Running out of inwards focus tube travel with new monochrome camera

[Richard_]

For the past few months I have been using my current setup just fine. I've recently changed from a OSC camera (ASI533MC-Pro) to a mono camera (QHY268M) and I seem to be having issues with back focal distance and the amount of focus tube travel left, to the point where I run out of it. I'd appreciate if someone could point out a stupid mistake I've made! See below picture illustrating the issue, along with a more descriptive background further on.

Current setup

• WO FLT 120 refractor (120mm aperture, F6.5)
• WO Flat 7 adjustable reducer (x0.8, bringing telescope to F5.2)
• ASI533MC-Pro camera
• Mini-EFW with UV-IR and L-eXtreme filter

New setup

• WO FLT 120 refractor (120mm aperture, F6.5)
• WO Flat 7 adjustable reducer (x0.8, bringing telescope to F5.2)
• QHY268M monochrome camera
• QHYCFW3 with LRGB-SHO 36mm filters

 

According to WO, my flattener requires a back focus distance of 71.4mm, which I achieved with my ASI533MC-Pro. To achieve this back focus distance, my ASI533MC-Pro already had 56mm backfocus using the spacers provided, then I added a 10mm spacer along with setting the flattener distance to 5.80mm. This brought me up to 71.8mm, just above 71.4mm but everything looked fine in terms of stars in corner of frames so I didn't bother fine tuning any further. To achieve focus, the focus tube had a distance of approximately 20mm. This has worked great for me up until the camera change.

I tried to repeat the above exercise with my QHY268M without much success. When using 71.8mm of back focus distance, the stars are much more elongated in the corners than the ASI533 but as this camera has a larger sensor, perhaps fine tuning is required. What I didn't expect was that my focus tube travel reduced all the way down to 2mm to achieve focus! Going by the star diagram from OPT, elongated stars means I need to move the camera away from the sensor which I can do by increasing the adjust distance on my flattener/reducer from 5.8mm to a larger distance. I actually tried distances between 5.5mm and 6.3mm but each time I increased the back focus distance, I needed to increase inward focus tube travel to achieve focus. Before I could get the right back focus distance, I ran out of inwards travel in the focuser. To give some perspective, at 20mm focus distance with my ASI533, my autofocuser is at about 20,000 steps. When achieving focus at 6.3mm back focus with my QHY268M, I had just 700 steps for my OIII filter, whilst LRGB were about 1,300 steps! The autofocus routine runs at 300 step between exposures so this becomes an issue with OIII.

I've added some diagrams and pictures of the two setups below, including distances etc. along with pictures of the stars using the "aberation inspector" in NINA for the different adjust distances.

1. Should the focal distance
2. Can anyone point out something stupid I've done?

 

 

 

 

[Clarkey]

A couple of things come to mind. Firstly, I assume the sensor distance from the camera front to the sensor is the same for the ASI and QHY? In theory the focus point with the same kit should be at the same place so the 20mm difference does not make sense. Secondly the star shapes seem to be getting worse as the distance increases (although I am looking on a small screen). Are you sure it does not need to be closer?

Probably not much help, but it is all I can think of.

Edited July 10, 2022 by Clarkey

[Richard_]

9 hours ago, Clarkey said:

A couple of things come to mind. Firstly, I assume the sensor distance from the camera front to the sensor is the same for the ASI and QHY? In theory the focus point with the same kit should be at the same place so the 20mm difference does not make sense. Secondly the star shapes seem to be getting worse as the distance increases (although I am looking on a small screen). Are you sure it does not need to be closer?

Probably not much help, but it is all I can think of.

That's right. Both ASI and QHY cameras provide spacers to achieve the suggested 55-56mm (depending on filter use or not) which is correct for most flatteners/reducers. My reducer requires 71.4mm so I just added an extra 10mm spacer to 56mm plus I set the adjust distance on my reducer to get into this 71.4mm ball park. Unless I've done something wrong in the setup, I was expecting to remove my ASI with 56mm back focus spacers and replace with my QHY with 56mm of back focus spacers as a straight swap and get the same focus. Focal distances should be driven by the optics, not the camera. 

Yeah I was thinking the same thing about the star shapes, it doesn't make sense that they get worse as the above diagram shows those star shapes is a result of camera being too close. 

I think my next step is to swap out to the ASI camera and see if I can repeat my old focus position of 20mm. If so, then that means the QHY does indeed require a different back focus / focal distance. If not, then I've made a mistake somewhere in my setup. 

[scotty38]

Am I understanding that you're also adjusting the FLAT 7A to try and gain focus? If so you can't do that, you have to set it to what it should be for the scope in question (in this case 1.4mm with additional adapter) and leave it. The back focus is then from the back of the Flat 7A using whatever adapters necessary. If that's what you're doing then ignore me 🙂

Edited July 11, 2022 by scotty38

[Richard_]

1 hour ago, scotty38 said:

Am I understanding that you're also adjusting the FLAT 7A to try and gain focus? If so you can't do that, you have to set it to what it should be for the scope in question (in this case 1.4mm with additional adapter) and leave it. The back focus is then from the back of the Flat 7A using whatever adapters necessary. If that's what you're doing then ignore me 🙂

Haha I won't ignore you pal! The WO flattener has an adjustable distance of up to 10mm. So with 55mm back focus and the extra 10mm on the flattener gives you 65mm back focus, which is short of the 71.4mm required. The $50 adapter literally just gives you an extra 10mm of adjustable distance, which is no different to adding a simple 10mm spacer, which is what I do (worked great with the ASI533). With the QHY268M, this back focus left me with elongated stars which implied I needed to add more back focus. 

I'm not trying to use this adjustable distance to focus the image, I'm using this adjustable distance to fine tune the back focus. Each time you adjust back focus, you need to re-focus the image and I'm finding that when I add back focus, my focus tube moves inwards to re-focus the image. I'm at the point where I can't move the tube inwards any further, yet my back focus is still not achieved. Does that make sense? 

Edited July 11, 2022 by Richard_
Wording

[AstroMuni]

3 hours ago, Richard_ said:

I was expecting to remove my ASI with 56mm back focus spacers and replace with my QHY with 56mm of back focus spacers as a straight swap and get the same focus. Focal distances should be driven by the optics, not the camera. 

Is the backfocus distance the same for both cameras?

[scotty38]

17 minutes ago, Richard_ said:

Haha I won't ignore you pal! The WO flattener has an adjustable distance of up to 10mm. So with 55mm back focus and the extra 10mm on the flattener gives you 65mm back focus, which is short of the 71.4mm required. The $50 adapter literally just gives you an extra 10mm of adjustable distance, which is no different to adding a simple 10mm spacer, which is what I do (worked great with the ASI533). With the QHY268M, this back focus left me with elongated stars which implied I needed to add more back focus. 

I'm not trying to use this adjustable distance to focus the image, I'm using this adjustable distance to fine tune the back focus. Each time you adjust back focus, you need to re-focus the image and I'm finding that when I add back focus, my focus tube moves inwards to re-focus the image. I'm at the point where I can't move the tube inwards any further, yet my back focus is still not achieved. Does that make sense? 

ok but I'm still not sure we're on the same page, we might be but I'm not sure. I know the 7A is adjustable, I have the 6A version and it's adjustable to take account of different scopes and as you say to (very) fine tune but it needs to be in the ball park of its design for the scope. For your setup it needs to be set to 1.4mm just like in my setup it needs to be 7.2mm. Is yours set at 1.4mm because if it's not then I am assuming that's why you're having issues. Of course I understand if you need it to be 1.3 or 1.4 or 1.5 etc etc to allow for the vagaries of measuring and engineering and so on but it should not be set at 5 or 10 if you see what I mean as it's then way out of the design parameters for that scope.

Once you've done that using the expensive or cheap adapter you then have the 71.4mm of backfocus from the end of the 7A to the sensor to find.

Edited July 11, 2022 by scotty38

[Richard_]

4 minutes ago, AstroMuni said:

Is the backfocus distance the same for both cameras?

They are I use the ASI533 with filter wheel so ZWO provide spacers to achieve 56mm. The QHY268M provide spacers to achieve 56mm too. 

[Richard_]

2 minutes ago, scotty38 said:

ok but I'm still not sure we're on the same page, we might be but I'm not sure. I know the 7A is adjustable, I have the 6A version and it's adjustable to take account of different scopes and as you say to (very) fine tune but it needs to be in the ball park of its design for the scope. For your setup it needs to be set to 1.4mm just like in my setup it needs to be 7.2mm. Is yours set at 1.4mm because if it's not then I am assuming that's why you're having issues. Of course I understand if you need it to be 1.3 or 1.4 or 1.5 etc etc to allow for the vagaries of measuring and engineering and so on but it should not be set at 5 or 10 if you see what I mean as it's then way out of the design parameters for that scope.

We're both on the same page the rotating part of the adapter can only move out so far. With the provided adapter, you cannot reach 71.4mm of back focus with the FLT120, so WO made an adapter to replace the one fitted to the reducer. The replacement adapter is about 10mm longer than the one already fitted to the reducer so you gain an extra 10mm back focus, but this can also be achieved with a regular 10mm spacer. 

https://williamoptics.com/products/accessories/longer-adjusting-adapter-for-flat68iii-flat7a

The main number is the 71.4mm back focus. It doesn't really matter if you use their adapter or not, so long as the overall back focus (including sensor depth, spacers and adjust distance) all sum up to 71.4mm. At least that's my understanding. 

