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Do filters add to back focus?


nightster

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I talked with my dealer to prepare the equipment list for the move to mono, the 460ex. As we were determining the backfocus for the spacer requirements, he told me that you need to add for the filters. He said 1/3 of the filter thickness. The Astronomiks list their glass at 1 mm, which is inconsequential. But my dealer said they are 3mm hence an additional 1mm backfocus. This is something I haven't read about before.

1. Has anyone heard of filters adding to backfocus?

2. Is the 3mm for Astronomiks correct?

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According to the QSI data sheet you have to subtract 1/3rd the thickness of the filter from your back focus distance. Also, if you have any additional filters in the imaging train. Huchtech idas LP filter for example, that has to be taken into account.

Steve

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so its nothing to do with chip distance to flattner , just focus ??edit to last post , I am a few thu over not under 55mm

If the filter is between the flatner and the chip I would increase the flatner to chip distance to account for the filter as specified above in a previous post.

Regards Andrew

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This is my understanding too.

Steve, if I do the ray diagram I can't see how it can be the opposite but both QSI and Optcorp say that it is. When Optcorp specified my extender for the TEC flattener they subtracted I/3 of the filter's glass thickness from the system FL. This is also what QSI say on the link above. I hate doing things by rote without understanding them but, sheep-like, I now follow the subtraction rule without knowing why.

I must say that the Optcorp extender has worked perfectly on the TEC flattener with exacting full frame chip, but this doesn't prove that they were right. I think I need to write to Ralf, the font of optical knowledge...  :grin:

so its nothing to do with chip distance to flattner , just focus ??

edit to last post , I am a few thu over not under 55mm

No, it applies to chip distance if we are talking about an add-on flattener that moves with the drawtube. It can be entirely ignored in inherently flat field instruments like Petzvals where all it will do is crop a mm off your backfocus. This shouldn't be tight enough to be affected.

Olly

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Ok, so it Adds to the Backfocus.  Since the Astronomiks are thick it add 1/3mm which is negligible.  But the Astrodons @3mm would add a full 1mm for a total of 56mm behind my specific flattener.  

To further my understanding, and I understand every flattener different, there has to be a small amount of leeway on the 55mm backfocus, right?

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And if I knew how to edit my post on SGL I could rescind the previous post as Olly says it subtracts from the backfocus.  My dealer told me it adds also.  The QSI report says "Back focus reduced by (glass thickness / 3)."   At this point this topic seems clear as mud.

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I am never clear what adding or subtracting backfocus means. I still believe the glass plate results in the focus being further from the objectve. I attach a sketch that show the effect. While not accurately to scale the key point is the light is refracted towards the normal as the light enters the plate and away when it leaves. This means the light converges less quickly when in the plate.

Regards Andrewpost-9303-0-84949800-1423852305_thumb.pn

Edited by andrew s
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Steve, if I do the ray diagram I can't see how it can be the opposite but both QSI and Optcorp say that it is. When Optcorp specified my extender for the TEC flattener they subtracted I/3 of the filter's glass thickness from the system FL. This is also what QSI say on the link above. I hate doing things by rote without understanding them but, sheep-like, I now follow the subtraction rule without knowing why.

This is a confusing subject and I have seen convincing arguments going both ways but I agree with the diagram above!

Although I don't use his filters, here's a quote from Don Goldman (Astrodon) who I believe knows a thing or two about filters but maybe hasn't got the price right yet! :grin: :-

"Camera manufacturers show a smaller optical backfocus than mechanical backfocus because they are measuring the distance from the imaging CCD focal plane. This includes detector chamber window and sometimes coverslips on the detector.

HOWEVER, that is not where WE measure backfocus from. We want to know how much space we need to add from the metal back of a telescope or from a field corrector/reducer to the imaging focal plane. This is how we select spacers, etc. So, our starting point is the scope.

THEREFORE, the addition of a 3 mm filter ADDS 1 mm [t * (n-1)/n] of backfocus between the scope and your camera (t is the filter thickness and n is the refractive index of the substrate of the filter - typically 1.5) So it ends up being t / 3. 3/3 = 1mm in this discussion. You need to ADD 1mm of space between your scope and your camera.

This is often confusing. When you place a filter in a beam of light that converges from left to right, the focus is extended FURTHER right INCREASING the backfocus distance as measured from the scope.

All Astrodon filters are 3 mm thick, so the same 1 mm must be ADDED."

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Steve, if I do the ray diagram I can't see how it can be the opposite but both QSI and Optcorp say that it is.

As has been posted above as I was writing, I'm in agreement.  It doesn't seem to make sense that the backfocus should reduce.

James

Edited by JamesF
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I agree entirely with Andrew S's ray diagram and this is what made me say that I can't understand the subtraction claims. It's exactly the ray diagram I originally had in my head.  It's just that when QSI say the reverse I feel outgunned. Now that Don Goldman agrees with the ray diagram I'm delighted. I've emailed my favourite optical engineer and am awaiting a reply.

This is a rum situation!

Olly

Edited by ollypenrice
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I would also tend to agree with Andrew and his ray diagram. This is GCSE physics, isn't it? Light rays entering the filter are bent towards the normal, then away from the normal on exiting the filter, so that both the incident light and emergent rays are parallel and the focus point is further away.

I read the QSI back focus document a while ago and didn't think too much about it but surely it's wrong to subtract. Interesting stuff!

Regards

John

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Terminology.....

If you want a fixed distance between the reducer/ corrector and the CCD chip say 50mm.

When you add a 3mm filter element, the physical distance would need to be 49mm and the "extra" 1mm coming from the filter change of focus position...

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Thanks, Merlin66. The QSI document now makes sense to me. So, in the case of a 3mm Astrodon, the filter adds 1mm to the light path so you have to subtract this 1mm from the physical backfocus measurement to maintain the fixed backfocus distance (in my case a x0.7 reducer).

Regards

John

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Not wishing to complicate things but I have been pondering what one should do if you have a corrector that is required to be a certain distance from the detector and you place a filter between them. If you assume that corrector is designed to have a certain optical path distance ( i.e. geometric distance * refractive index) from the corrector to CCD then if you place a filter of say Xmm thickness and of refractive index n between them, without increasing the geometric distance, it adds X*(n-1) to the optical path length - assuming the index of air to be ~1. So the geometric distance between the corrector and CCD needs to be reduced by this amount does it not so the total optical path length stays at the design distance! Refocusing will also be required. 

I still stand by what I posted before but for this particular point.

Regards Andrew

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Merlin66's argument is compelling (see what I mean about great arguments out there!!). This would seem to confirm the QSI and Optcorp stance but if the light path has now been modified, how relevant is the original 50mm PHYSICAL spacing in the example above?

Fascinating stuff.......

Sent from my iPhone from somewhere dark .....

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