Getting the backfocus just right.

[powerlord]

This was shot with my altair 80ED with 0.8FF. It was shot with the zwo Ha filter, and with the backfocus recommended by zwo(*). that is:

asi1600 + EFW + M42 adapter + 11mm +16.5mm : 0.8FF/OTA

6.5 + 20 + 2 + 11 + 16.5 = 56mm

Perhaps they are saying 1mm for the filter itself, but that seems too much ?

Comparing the edges of the image with a pic online (**) supposedly showing the pattern you get with the sensor too far from and too close to the lens, this looks like it is too far from the lens ?

Am I on the right track ? I believe 0.3mm is generally allowed for a NB filter, so I should remove 0.7mm from the chain somehow ?

 

*  https://astronomy-imaging-camera.com/tutorials/best-back-focus-length-solutions-55mm.html

** https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus

[Same old newbie alert]

Have you measured the adapters as a 10mm spacer is rarely 10mm

And 1/3 rd of the thickness of the filter, doubt a zwo is 3mm thick

Edited February 13, 2022 by Same old newbie alert
Added info

[powerlord]

yeh they all measured/verified. yup my expectation is filter is 1mm thick, so need 0.3. just seems weird ZWO should recommend a full mm.

i was thinking I replace the 16.5 with a 15mm and then screw it out a bit and start taking test subs at slightly different distances out. find what works and then measure the distance I've screwed it out.

[michael8554]

14 hours ago, powerlord said:

This was shot with my altair 80ED with 0.8FF. It was shot with the zwo Ha filter, and with the backfocus recommended by zwo(*)

ZWO have a fixation with advising cameras be setup with 55mm backfocus, even without a FF or FR.

It's the FR or FF that determines the backfocus, so you should use the backfocus recommended by the FF maker, not the camera maker.

Because not all FF/FRs have 55mm backfocus  eg Meade/Celestron 0.63 FR is 105mm.

I couldn't find a 0.8FF for the 80ED, only a 0.85FF, there is a 0.8FF for the 60EDF ?

But you're in luck, both have a 55mm Backfocus.

Michael

[teoria_del_big_bang]

14 hours ago, powerlord said:

yeh they all measured/verified. yup my expectation is filter is 1mm thick, so need 0.3. just seems weird ZWO should recommend a full mm.

i was thinking I replace the 16.5 with a 15mm and then screw it out a bit and start taking test subs at slightly different distances out. find what works and then measure the distance I've screwed it out.

Which filters exactly are you using, I have seen ZWO filters at 1mm thick but most are 2 mm thick but I do not think any are 3 mm thick..

So that would mean a BF of 55.7 mm is required.

But as mentioned on previous posts above do not rely on manufactures specification of either FW thicknesses or the spacers. You need to measure with digital verniers. I have found filter wheels to be over 1 mm different to the thickness they specify and spacers certainly 0.5 mm. I think the ASI1600 was pretty accurate on mine anyway.

Your idea of screwing one spacer out a bit sounds a bit risky at what if the weight of the  FW starts the whole thing unscrewing even more. I prefer to get a set of thin (sub 1mm) spacers and keep adding them and moving camera out bit by bit until you end up past the optimum BF and then look at stars in corners and see when they change from looking too close to too far away and hopefully one will be just right with round stars.

I bought a couple of sets of THESE

Steve

 

[powerlord]

2 hours ago, michael8554 said:

ZWO have a fixation with advising cameras be setup with 55mm backfocus, even without a FF or FR.

It's the FR or FF that determines the backfocus, so you should use the backfocus recommended by the FF maker, not the camera maker.

Because not all FF/FRs have 55mm backfocus  eg Meade/Celestron 0.63 FR is 105mm.

I couldn't find a 0.8FF for the 80ED, only a 0.85FF, there is a 0.8FF for the 60EDF ?

But you're in luck, both have a 55mm Backfocus.

Michael

I get that. But ZWO are saying "use this for 55mm backfocus" - and then show an imaging chain that is 56mm.

Yeh my FF is 55mm.

[powerlord]

2 hours ago, teoria_del_big_bang said:

Which filters exactly are you using, I have seen ZWO filters at 1mm thick but most are 2 mm thick but I do not think any are 3 mm thick..

So that would mean a BF of 55.7 mm is required.

But as mentioned on previous posts above do not rely on manufactures specification of either FW thicknesses or the spacers. You need to measure with digital verniers. I have found filter wheels to be over 1 mm different to the thickness they specify and spacers certainly 0.5 mm. I think the ASI1600 was pretty accurate on mine anyway.

