Do filters add to back focus?

[NickMorris]

Thanks John,

The part I don't understand is what to choose between geometric back focus or optical back focus.  The distance between the ccd chip and reducer or reducer/flattener is the only thing of importance to me to get the spacing correct and therefore have perfect stars in the corners.  Where the focuser needs to be in order to bring this into focus is irrelevant to me as long as it is within the focus travel range.  It seems to me that back focus is a poor choice of terminology and manufacturers should just call chip/corrector distance!

cheers

Nick

[nightster]

Retire at will, Andrew, as is your perfect right, but please don't do so before accepting my heartfelt thanks for the most edifying of posts! I must admit to functioning at about 15% of an already limited mental capacity following an accident which leaves me unable to sit in comfort for more than a few minutes, so my mind is not really on the task. I'll go through all this in a month or so when I'm back to what passes for normal!

Olly

Olly, you are such an excellent teacher of these most difficult skills and theories that I, as well as others, get excited to see a reply from you. I appreciate the time you give a mere beginner such as myself.  I hope you recover quickly and fully.

Thank you,  

[sharkmelley]

I'm afraid the terminology "geometrical" and "optical" distance just confuses me.  However, I do have experience of modifying DSLR cameras.  What I know is that when the internal IR filter is replaced by a thicker piece of glass, the sensor has to be moved backwards within the camera to retain the correct focus.  Gary Honis describes the calculation of the spacers required here: http://dslrmodifications.com/rebelmod4.html

So, if instead you place a 3mm filter between the corrector and camera, you are increasing the total thickness of glass, so you have to add a 1mm spacer between the corrector and camera.  This agrees with Andrews diagram where the addition of extra glass causes the rays to come to focus further back and so an extra spacer is required for the sensor to coincide with that new focal point.

So if you add a 3mm filter you must add a 1mm physical spacer to compensate. Just don't ask me to translate this argument into terms involving  geometric or optical back focus because I'll end up confused

Mark

[wxsatuser]

The filter only has the effect of reducing the CCD's

Thanks John,

The part I don't understand is what to choose between geometric back focus or optical back focus.  The distance between the ccd chip and reducer or reducer/flattener is the only thing of importance to me to get the spacing correct and therefore have perfect stars in the corners.  Where the focuser needs to be in order to bring this into focus is irrelevant to me as long as it is within the focus travel range.  It seems to me that back focus is a poor choice of terminology and manufacturers should just call chip/corrector distance!

cheers

Nick

Back focus could be a poor choice, in the normal camera world it's FFD ( Flange Focal Distance ) or several other variations.

I think all we have to know is that to bring the altered focal point back to the focal plane is increase the optical path.

Edited February 17, 2015 by wxsatuser

[sloz1664]

Just to throw another confusing point in this debate. The distance from a FF/FR to the CCD plane is critical to obtain a flat field i.e. nice round stars to the very edges of your image.

Does the fact that adding filters to the imaging train alters this distance and could cause the flat field from not being flat?

Steve

[Merlin66]

Steve,

You need to re-read the previous messages...

To maintain the design optical distance you need to add the filter compensation.

My last words......

[steppenwolf]

You know what folks, we have essentially all gone round in a circle or two and yet we still (as far as I can see) haven't reached a satisfactory conclusion. In fact, we have only really agreed on one thing and that is that inserting a filter 'spreads' the light cone 'outwards' (my terms for simplicity) thus moving the focal plane further out and away from the light source and that to achieve focus, this additional light cone length needs to be accounted for. What we haven't ALL agreed on yet is what effect this has on the critical spacing of the sensor from the mounting face of any reducer/corrector in the field. Part of this problem is down to terminology (ain't that often the case?).

Sadly, I don't have the final answer although my own current view flies against the advice from QSI - even though I use their camera - and that is simply because I believe that the critical distance requirement of correctors is based on an optical distance viewpoint (that assumes an air-filled void with a refractive index of 1 in the space between the corrector and the sensor) not a mechanical distance viewpoint and if we move the optical 'path' outwards then to me it follows that we need to move the optical 'distance' outwards to maintain the status quo!

Not necessarily my last words ......

[NickMorris]

Well said Mr Steppenwolf,

Perhaps that should be the new acronym: CDR - critical distance requirement.  I like it and less confusing than back focus distance.......

[strutsinaction]

Sadly, I don't have the final answer although my own current view flies against the advice from QSI - even though I use their camera - and that is simply because I believe that the critical distance requirement of correctors is based on an optical distance viewpoint (that assumes an air-filled void with a refractive index of 1 in the space between the corrector and the sensor) not a mechanical distance viewpoint and if we move the optical 'path' outwards then to me it follows that we need to move the optical 'distance' outwards to maintain the status quo!

Steve - isn't that what QSI are saying though? They state that the backfocus of the camera should be adjusted by -1mm if a 3mm filter is fitted. So, when you do the arithmetic to maintain the 72.2mm backfocus distance the contribution made by the camera is now reduced by 1mm to 49.2mm so you need to add a 1mm spacer to compensate, thereby moving the' optical distance outwards to maintain the status quo'.

Regards

John

[strutsinaction]

If you're interested, there's a similar thread on CN asking the same question:

http://www.cloudynights.com/topic/475406-considering-filter-thickness-when-calculating-backfocus/

Look for the post with the parts list where the supplier has quoted the backfocus contribution of the 3mm filter as -1.

Regards

John

[andrew s]

Ok as it was my fault for introducing the term optical distance so I will redefine it here as it is now being mixed up with the geometric distance. 

The geometric distance is the distance you would measure with you ruler, calipers etc. (I am here ignoring the effects of special & general relativity Olly!)

