View Full Version here: : Backfocus measurements - refractors

The Mekon
11-09-2017, 12:08 PM
I have long held that most modern refractors are manufactured with tubes that are way too short for visual use. For example my 106mm Sharpstar needs the focuser racked out all the way when using a Type 6 Nagler in a 2 prism and even then I need to add a 3mm spacer to get focus.
This year I ordered a 132mm F6.9 CFF. Prior to ordering, the first question I asked Catalin Fus was about back focus and whether the scope would come to focus using a prism diagonal. He answered that the back focus was 180mm so any diagonal would need to use up a minimum 66mm of the light path with the Feathertouch 114mm travel focuser fitted. So focus seemed easily achievable with most prisms.
Last night I took some measurements of my 132mm refractor to find out just what the true backfocus value is.
Using an extension tube of 115mm (checked with verniers) I observed Achernar straight through with various eyepieces, noting in each case the reading on the focuser gauge.
7mm Nagler focuser racked out 92mm
17mm Plossl focuser racked out 85mm
17.3 Delos focuser racked out 80mm
This would seem to indicate a back focus of between 195mm and 207mm, not 180mm
Question: am I missing something here? Perhaps I should check the back focus using the projection method as well?
I would like some comments before I go back to the manufacturer to say that the backfocus claim is all wrong.
The reason I am interested in backfocus figures is that I want to use a Baader T2 prism, which has a light path of around 70mm, this would leave me around 20mm-25mm short of focus. My 2 mirror diagonal has a measured light path of 108mm, my 2 prism diagonal one of 88mm. I much prefer prisms last night the companion of Antares was easy with the prism, but quite hard to spot with the mirror.

11-09-2017, 01:44 PM
Interesting discussion...

I assume the backfocus is measured from the outer end of the fixed body of the focuser....

I don't know what the backfocus of my TS102 is - that's not true, I have a PST Mod adaptor I made to allow interchangeability with the Baader Herschel wedge. This adaptor positions the etalon at 200mm inside the prime focus...
Just measured:
The backfocus from the rear OTA flange, where the focuser body fits, is 250mm. The focuser body is 80mm (including the flange on the drawtube) this leaves a clearance of (250-80)= 170mm for all the diagonals, eyepieces etc.
The Baader Herschel wedge with a 20mm Plossl sits at +26mm.....
(This infers the wedge is about 145mm light path.)

The most accurate method is to use a solar filter (or stop down the objective) and focus the solar image on a card......

11-09-2017, 02:02 PM
Hi John,
I think the most accurate way is to measure from your front objective back.
You know the focal length of the telescope = total length. So to achieve focus measure from the front of objective back add on the focal length of your diagonal + eyepieces you want to use.
I bought the Baader 2" click lock and worked out spot on.


11-09-2017, 02:15 PM
I agree. IF you know exactly the focal length of the objective and know where to measure from (front/middle/rear/optical centre) on the objective.
The actual FL may not be exactly what it says on the lid....

11-09-2017, 02:43 PM
John, If your extension tube is actually 115mm (As long as you didn't include the inserted nosepiece in you measurement :D) then you scope has a back focus of 201mm which agreed with the 12mm difference between the Nagler T6 and Delos 17.3 (actual difference should be 12.2mm or 0.48")
I was under the impression that CFF measured the back focus from the back of the fitted adapter on the draw tube.

The Mekon
11-09-2017, 03:36 PM
Thanks Matt for confirming these eyepiece figures. My extensions are a 35mm + 80mm and I am not including the nosepiece.
The backfocus should be measured as you say, from the fitted adaptor (3.5" to 2") in the draw tube.
I have just built a projection rod to bolt to the scope and I will project the sun or moon sometime.

Darren, I do not see how measuring the focal length from the objective will help - it is difficult to know the exact point on the objective to measure and where it is. But you are right in thinking I need to know the F.L. - I am starting to believe it is closer to 935mm rather than the stated 915mm.

11-09-2017, 05:11 PM
I think there is some variation in FL in every scope, in particular when lenses are hand-figured for optimal optical performance.

Another way to quite accurately measure FL is to plate solve an image with a star field, but the image should be taken without a corrector as I feel some correctors may slightly alter f-ratio.

The Mekon
13-09-2017, 07:57 AM
Sauvi, yes as you say but I would have to image! Would it be easy to pick up the difference between 915mm and 935mm given a DSLR image?

This morning I rigged up my scope with measuring rod attached and projected the sun. Sure enough the sharpest projected image measured at a backfocus position of 198 - 200mm (difficult to be too precise using this method.
I will be contacting Catalin at CFF tonight.

13-09-2017, 12:07 PM
I believe that dslr is fine, as long as we know pixel size?

Thank you for keeping us updated John.

EDIT: Here is a link to a similar discussion on CN: https://www.cloudynights.com/topic/511109-effective-focal-length/

13-09-2017, 12:16 PM
Plate solving a DSLR image isn't any problem at all, just upload it to Astrometry.net and it'll do it all for you quickly enough.

14-10-2017, 04:39 PM
Where did you end up with your back focus clearance?
Just interested.

The Mekon
14-10-2017, 05:30 PM
Thanks for the interest Ken.
Catalin from CFF, advised that the advertised back focus was not fixed, but should have been within a certain range. The 132mm that I ordered was one of a run of 10 that had the tubes assembled first and then the objectives made to order (I had thought it would be the other way around).
A result of this was that my lens came up around F7 rather than F6.9 so increasing the back focus to around 200mm. I am not complaining - the scope is superb and I have made a number of repeat observations that I made once with my AP 130EDT over 20 years of ownership. I reckon this scope is better then the Astro-Physics.
To get around the longer back focus I have ordered a Baader 2" BBHS mirror diagonal rather than the smaller T2 prism. This will give plenty of leeway on the focus travel.