Your idea of screwing one spacer out a bit sounds a bit risky at what if the weight of the  FW starts the whole thing unscrewing even more. I prefer to get a set of thin (sub 1mm) spacers and keep adding them and moving camera out bit by bit until you end up past the optimum BF and then look at stars in corners and see when they change from looking too close to too far away and hopefully one will be just right with round stars.

I bought a couple of sets of THESE

Steve

 

prob them then. Ive not measured the filter thickness. that'd mean I'm only .3mm off though. Doesn't seem likely that that amount of image aberations could be due to  that small amount of difference, especially with a wee asi1600 sensor ?

Maybe more likely focus tube is a wee bit tilted or something ?

stu

[teoria_del_big_bang]

15 minutes ago, powerlord said:

prob them then. Ive not measured the filter thickness. that'd mean I'm only .3mm off though. Doesn't seem likely that that amount of image aberations could be due to  that small amount of difference, especially with a wee asi1600 sensor ?

Maybe more likely focus tube is a wee bit tilted or something ?

stu

Yes I agree 0.3 mm error in BF should not be really noticeable, but if you go on the manufacturers dimensions for spacer and FW dimensions from experience you could be over 1mm in error, you shuld always measure these things yourself (not the filter I do just go on manufacturers spec for those and add 1/3 on to BF.)

I must admit I only read your OP and did not look at the images myself, if the stars are elongated or teardrop shaped in some corners and not other then yes that will be tilt but if all four corners are comparable then it is BF (my understanding anyway 🙂 )

Steve

[Clarkey]

I would not take the back focus as gospel. Some reducers flatteners seem to worm as published others need adjustment. My f4 coma corrector is extremely picky, but other flatteners seem to work at 55mm or there about. I would just experiment to see what works.

[Same old newbie alert]

I'd also recommend some spacers than your unscrewing method but I'd recommend solid fittings rather than plastic.. I had slight tilt using delrin type, so I measure the thickness and they differed from one side to the other...

So I bought some baaders...job done, no more tilt

[powerlord]

So er.. I think I might have screwed up here. Looking for advice.

I'm using this:

https://www.altairastro.com/lightwave-08x-reducer-290-p.asp

which I think is the same as this:

https://www.astronomics.com/astro-tech-0-8x-reducer-field-flattener-for-f-6-refractors.html

Now, I'd assumed I was aiming for 55mm to sensor from red line below.. but astronomic mention 'shoulder' and I think that corresponds the the actual optic lens in reducer maybe. green below ?

So I should really be aiming to get distance from green line (i.e. lens) to sensor to be 55mm ?

[Alien 13]

I have personally found that even an error of 0.1mm is noticeable in an image, have lots of thin shims at the ready..and as peviously mentioned check actual distances with a caliper.

Alan 

Edited February 19, 2022 by Alien 13

[ollypenrice]

Astrodon filters are 3mm thick and Baaders 2mm.  1mm strikes me as being unlikely...

Olly

[scotty38]

NINA are not far off releasing a plugin to assess backfocus, tilt etc and remove the trial and error.... Should be huge.....

[jacko61]

6 hours ago, powerlord said:

So er.. I think I might have screwed up here. Looking for advice.

I'm using this:

https://www.altairastro.com/lightwave-08x-reducer-290-p.asp

which I think is the same as this:

https://www.astronomics.com/astro-tech-0-8x-reducer-field-flattener-for-f-6-refractors.html

Now, I'd assumed I was aiming for 55mm to sensor from red line below.. but astronomic mention 'shoulder' and I think that corresponds the the actual optic lens in reducer maybe. green below ?

So I should really be aiming to get distance from green line (i.e. lens) to sensor to be 55mm ?

I'd say you should be measuring from the red line.  the altair site says this.. For DSLR cameras, a T-Thread adaptor of approx. 8-10mm thickness is required.

Given a DSLR has 44mm from the sensor to the lens mounting plate there's no way you could have 55mm from the green line. 

the astro-tech says . Back focus is 55mm from the shoulder of the reducer to the image plane, the standard spacing for 35mm and DSLR camera bodies

The shoulder is the edge next to the thread - again, your red line. The standard spacing for dslr cameras is 44mm from  the sensor plus the 11mm T ring 

Graeme

Edited February 20, 2022 by jacko61
spelling

[powerlord]

thanks Graeme, you are correct. I just heard back from Altair who said the same. drat - thought that might be an easy fix. well, just need to move it out and in a bit. got a set of metal spacers from FLO