the filters which are 1mm.

Now I read somewhere that I need to allow .33 of the filter in the back focus calculations. But can't remember or find it again but do I add or subtract that from the adapter.

eg.. 100mm-4mm-61.5 = 34.5mm +/- .33mm to allow for the filters and is it that critical.

I hope this makes sense

Thanks for any adivce

22-09-2017, 11:28 AM
Adding filters ADDS to the optical back focus but SUBTRACTS from the mechanical.

A flattener that has a 55mm back focus, when adding a 1mm filter you calculate it from a 55.33mm optical back focus.

23-09-2017, 07:56 AM
Thanks Colin.

So if I read that right I need to get a 34.2 mm adapter to bring the back focus back to 100mm.


23-09-2017, 08:05 AM
Yes this is known as the "Rule of 1/3rds". Each 1mm of glass lengthens the focal length by 0.33mm. You don't actually add the filter thickness as it sits in the light path, only the optical effect of it being there. Don't forget that any camera sensor compartment cover glass will also enter into calculation.
Also note thatsome flattener backspacing is measuredfrom the shoulder of the flattener, not the end of the threaded section, better check on that.

23-09-2017, 10:01 AM

On your figures, you must ADD the 0.33mm for the filter glass to the adapter thickness - ie the adapter thickness would be 34.83mm.

As to how critical it is, it depends on the telescope f ratio. For a very fast system it can be super-critical.


23-09-2017, 02:33 PM
Hi Glen,

The 100mm in my case is from the glass, which is 4mm inside the housing. see image1 below and the 61.5mm according to Moravian website is from the sensor to edge of OAG see image 2 below. I can't find any mention of cover glass or if its included in the specs or not.

Hi Mark thanks very much for that I obviously would have gotten the wrong lenght. Its a F7.
Rereading the manual sugests a +/-1mm to obtain a flat field, so mustn't be super critical.


23-09-2017, 03:00 PM
It is easiest to give an example so I'll go with my first setup as I can remember all of the figures :P
The flattener/reducer had a back focus of 55mm (pretty standard). I had a QHY9, CFW and Astrodon filters which are 3mm thick.

The QHY9 had 17.5mm back focus.
Filter Wheel 17mm back focus.

As you add 1/3 of filter thickness to optical back focus the calculation ends up being:
55+1-17-17.5= 21.5mm

So I need a 21.5mm spacer.

23-09-2017, 03:01 PM

The Moravian backfocus data is quoted as 'optical'. ie the cover glass is already accounted for, and you should not allow for it in the adapter calculation.

Regarding the +/- 1mm: at f/7 that sounds reasonable. Another factor (hopefully negligible!) that I did not mention before is that the optimum backfocus can also be affected by the deviation from flatness of the flattener!


23-09-2017, 05:29 PM
Thanks Guys,

So 34.83mm is what I need.

That is

100+.33 -4 -61.5 = 34.83

Very much appreciated.