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Please Help! QHY268C with Canon Lens and Filter Drawer Back-Focus.

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#1 benringel22

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Posted 15 February 2021 - 11:30 PM

Hi everyone,

 

I have a back-focus issue and would appreciate any wisdom others more experienced than myself can offer.

 

I recently got a QHY268C, and would like to use it with a Canon lens and my Starizona Filter Slider (for an IR-cut filter or other LP filters). I will need to order some sort of adapter (for example this one) to connect the lens to the camera.

 

My issue is that I do not have enough back-focus for the filter drawer and lens adapter. Does anyone have an idea of how I could incorporate this filter slider into this camera-lens combination? The back focus dimensions are below.

 

Relevant back-focus dimensions:

- Camera sensor to "flange": 23.5mm

- Canon lens adapter: 10mm (smallest I have found)

- Starizona filter slider: 17.5mm

- Canon lens back-focus: 44mm

- Current minimum flange-sensor distance: 51mm (7mm over assuming no other adapters are required!)

 

If anyone has any creative solutions or suggestions I would be very grateful to hear them.

 

Thanks in advance!

 

~ Ben

 



#2 Tapio

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Posted 15 February 2021 - 11:56 PM

That is the problem when using DSLR lenses with astro cameras (with long back focus).

Maybe if you can live without camera angle adjuster your camera bf reduces to 17.5mm.

What filter are you going to use ?


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#3 benringel22

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Posted 16 February 2021 - 12:06 AM

That is the problem when using DSLR lenses with astro cameras (with long back focus).

Maybe if you can live without camera angle adjuster your camera bf reduces to 17.5mm.

What filter are you going to use ?

Thanks for the reply!

 

I would love to not use the angle adjuster to gain the 6mm, but I don't understand how to attach anything to the camera without it, as it contains the flange for attaching spacers and extension tubes. Do you know of a way of bypassing the angle adjuster for the QHY268C?

 

Also, I will at least need to use an IR-cut filter, but I would also like to use my broad band LP filter and occasionally my Optolong L-Extreme dual narrow band filter when the lens is at higher f-stops.

 

Thanks for the help!

~Ben



#4 1DegreeN

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Posted 16 February 2021 - 09:29 PM

I have the 168C and it is similar in construction (so far as I can tell) to the 268C and also includes in the box the centre angle adjustment adapter with its female M54 thread interface. On the QHY web page for the 268C specifications it implies that the CAA is optional but doesn't give any examples of how to connect to the camera without it. On my 168C, the circular dovetail flange (which the CAA is tightened on to) is also threaded - for M54 it appears although I've never tried. However there doesn't seem to be much depth to the thread and the heads of the 4 screws that secure the dovetail flange to the face plate of the camera protrude a couple of mm and may interfere with any attempt to attach an adapter. If you can do without the CAA then you save 6mm backfocus and if your filters are 3mm thick you have to add 1mm (one third the filter width) to backfocus so you would then be spot on, assuming you didn't need any additional adapters (e.g. thread converters). If you have anything with a male M54 thread then I guess the first step is to try screwing it into the dovetail flange of your 268C and making sure it can fit securely and measure the thread depth for your backfocus calculation.

 

Not sure I understand your comment re filters - you only need 1 filter: an LP or dual narrowband filter will also cut out the IR.

 

For my 168C, I use a Cyclops Optics BladeC adapter to connect the camera (with CAA) to my Canon lenses. It can hold a 2" threaded filter but it wouldn't be compatible with your filter slider. They support the 268C too, may be an option to consider if you can't source an alternative solution.


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#5 benringel22

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Posted 16 February 2021 - 11:21 PM

Thanks for the suggestions!

 

I have the 168C and it is similar in construction (so far as I can tell) to the 268C and also includes in the box the centre angle adjustment adapter with its female M54 thread interface. On the QHY web page for the 268C specifications it implies that the CAA is optional but doesn't give any examples of how to connect to the camera without it. On my 168C, the circular dovetail flange (which the CAA is tightened on to) is also threaded - for M54 it appears although I've never tried. However there doesn't seem to be much depth to the thread and the heads of the 4 screws that secure the dovetail flange to the face plate of the camera protrude a couple of mm and may interfere with any attempt to attach an adapter. If you can do without the CAA then you save 6mm backfocus and if your filters are 3mm thick you have to add 1mm (one third the filter width) to backfocus so you would then be spot on, assuming you didn't need any additional adapters (e.g. thread converters). If you have anything with a male M54 thread then I guess the first step is to try screwing it into the dovetail flange of your 268C and making sure it can fit securely and measure the thread depth for your backfocus calculation.

 

For my 168C, I use a Cyclops Optics BladeC adapter to connect the camera (with CAA) to my Canon lenses. It can hold a 2" threaded filter but it wouldn't be compatible with your filter slider. They support the 268C too, may be an option to consider if you can't source an alternative solution.

 

I unfortunately have not been able to find any way at all to use the QHY268C without the CAA, and I have read online that it is a compulsory component, meaning that you must sacrifice the 6mm. I have discovered a way to thread a two inch filter using the given spacers and some M48 extensions. I have attached a picture of this setup below for anyone else having the same struggle.

 

It is good to know about the Cyclops Optics BLADE-C; thank you for pointing that out! This may offer slightly better modularity if it is easier to change the filter without disassembling the entire spacer assembly.

 

My current QHY268C/EOS lens back-focus solution:

QHY268C with EOS Lens Back-Focus Arrangement

 

 

Not sure I understand your comment re filters - you only need 1 filter: an LP or dual narrowband filter will also cut out the IR.

 

I was probably unclear about the filters; sorry about that. I will need at the very least the UV IR-cut filter, or any of the other filters I mentioned, but definitely only one. lol.gif

 

Overall, I have realized that it doesn't currently seem there is any solution out there to incorporate any filter drawer with this camera and an EOS lens. The additional 6mm of the CAA is devastating towards this cause. This may be the biggest difference between the QHY268C and the ZWO ASI2600MC Pro, since the ZWO ASI2600MC Pro allows for threading directly to the front flange, and there is even a dedicated filter drawer EOS adapter that would be absolutely perfect... If the ability to change filters while using a camera lens without disassembling the all your spacers (which will take quite a lot of time in the dark) is valuable to you and some of the added features of the QHY268C are not, I definitely recommend you get the ZWO ASI2600MC Pro.

 

If anyone else has thoughts or comments please let me know. I have a lot to learn!

 

~ Ben



#6 t-ara-fan

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Posted 18 February 2021 - 02:47 AM

The QHY website says backfocus is 17.5mm. The tilt adjuster is optional and adds 6mm. Just take off the tilt adjuster.

#7 benringel22

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Posted 18 February 2021 - 11:13 AM

The QHY website says backfocus is 17.5mm. The tilt adjuster is optional and adds 6mm. Just take off the tilt adjuster.

Unfortunately this is not possible. The tilt adjuster is not optional, but is required to thread anything to the camera. QHY's manual is actually very misleading about this, as I thought that this would be the case before purchasing the camera when it is definitely not.

 

This can be seen in the below drawing from the manual. The shoe that is shown on top of the camera contains no threads and no bolt holes to attach a plate/spacer to. This means that spacers and extension tubes can be attached exclusively using the tilt adjuster, and that the 6mm must be lost (unless you were to custom machine something).

 

QHY268C Drawing From Manual

 

As I said before, if this is important to anyone, I do not recommend buying this over the ZWO ASI2600MC Pro.



#8 1DegreeN

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Posted 18 February 2021 - 09:03 PM

It may be worthwhile asking this question over on the QHY forum. I notice on the web page it states that the telescope interface for the 268C without CAA is M48 using the "standard adapter" which I interpret as meaning the circular dovetail flange should have internal M48 threads. If in fact there is no threading then QHY need to clarify  - and/or ask the same question to the vendor who sold you the camera.

On my 163M, the standard dovetail flange is attached to the faceplate by 4 small screws and can be removed and replaced with another adapter fixed by those same 4 screws e.g. I have an adapter with male M54 threads. These are intended particularly for their mono cameras as a low BFL option for attaching to a filterwheel but they may have something similar for the 268C.


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#9 benringel22

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Posted 18 February 2021 - 11:14 PM

It may be worthwhile asking this question over on the QHY forum. I notice on the web page it states that the telescope interface for the 268C without CAA is M48 using the "standard adapter" which I interpret as meaning the circular dovetail flange should have internal M48 threads. If in fact there is no threading then QHY need to clarify  - and/or ask the same question to the vendor who sold you the camera.

On my 163M, the standard dovetail flange is attached to the faceplate by 4 small screws and can be removed and replaced with another adapter fixed by those same 4 screws e.g. I have an adapter with male M54 threads. These are intended particularly for their mono cameras as a low BFL option for attaching to a filterwheel but they may have something similar for the 268C.

That is super helpful!

 

You are 100% right, the webpage does say that it should be possible... As shown in the image below, the circular dovetail on mine does not have threads on the inside (which would be perfect).

 

QHY Front Flange/Dovetail

 

QHY268C Front Flange/Dovetail

 

I've noticed that this is the same for other QHY268C-PH cameras in online reviews such as this one, however, in this review of a different version of the QHY268C, the flange does seem to contain threads. Is this a different flange that I can simply purchase and replace my current one with? Also, are these four small screws the ones shown in the picture above?

 

Also, it is a good idea to post this to the QHY forum, and I will do that as soon as I am able. Thanks so much for this information; this might be a breakthrough so I am very grateful.

 

~ Ben



#10 cccha

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Posted 24 February 2021 - 06:22 AM

Hello In fact, QHY provides a very complete adapter ring system, through the 6 screw holes on the center inclination adjustment ring, you can combine multiple sizes of interfaces. Including Canon or Nikon solutions, you can check here whether the combined drawings can meet your requirements

 

https://www.qhyccd.c...atid=193&id=114



#11 1DegreeN

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Posted 24 February 2021 - 09:20 PM

Hello In fact, QHY provides a very complete adapter ring system, through the 6 screw holes on the center inclination adjustment ring, you can combine multiple sizes of interfaces. Including Canon or Nikon solutions, you can check here whether the combined drawings can meet your requirements

 

https://www.qhyccd.c...atid=193&id=114

But with the QHY adapters there doesn't seem to be any way to include a filter drawer, specifically Ben's Starizona filter drawer. Is there any alternative to the center inclination adjustment ring i.e. any way to connect directly to the face of the camera? This would save 6mm of BFL.


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#12 benringel22

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Posted 25 February 2021 - 04:58 PM

But with the QHY adapters there doesn't seem to be any way to include a filter drawer, specifically Ben's Starizona filter drawer. Is there any alternative to the center inclination adjustment ring i.e. any way to connect directly to the face of the camera? This would save 6mm of BFL.

That is exactly right.

 

I posted on the QHY forum and confirmed that there is no way to use the camera without the CAA. This means you should not plan to use the first reported back focus of 17.5mm, as the actual back focus distance will be 23.5mm with the compulsory CAA.

 

If having 17.5mm of back focus is important to anyone, you will have to go with the ZWO ASI2600MC Pro, which allows the camera to be used at 17.5mm of back focus.

 

~ Ben




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