QHY 268 Back Focus?

[DaveS]

Quick question.

Does anyone know the real BF of the QHY268 in use?

I've seen either 17.5 or 23.5mm, and I think the latter is with the dovetail fitting that is needed for attaching it to, well, anything really.

I don't have one yet, but I'm trying to work out various adaptor fittings, and the spacing could make a difference to the filter size requirements.

Think the QHY600 is similar.

[DaveS]

Well, in case anyone is interested (Judging by the resounding silence probably not) I now have it on good authority that the practical figure is 23.5mm

The figure of 17.5 which is widely stated is from the sensor to the end of the dovetail, which doesn't have a thread, so you have to use the supplied quick attachment fitting.

Hope this is of use to somebody.

[AbsolutelyN]

Very interesting. I was very tempted to a 268 and had concluded it was 17.5mm but could not figure out how to attach it to a rasa 8. 

[Allinthehead]

@DaveS https://www.astrobuysell.com/uk/propview.php?view=165814

[DaveS]

Thanks for that.

At the mo I'm still debating as I'm living on savings, don't get my first pensions (Quite small) 'till March next year so am trying to avoid depleting too much.

[kirkster501]

On 11/09/2020 at 19:34, DaveS said:

Thanks for that.

At the mo I'm still debating as I'm living on savings, don't get my first pensions (Quite small) 'till March next year so am trying to avoid depleting too much.

Live within your means Dave.  The camera can wait if needs be.  I am thinking longer term about my astro gear too since I want out of the rat race when I'm 55 (in three years).

[DaveS]

3 hours ago, kirkster501 said:

Live within your means Dave.  The camera can wait if needs be.  I am thinking longer term about my astro gear too since I want out of the rat race when I'm 55 (in three years).

Yes, just so.

I think I will wait at least until march before considering the first one. After that@ We'll see.

[RichardHurst]

On 10/09/2020 at 17:55, DaveS said:

Well, in case anyone is interested (Judging by the resounding silence probably not) I now have it on good authority that the practical figure is 23.5mm

The figure of 17.5 which is widely stated is from the sensor to the end of the dovetail, which doesn't have a thread, so you have to use the supplied quick attachment fitting.

Hope this is of use to somebody.

Its 17.5mm back focus or 23.5mm when using the centering/tilt adjustment ring

Edited September 15, 2020 by RichardHurst
spelling

[DaveS]

1 hour ago, RichardHurst said:

Its 17.5mm back focus or 23.5mm when using the centering/tilt adjustment ring

Well, yes this is what I posted. However, be it noted that the QHY 268, like the 600 does not have an internal thread in the dovetail. Therefore, unless you are prepared to source or machine your own quick release fitting you are pretty much stuck with the QHY offering and the 23.5mm back focus.

[AndyThilo]

If it's 17.5 with the dovetail, does that mean I can use my 16.5mm ZWO filter drawer + 1mm spacer?

[DaveS]

No, because there is nothing to connect the filter drawer to the camera without the QHY adaptor ring.

[DaveS]

Camera drawing Here

Filter wheel showing dovetail adaptor Here

[AndyThilo]

It comes with the dovetail to M54 adapter? Can't I just a M54 to M48 adapter or get the M54 filter drawer instead

Camera is 17.5mm, M54 ring is 6mm? Total 23.5mm So I've got another 31.5mm to play with? I'm really confused lol

[tooth_dr]

36 minutes ago, AndyThilo said:

It comes with the dovetail to M54 adapter? Can't I just a M54 to M48 adapter or get the M54 filter drawer instead

Camera is 17.5mm, M54 ring is 6mm? Total 23.5mm So I've got another 31.5mm to play with? I'm really confused lol

I've been looking at this camera, and although it works with one of my scopes, it wont work with my other scope due to an OAG and lack of back spacing.

But what you are proposing is 100%.  I think Dave was assuming you were trying to connect your adapter directly to the QHY268, which simply isnt possible.

In your case you have 31.5mm to work with, so the 16.5mm filter holder and then take up the rest with spacers and adapters?

Now you have got me thinking - I only ever really image Ha and Lum, so I could just buy a filter drawer system for the other scope, which is thinner than the EFW, and would free up 22-16.5mm = 5.5mm

[AndyThilo]

1 hour ago, tooth_dr said:

I've been looking at this camera, and although it works with one of my scopes, it wont work with my other scope due to an OAG and lack of back spacing.

But what you are proposing is 100%.  I think Dave was assuming you were trying to connect your adapter directly to the QHY268, which simply isnt possible.

In your case you have 31.5mm to work with, so the 16.5mm filter holder and then take up the rest with spacers and adapters?

Now you have got me thinking - I only ever really image Ha and Lum, so I could just buy a filter drawer system for the other scope, which is thinner than the EFW, and would free up 22-16.5mm = 5.5mm

Yeah I’ll need to get an M54 to M48 adapter and then some spacers. I’ll have to measure it all up but in theory I’ll need a 15mm spacer. I have an adjustable baader spacet which might work. I swap my imaging train between my GT81 and RC8 so need a sturdy solution. 
 

[AndyThilo]

Well I think all I need is a new filter drawer, my ZWO one is M42 to M48. I really don't want M42 so I'll get the ZWO M54 version..

[AndyThilo]

Well 268C finally in my hands and back-focus all sorted. The sensor depth is where the centre of the desiccant screw is, so 55mm from there gives the correct back-focus spacing. If you need to use all the spacers, you'll need longer M3 screws. They only supply screws up to 16mm for some reason and you need 20mm for this config.  Hope this helps someone. 

Photo below showing this:

 

And my final spacing:

[Xsubmariner]

Nice simple solution, what spacing have you provisioned for the filters you will be using?

 

[AndyThilo]

I don't bother with that :D.

 

 

I hadn't

[Gianluca67]

I am also interested in this camera, the mono version. I would like to use it with my SX filter wheel but I figure out that the camera has no thread so I am prepared to have a sort of low profile adapter built by a friend of mine that works as lathe operator. However to match this camera to my rig I would need the short back focus version (advertised to be 6.7 mm only). QHY says that the real back focus of this version is actually 12.7 mm with their adapter that perhaps could be threaded to my SX wheel. QHY also states that there is bigger risk to get condensation over the glass regardless of the anti-dew heating system. I am concerned about that. Opinions?

Thank you

[DaveS]

The QHY dovetail adaptor has a M54 thread. SX do a M54 adaptor for the filter wwheel.

[Gianluca67]

Thank you. The dovetail adaptor has a length of 6 mm? 

[DaveS]

To the best of my knowledge, yes.

[Gianluca67]

Does anyone know how to use the centering/tilt adjustment ring? I think it is impractical to align the sensor during an observing session especially if rotating the camera results in chip misalignment. Do you also now the dimensions of the sensor in mm excluding optical black and overscan area?

[kirkster501]

I have several articles about how I do this:

https://skastro.net/unboxing-and-initial-setup-of-qhy268c/

note, website in development.