The optical distance is the integral (i.e. sum) of the geometric distance multiplied by the refractive index n. 

So if you have 20mm air (n~1) then 3mm of glass (n ~1.5) then 10mm of air the the geometric distance is 20+3+10 = 33 while optical distance is 20*1 + 3*1.5 +10*1 = 34.5.

Don't forget though adding the 3mm of glass instead of 3mm of air also changes the focal length.

Normally in optics it is the optical distance ( referred to as the Optical Path Length) that matters but if the medium is air then they are about the same.

What was to be my last post on this above gives my best view on the issue of adding a filter to an existing system. I hope this makes it clearer.

I have also looked up the design of some reducers/field flateners and as far as I can tell they are designed assuming only air between them and the focal plane.

I can see no way they could account for all the specific designs of CCD with there differing distances window & cover glass thicknesses.

My conclusion from this would be to not modify the system when adding a filter and see if it is ok in use. If not and it is simple to add a 1mm spacer try that next and only if it gets worse try reducing it or do it the otherway round if it is simpler.

I have a strong suspicion we are all dancing on the head of a pin but none of us are angels !

Regards Andrew retired.

Edited February 17, 2015 by andrew s

[Tonk]

"Do filters add to back focus?"

They ADD to metal back distance (MBD) of your scope or reducer or they SUBTRACT from the camera back focus cost . You calculate the spacer requirements needed by either the following methods:

(First calculate the filter offset via t * (n - 1) / n  where  t is filter thickness and n is filter substrate refractive index - note that most filters have n close to 1.5 so the equation can be treated as t / 3)

1. Adding the filter offset to the metal back distance of your scope or reducer and using this adjusted MBD as the base for subtracting the back focus costs used up by filter wheel and camera

OR

2. Subtracting the filter offset from the camera back focus cost and using the original scope/reducer MDB as the base to subtract back focus cost due to filter wheel  and due to the filter-adjusted camera back focus cost. (Some camera manufacturers instruct you to use this  approach - particularly if the camera has an internal filter wheel where you add your own filters)

The two methods are equivalent - one instructs you to add the filter offset  to something - the other to subtract it from something else - somehow this subtle point has lead to 3 pages of tail chasing - lol

The equivalent equations in words

1. Spacer length = metal back distance + filter offset - (filter wheel back focus cost + camera back focus cost)

2. Spacer length = metal back distance - (filter wheel back focus cost + (camera back focus cost - filter offset))

Camera back focus cost is measured from front flange to sensor surface

Edited July 26, 2017 by Tonk

[wxsatuser]

It's simple.

After any filter just add 1/3 the glass thickness to the image train, a 3mm thick filter adds 1mm.

 

[Tonk]

I think the problem was that the instructions of camera manufacturers where misinterpreted as subtracting from the scope/reducer metal back distance when its actually "subtract from the camera back focus cost". Don Goldman (Astrodon) pointed this common trap out many years ago.

Edited July 26, 2017 by Tonk

[ollypenrice]

It seems to me that the camera manufacturers are imposing on us a kind of mental double negative which isn't very helpful. I have had to look this thread up again since I'm back in the reducer spacing jungle! Groan.

I'm going to follow Don Goldman and add 1mm of metal to the hardware connecting flattener to camera. That is, I'm going to make it an 86mm separation rather than a filterless 85mm.

Olly

[x6gas]

I can see why the confusion arises here but I think it's terminology about adding and subtracting and negative numbers...

So for my FSQ85 reducer I need a spacing of 72.2mm.  If I have a 3mm astrodon filter in the image train it pushes the image cone outwards by 1mm so the spacing requirement is increased by 1mm to 73.2mm.  You are ADDING to the back focus requirement.

So to make that 73.2mm I have:

Atik OAG: 24mm

Atik EFW2:  21.8mm

Atik 490: 13mm

Spacer:  14.4mm

=73.2mm - the impact of the astrodon filter has been taken into account by increasing the required back focus distance.

Now alternatively you can include the impact of the filter on the imaging train calculation by including it as follows:

Atik OAG: 24mm

Atik EFW2:  21.8mm

Astrodon filter: -1mm

Atik 490: 13mm

Spacer:  14.4mm

=72.2mm

In this instance you are SUBTRACTING the impact of the filters on the elements contributing to your backfocus.  It might help the penny drop if you calculate the spacer required in both of the scenarios above.

Spacer = 14.4mm = -73.2mm + 24mm + 21.8mm + 13mm = -72.2mm + 24mm + 21.8mm - 1mm + 13mm

Is that clear?

[gspowers]

Folks I have a random, but topical question on BackFocus, given that i have a Reducer/Flattener asking for 55mm, and Camera Sensor to Flange distance of 17.5 leaving me to fill a gap with 37.5 mm of spacers what sort of 'Tolerances' am i working with, will simply purchasing an Astro 7.5mm and 30mm tube spacer do the job and that the Astro industry has the tolerances all buttoned up with acceptable parts or will there inevitably be some fine tuning required when the images start to tell a story. OR, is the Reducer/Flattener more tolerant of a little stackup error in the parts.  for me it comes down to whether I go with an adjustable Tube spacer or Fixed . 

some insight appreciated, apologies to all if i missed the pertinent sentence some place.

Gary ( am kinda just setting up with the hobby )

[x6gas]

To be honest Gary it depends on your setup.  Longer focal lengths will probably be more forgiving but it depends on other aspects too.

I suspect that the spacers you suggest will be absolutely fine but the thing to do is to actually image with that spacing and check the results which will tell you whether the spacing is too close:

or too far: