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Do Filters Change Back Focus Significantly.

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#1 diligentskies



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Posted 25 October 2021 - 10:58 PM

Hello ALL,


I have an OTA that calls for a back focus of 62.1mm.

That is according to this specification, "A total back focus distance from my reducer housing to my camera sensor should be 62.1mm."


With 17.5mm of built-in back focus from my camera, "The total remaining back focus distance of my imagining train build-out is 44.6mm."


I am using a ZWO filter drawer and spacers between the drawer housing and camera to gap-out this distance.


Such that when assembled and everything is reasonably tightening down, "A digital caliper measures this distance at 44.3mm."

So I am very much, "In the ball park of nailing the back focus distance for this OTA for this given reducer just short of the back focus specification."


So my question is, "Does the incorporation of additional glass due to filters change my back focus in any meaningful manner?"


In theory as a starting basis. "The typical back focus without a reducer would be 55mm, so in theory this reducer adds a little over 7mm to the back focus."


So my conclusion is, "Yes additional glass does add distance to the back focus."


With that observation.


What of filters???


"Since the use of filters should add some measurable distance to the total back focus build-out of my imagining train."

Because filters are an incorporation of additional glass in the imagining train.


However for me, this time around a new parameter has been introduced.

Where exactly is the, "This additional glass being placed in the imagining train build-out."


Is it bias towards the reducer or sensor???


Meaning if my OTA specs calls for a back focus of 62.1 mm. That specification is based on there is, "NO ADDITIONAL glass placed within that distance."

However if filters DO contribute to changing the back focus then, "The location of where these filters are placed in the imagining train build-out should become an additional governing parameter."


In theory, the manner in which I am understanding this optical problem is such, "Yes filters do contribute to lengthening the back focus distance."


Essentially the question then BECOMES, "BY HOW MUCH?' and IF SO DUE THESE FILTERS, "Significantly contribute to CHANGING THE OVER ALL back focus in any measurable manner."


My point is if back focus distance is no longer a issue of just, "The thickness of the filter but also an issue of where the filter is placed within the imagining train AND...


I have opted to use a single and consistent manufacturer (Optolong's brand of narrow band filters for H-alpha, O-III and Sulfur-II under the belief that they are reasonably parfocal).


If there is NO settled RULE OF THUMB for a given class(brand) of filter's contributions towards changing the over all back focus according to a given back focus spec.

THEN Experimentally, "WHAT SHOULD BE MY GUIDING OBSERVATIONALS to determining the adding additional spacers adjustments in increments of 0.5mm to EFFECT optimum back focus?"




Edited by diligentskies, 26 October 2021 - 12:00 AM.

#2 bobzeq25



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Posted 25 October 2021 - 11:17 PM

Neglecting the fact that backfocus specs are often approximate, and that people often get better results by varying it.


Backfocus increases by 1/3 the filter thickness.  I believe that's independent of where the filter is placed.  I can't imagine a setup where you'd put the filter _before_ the reducer, that's inconvenient and likely to introduce vignetting.

Edited by bobzeq25, 26 October 2021 - 10:01 AM.

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#3 scadvice



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Posted 25 October 2021 - 11:27 PM

Yes, I agree with Bob.

#4 Sean1980


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Posted 26 October 2021 - 12:37 AM

Agree with Bob on the 1/3 Filter thickness adjustment for backfocus. Would also suggest that it is best to always place the filter as close as possible to the sensor in the imaging train.

Fabrication spec of the backfocus can also be off by quite a bit in a real world scenario (allowing for production tolerances) so to get the best result some tweaking in use will be needed to get the optimal result.

Something like CCD inspector can help with that or any of the alternatives that can measure the field you are capturing

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