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Old 03-06-2015, 09:47 PM
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Optical Back Focus vs Physical Adapter Size?

I think there is something basic that I may not be understanding. Case in point concerns my Moravian G2-8300 camera with the built in filter wheel.

On the Moravian site it indicates that the BF to the camera window is 29mm. To this I want to add a low profile t adapter which has a size of 7.5mm. That is, on the camera side it is flat and bolts to the front plate. On the telescope side there are female t threads. So, it would appear to me that anything mounted and fully screwed into the t-thread would be extended by exactly 7.5mm making the BF 36.5mm. But no. On the Moravian site the value is given as 33.5 mm. Where did 3mm go?

So, I wrote to Moravian about this and received the following advice:

The low-profile T-thread adapter height is 7.5mm, this is true. But it definitely does not mean the BFD is 7.5mm. For BFD of various camera configurations refer to

http://www.gxccd.com/art?id=396&cat=1&lang=409

What am I missing?

Thanks,

Peter
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Old 09-06-2015, 10:29 AM
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An example. The RHA has a backfocus of 162mm. Being precisely made I assume that is accurate to +/- 1mm.

The measured distance it comes to focus at is 165mm on my scope.

The difference is the +1mm for the 3mm filters and about the same again for the CCD optical window and the remaining 1mm is probably manufacturing tolerance.

There is a note on the astrodon website explaining that 1/3rd the thickness of filters needs to be added to backfocus calculations.

The term "Optical backfocus" applies to this being accounted for. So any glass the light goes through add 1/3rd the thickness. So the CCD window is probably 3mm and your filters may be 3mm (Astrodon or Baader, Astronomik are only 1mm). So you would add 2mm to the mechanical backfocus to get the optical back focus which is the one you need to work off.

From the Astrodon Website:




  • Please note that camera manufactures often specify the MECHANICAL back focus of their products. 3 mm thick filters will ADD 1 mm (0.04") to that mechanical backfocus as measured from the telescope or from the MonsterMOAG prism. The camera window, often 3 mm thick will add another 1 mm (0.04"). So, You may have to add 2 mm (0.09") to the mechanical backfocus.
  • There may be some confusion as camera manufacturers measure backfocus from the focal plane of the CCD to the outer surface of the camera. When they account for the thickness of the filters, the SUBTRACT the 0.04", which is correct as measured from the CCD. However, most people measure backfocus from the back of their scope of from a corrector, and then add/subtract spacers to arrive at the correct backfocus. In this case, as measured from the scope, the 0.04" must be ADDED. A subtle point, but does get people in trouble from time to time.
  • Also, please note that different filter wheels from the same manufacturer may have different backfocus, as indicated in the list above.
    Greg.

Last edited by gregbradley; 09-06-2015 at 10:47 AM.
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Old 09-06-2015, 07:16 PM
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Greg,

I appreciate what you have written. That makes a lot of sense, and of course, one must account for glass thickness in calculating BF.

But on the Moravian site they simply state that the distance from the CCD to the surface of the camera is 29mm. There is no mention of glass. Then in the next image they add what seems to be the low profile t-adapter that is 7.5mm from the bottom flush end of the adapter to the top with female threads. Again, no mention of glass...yet, this adds up to 33.5mm. This isn't confusing (or wrong!)?

Peter
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Old 09-06-2015, 08:16 PM
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Yeah its either wrong or that adapter sits inside a bit.

Greg.
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Old 10-06-2015, 09:57 AM
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Quote:
Originally Posted by gregbradley View Post
Yeah its either wrong or that adapter sits inside a bit.

Greg.
Well, it's wrong as I found out today that it isn't the 7.5mm t-thread adapter shown on that web page. It would be so much more helpful if Moravian actually gave specs for all of their adapters.... But another confusing statement from them said that all the distances account for "all glass." That seems impossible given their cameras with internal FWs. How could they possibly know the thickness of different filter thicknesses one might choose?

Peter
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