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Correct image train and filter placement

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#1 ERHAD

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Posted 14 January 2020 - 02:12 PM

I am about to pull the trigger and buy a TS-Optics  65mm astrograph. It is an F/4.5 -  5 element APO with integrated corrector. The correct distance to the sensor plane is 55mm from the rear M48 port. I intend to put a filter drawer between the corrector and the camera. I basically have two questions regarding the filters and its placement:

 

-1.25' or 2' filters for a ZWO 183, a ZWO 294 and for APS-C sensor? How much of a liability would 1.25 filters be?

 

-Regarding the filter placement, TS has suggested buying a filter drawer with an M48 thread so it could be screwed directly to the corrector, then T2 extensions to complete the 55mm to the sensor. My questions is: is it important where you put the filters in the image train, and would it be better to put the filters closer to the camera (if it makes a difference at all)?

 

Thanks all for your kind input.

 

E.



#2 erictheastrojunkie

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Posted 14 January 2020 - 02:42 PM

Put the filters closest to the camera as possible, I use 1.25" with my 183mm threaded directly into the ASI filterwheel and I have no vignetting at all. 


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#3 Der_Pit

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Posted 14 January 2020 - 02:43 PM

The further away from the sensor the filter is, the larger it has to be to not produce vignetting.  A F/4.5 beam at 50mm from the sensor already needs  11mm diameter for the optical axis, to not be vignetted in the corners you need to add the sensor diameter to this.  E.g., for APS-C (23.5×15.7) the diagonal is 28.2mm, so you'd need 39.2mm clear aperture -> 2" filters.  The 294 has a diagonal of 23.2 -> 34.2.  So you could just go with 36mm unmounted.

 

edit:

Oh, and keep in mind that the filters will count as backfocal distance with a third of their thickness(*).  So BF fill change when you take out filters, or have several of different thickness.

 

(*)i.e., a 3mm filter will make 56mm distance work like 55mm backfocus


Edited by Der_Pit, 14 January 2020 - 02:47 PM.

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#4 ERHAD

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Posted 14 January 2020 - 03:37 PM

The further away from the sensor the filter is, the larger it has to be to not produce vignetting.  A F/4.5 beam at 50mm from the sensor already needs  11mm diameter for the optical axis, to not be vignetted in the corners you need to add the sensor diameter to this.  E.g., for APS-C (23.5×15.7) the diagonal is 28.2mm, so you'd need 39.2mm clear aperture -> 2" filters.  The 294 has a diagonal of 23.2 -> 34.2.  So you could just go with 36mm unmounted.

 

edit:

Oh, and keep in mind that the filters will count as backfocal distance with a third of their thickness(*).  So BF fill change when you take out filters, or have several of different thickness.

 

(*)i.e., a 3mm filter will make 56mm distance work like 55mm backfocus

Thanks for your answer. This concept of filter glass affecting the BF distance is new to me (so much to learn). So, what you are saying is that, even if I have already taken the filter drawer into consideration when calculating my BF, the filter glass, because of its own nature, will "eat" 1/3 of its thickness to the backfocus?

 

My filters are 2mm thick. So, if my calculations are correct I have: 17mm (filter drawer), 20 mm (T2 extension), 17,5mm (camera sensor "depth"). That comes up to 54.5mm total back focus. I would then have to subtract (according to your message) .6mm for the filter, leaving me with 53.9mm. So I need an additional mm to add, somehow, to the train?

 

Thanks!

 

E.


Edited by ERHAD, 14 January 2020 - 03:38 PM.


#5 Der_Pit

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Posted 14 January 2020 - 04:28 PM


Thanks for your answer. This concept of filter glass affecting the BF distance is new to me (so much to learn). So, what you are saying is that, even if I have already taken the filter drawer into consideration when calculating my BF, the filter glass, because of its own nature, will "eat" 1/3 of its thickness to the backfocus?

Yes, exactly.

The 'nature' BTW is the refractive index, and the focus shift scales as (n-1)/n; with typically n~1.5 (glass) that gives the factor of a third:

focusshift.jpg

 


My filters are 2mm thick. So, if my calculations are correct I have: 17mm (filter drawer), 20 mm (T2 extension), 17,5mm (camera sensor "depth"). That comes up to 54.5mm total back focus. I would then have to subtract (according to your message) .6mm for the filter, leaving me with 53.9mm. So I need an additional mm to add, somehow, to the train?

Indeed.  You could try to replace your 20mm T2 extension with a Baader VariLock or something similar.


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#6 ERHAD

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Posted 14 January 2020 - 04:44 PM

Yes, exactly.

The 'nature' BTW is the refractive index, and the focus shift scales as (n-1)/n; with typically n~1.5 (glass) that gives the factor of a third:

attachicon.giffocusshift.jpg

 


Indeed.  You could try to replace your 20mm T2 extension with a Baader VariLock or something similar.

Excellent, that makes sense, and thanks for pointing me in the direction of the Varilock.

 

I just have one more question, if I may: how critical is the distance from the corrector and what would be considered as an acceptable deviation? Would, say, 0.5 or even 1mm be too detrimental, and if so, would it be better to err on the long side or better have a slightly shorter BF?

 

Thanks so much for the great help.

 

E.



#7 Der_Pit

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Posted 14 January 2020 - 05:26 PM

Always glad if I can help grin.gif

No real answer though on the needed accuracy.  The corrector I use has a much longer BF (120mm), and is very uncritical (5mm are not really noticeable).  But I think the shorter the BF itself is, the more critical is the needed accuracy.  I've read posts here from users of 55mm BF correctors that say 1mm is clearly visible and you can locate the optimum to .5mm.  But maybe you get more concise answers from others to this question.

 

In any case, have fun with the new toy!


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#8 P_Myers

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Posted 15 January 2020 - 10:38 AM

 you're allowed (+/- 1mm) when calculating BF    before you get a 1mm spacer   see if you can achieve focus at the moment

 

If not then purchase a 1mm spacer


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