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Back Focus with Flattener/Reducer (I'm lost...)

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#1 avaniperen

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Posted 14 January 2021 - 03:30 AM

Good Day Dear Astrophotographers,

 

After spending years on star trail photography, I made the decision that the pandemic was an good time to expand my interest and invest in a decent camera, telescope, mount, etc. Hence, i bought a used mount, a new ASI533MC, a new Zenithstar 61 II APO with adjustable flat61 for Z61, a Zwo 1.25" filterwheel with Optolong L-Enhance and Zwo Dou-Band filters.

 

Everything came in perfectly, and I'm ready for some clear skies. However, before i'm going out and about, I'm a bit lost in the back focus measurements. I do understand that Zwo needs the 56 mm back focus, like mentioned on their website (see first attachment). The thing that I'm struggling with is from where to where do you measure, I mean from the end of the telescope of from the end of the reducer? And flat on the surface of the camera or do I need to add a few mm's? And last, I did see on other topics that you need to calculate the filter as well however some users do add 1 mm others 0.3 mm...

 

I have included a picture of my configuration and I would appreciate it if you could maybe draw in some markers of the start and end of the measurements.

 

Thanks in advance, I really appreciate your time with reading my post!

 

Alexander

 

 

Attached Thumbnails

  • Screenshot 2021-01-14 at 11.04.52.png


#2 avaniperen

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Posted 14 January 2021 - 03:32 AM

Sorry, I forgot to add the picture of my telescope and camera...

 

 

Attached Thumbnails

  • Zenithstar 61 II APO.jpg


#3 Cometeer

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Posted 14 January 2021 - 03:35 AM

The specs for the Flat61 says it requires 67.7mm of backfocus with the Z61. This backfocus distance is measured from the base of the threads on the reducer. The backfocus is required by the reducer, not the ZWO camera. 55/56mm is a fairly common backfocus requirement, so that's why ZWO provides the necessary parts to achieve that. 

 

The ZWO diagram above shows you how to achieve 56mm of that that 67.7mm backfocus. Since the Flat61 reducer is adjustable, you can adjust it to add an additional 11.7mm of backfocus. Add an additional 1/3 the thickness of the filter (ex: add 1mm if you are using 3mm thick filters).


Edited by Cometeer, 14 January 2021 - 03:39 AM.


#4 Ernest_SPB

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Posted 14 January 2021 - 03:52 AM

I am not sure, but it seems required back focus (flange distance from M48x0.75 of Flat61) shall be equal 62.9 mm (see https://williamoptic...Test Report.jpg). It is close to 56 mm specified above. Difference in 6.9 mm is tolerable, but you can look for corresponding M48 (or T2) extension ring. Correction to glass of filter (1/3 of its thickness) is neglectable.


Edited by Ernest_SPB, 14 January 2021 - 03:54 AM.


#5 Jarno

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Posted 14 January 2021 - 05:29 AM

As mentioned by cometeer, the backfocus distance is a requirement of the flattener. You need your camera to be in the Goldilocks zone: too close and you won't get enough correction, too far and you'll get overcorrection. In my experience you don't have to get the distance correct down to the last fraction of a mm because there are always tolerances:

1. The backfocus distance the flattener requires also depends on the focal length of your telescope. That focal length can also be a bit off and might even change with temperature.

2. The metal of your connectors will expand or shrink with temperature.

3. Can you really screw on all parts to the end of their threads? I often find they don't screw on completely due to anodizing and/or tolerances which makes the parts a few mm longer than nominal.

 

So don't worry too much about the precision, some fine tuning WILL be required. As long as you are within plus or minus a few mm it'll be allright. And even if it isn't, the worst thing that'll happen is that the stars in your corners won't be perfect. In that case just make a few test images with different distances until you get close to optimal. You might want to crop off a bit around the edges anyway to get rid of vignetting.

 

Jarno



#6 avaniperen

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Posted 14 January 2021 - 03:53 PM

Thank you so much for all your feedback! Lessons learned 1) always reach out to others, I totally thought I had it under control, 2) don't assume that an adjustable flattener means that you can only adjust the sensor frame with the object in the sky... I did not know that I could adjust the Flat61A for the back focus...

 

Again, thanks for your support!

 

Alexander


Edited by avaniperen, 14 January 2021 - 03:54 PM.


#7 RogerM

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Posted 14 January 2021 - 07:30 PM

Alex,

 

My setup is nearly identical to yours (exception is the camera model).

 

    Camera backfocus: 6.5mm

Filter wheel thickness: 20 mm

----------------------------------

                                   26.5 mm total

 

For the Flat61A:

  Required backfocus: 67.7 mm

                                 - 26.5 mm  (subtract camera/filter wheel distance)

                       =============

                        Need:  41.2 mm

 

Ext Rings:  16.5 mm

                   21.0 mm

           ==========

                  37.5 mm

 

Remaining backfocus distance:  41.2 mm

                           Ring distance: -37.5 mm

                          ===================

Remaining Backfocus distance:   3.7 mm <-- subtract 1/3 filter thickness per cometeer's comment and adjust the flattener to this number.



#8 avaniperen

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Posted 15 January 2021 - 03:46 AM

Good Morning Roger,

 

Thank you for your reply, your feedback makes it very clear to me. One small thing that I would like to bring up:

  1. I subtract 1/3 filter thickness? Some users suggesting that you should add the  1/3 thickness  of the filter.

So my calculations would be:

67.7 mm - 26.5 mm - 37.5 mm - 0.8 mm (1/3 of filter) = 2.9 mm

67.7 mm - 26.5 mm - 37.5 mm + 0.8 mm (1/3 of filter) = 4.5 mm

 

Thanks for your feedback,

Alexander



#9 Cometeer

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Posted 15 January 2021 - 02:23 PM

The wording can be confusing. Filters SUBTRACT from the optical backfocus distance to the camera, so to fix that, you ADD to the mechanical backfocus requirement of the reducer.

A filter shortens the optical path by about 1/3 its thickness. To compensate you should lengthen the adapter by the same amount.

Edited by Cometeer, 15 January 2021 - 02:24 PM.


#10 RogerM

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Posted 15 January 2021 - 03:42 PM

Good Morning Roger,

 

Thank you for your reply, your feedback makes it very clear to me. One small thing that I would like to bring up:

  1. I subtract 1/3 filter thickness? Some users suggesting that you should add the  1/3 thickness  of the filter.

So my calculations would be:

67.7 mm - 26.5 mm - 37.5 mm - 0.8 mm (1/3 of filter) = 2.9 mm

67.7 mm - 26.5 mm - 37.5 mm + 0.8 mm (1/3 of filter) = 4.5 mm

 

Thanks for your feedback,

Alexander

 

 

The wording can be confusing. Filters SUBTRACT from the optical backfocus distance to the camera, so to fix that, you ADD to the mechanical backfocus requirement of the reducer.

A filter shortens the optical path by about 1/3 its thickness. To compensate you should lengthen the adapter by the same amount.

 

Thanks for the correction, Cometeer!  Yes, I don't know why I said subtract as the filter itself shifts the focus point further away from the reducer's flange.  So yes, the final filter thickness value is added to the optical path length...

 

Reducer backfocus: 67.7 mm

your filter thickness:   0.8 mm

======================

     Total Backfocus: 68.5 mm

 

    Camera/FW backfocus: 26.5 mm

                         Ext. Rings: 37.5 mm

                    ==================

                                            64.0 mm

 

Residual backfocus: 68.5 mm - 64.0 mm = 4.5 mm




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