[scotty38]

15mm spacer according to the WO page so if you only added 10mm and wound the adapter out a further 5mm to compensate then you have messed things up as you've changed the "focus" of the flattener.

What I'm saying is 7A at 1.4mm plus 45mm extension does not equal 7A at 6.4mm plus 30+10mm extension.

[Richard_]

20 minutes ago, scotty38 said:

15mm spacer according to the WO page so if you only added 10mm and wound the adapter out a further 5mm to compensate then you have messed things up as you've changed the "focus" of the flattener.

Ah apologies, yes it's 15mm not 10mm! So in my calculations, I made up that extra 15mm with a 10mm spacer and adding on the 1.4mm adjust distance until back focus was achieved. For my ASI533, this was an adjust distance of 5.8mm. 

22 minutes ago, scotty38 said:

What I'm saying is 7A at 1.4mm plus 45mm extension does not equal 7A at 6.4mm plus 30+10mm extension.

Wait, is that right? I only thought the adjust distance was to "add on" to the existing 55/56mm made up with spacers provided with the camera. Looking at the diagram in the link I provided, rotating the spacer just moves the camera back and forth from the flattener, it doesn't actually move any optics. How is that different to using a spacer? 

[scotty38]

I would have to check but I am pretty sure adjusting the flattener adjusts the optics. As long as it's at 1.4mm you should be fine however you achieve the rest as you say.

[AstroMuni]

1 hour ago, Richard_ said:

They are I use the ASI533 with filter wheel so ZWO provide spacers to achieve 56mm. The QHY268M provide spacers to achieve 56mm too. 

See if this article throws some light on your issue? https://www.cloudynights.com/topic/783742-help-with-back-focus-measurements-mating-a-qhy268mm-with-an-fsq85-±-x-101-flattener/

[Richard_]

Alright, I think we "may" have a case of ID-10T going on 😂

I just checked my ASI533 and wondered why there was an extra spacer on there. Oh wait, it's the 10mm spacer, so what's on the QHY268M? The goddamn 20mm spacer! So instead of hitting 71.4mm back focus I was actually getting +10mm so was hitting 81.4mm back focus! 

I think I know how it happened too. The only dust covers I have are for normal M48 spacers (51mm OD). If you look past the 20mm spacer on the QHY filter wheel, you can see much wider spacers which are wrapped with velcro ties (71mm OD). I don't have any dust caps to fit that. I now remember putting the 20mm spacer on just so I could put a dust cap on it which I would then swap out with the 10mm spacer when putting on the telescope. I must've forgotten to do the latter and thought I had a 10mm on this whole time. 

I'll be eagerly awaiting the next clear night to see if this has resolved the issue. Remember kids, measure thrice, not twice 😅

Thanks everyone for your support! Hopefully this one gets put to bed over the next few days. 

 

Left: QHY camera, filter wheel, spacers and the 20mm spacer

Right: ASI camera, filter wheel, 11mm, 16.5mm and 10mm spacer. 

[Clarkey]

I'm glad I'm not the only one.....😂😂

[scotty38]

Fingers crossed....

[SoftwareDeveloper]

Hello, this is my first post to SGL, so be kind with replies! TIA..

Richard, and other interested people-

Its possible to measure the focal length of your telescope using astrometry.net. Take a photo of any star field with each camera and upload it to astrometry.net. In the results for each image (typically) on the fourth line of the calibration result summary is a field "Size: WIDE x HIGH deg". Wide is the horizontal (wider) field of view for the image.

Using the cameras pixel width: 3008 for the ASI533MC, and the pixel width in micrometers: Pixel Size: 3.76µm (from the camera spec web page) compute the horizontal width in millimeters for your camera: xWIDTHmm = 3008 * 0.00376mm -> Width_mm=11.31008. Be careful not to use the diagonal size of the focal plane that would mess up the calculation.

Now use the following equation:
FocalLength_mm = Width_mm / (2.0 * (tan(FOV_X) / 2.0))

With either camera, the measured focal length should result in the same value (within about a millimeter). If not, there is a problem. If the measured focal length is significantly (more than 1% ish) different than the manufacturers spec for the telescope, again something may be wrong.

Note: these measurements should be made with the filter wheel to an OPEN position. Filters change the focal length due to the refractive index of the filter material being different than that of air.  Also if your calculator can only work in radians, be sure to convert FOV_X to radians before calling the tan function, otherwise some strange results would occur..

That may or may not help with your picking spacers for the your cameras, but it does check the basic math.

Hope that provides a useful tool for evaluating issues like this.

End of first post!

 

[iapa]

QHY268M Back focus is 12.5mm per Modern Astronomy.

the ASI533MC has 6.5mm back focus per FLO.

So that the source of 6mm of your difference.