FF/FR Backfocus help

[Adam1234]

Hi all, I'm after a bit of advice of getting the correct backfocus with the 0.85x field flattener/focal reducer for the ED80. 

I had a random clearish sky last Thursday amongst all the cloudy days we've had recently...so naturally I jumped at the opportunity to test out the backfocus on the FF/FR. I've still got some work to do to get it working at it's best and get pinpoint stars it seems, as I still have comet shaped stars around the edges.

Currently I have it set up as follows per the instruction manual for the ZWO ASI 1600mm Pro:

FF/FR > M42-M48 16.5mm extender > T2 11mm extender > M42-M42 adapter (male-male) > ZWO filter wheel > ZWO ASI 1600mm pro

This gives me the 56mm backfocus that ZWO says is required (55mm + 1mm for filters?). I have measured the distance from the FF/FR to the front of the camera using calipers as 49.5mm.  + 6.5mm to the sensor gives me 56mm. The below image obviously shows that I need to refine the backfocus. It looks like the stars at the edges are going towards the centre, so am I right in saying that I need more distance between the FF/FR and the camera sensor? If this is correct (I hope so as I don't know how I will get less distance), how much more distance do you guys think is needed?  

Thanks, Adam

 

[MarkAR]

Definitely too close, as for how much I think it'll be down to trial and error.

 

[Adam1234]

21 minutes ago, MarkAR said:

Definitely too close, as for how much I think it'll be down to trial and error.

 

Thanks. At least I know which direction to go now. Trial and error it is! I've got some thin spacer ring things to increase the distance, what increments do you reckon to go in? 0.5mm a time?

[MarkAR]

If you have a 5 or 10mm spacer then try that first. It looks like it would be more than the odd millimetre out.

Edited July 5, 2020 by MarkAR

[Adam1234]

16 minutes ago, MarkAR said:

If you have a 5 or 10mm spacer then try that first. It looks like it would be more than the odd millimetre out.

Ok thanks, I've got an 8mm spacer, so I'll try that and then go from there

[geeklee]

@Adam1234 You've got a pretty definitive answer from @MarkAR but I wanted to check the measurements as I've put two cameras on my 80ED with SW FR/FR and been a little liberal with spacing on a mono with filters and seen nothing like the above.  Even seeing some other pictures of incorrect spacing in the past has shown minor trails. The SW FF/FR seems pretty plug and play on small to mid size sensors.

Your description and measurements sound absolutely spot on and based on ZWO's guidance.  Here's my OSC this time, that needs 55mm spacing.  Does the 48.5mm distance mark look the same as your 49.5mm (that takes into account the filter with ZWO guidance for wheel)?  FR/FF look the same?

Just belt and braces...

 

Edited July 5, 2020 by geeklee

[Adam1234]

49 minutes ago, geeklee said:

@Adam1234 You've got a pretty definitive answer from @MarkAR but I wanted to check the measurements as I've put two cameras on my 80ED with SW FR/FR and been a little liberal with spacing on a mono with filters and seen nothing like the above.  Even seeing some other pictures of incorrect spacing in the past has shown minor trails. The SW FF/FR seems pretty plug and play on small to mid size sensors.

Your description and measurements sound absolutely spot on and based on ZWO's guidance.  Here's my OSC this time, that needs 55mm spacing.  Does the 48.5mm distance mark look the same as your 49.5mm (that takes into account the filter with ZWO guidance for wheel)?  FR/FF look the same?

Just belt and braces...

 

Hi Lee, here's my set up & measurements

[Adam1234]

Ok so I've added in an 8mm spacer, bringing my backfocus to around 64mm and it still looks like the stars are pointed towards the centre (i.e. more distance needed). hmmm....

[david_taurus83]

I've seen both kinds of aberration either side of optimum.. That chart is useless imo. Your clearly too far away and you still have stars pointing inward.

You need small adjustments. Try it the other way first. 16.5 + 21 + 11 + camera. No filterwheel. See what its like at 55mm.

 

[geeklee]

38 minutes ago, Adam1234 said:

Ok so I've added in an 8mm spacer, bringing my backfocus to around 64mm and it still looks like the stars are pointed towards the centre (i.e. more distance needed). hmmm...

Something still doesn't seem quite right. This is a well known, matched FF/FR with a clear, documented back focus requirement of 55mm (little more with filter in path to sensor).  What about taking things out of the equation next - filter wheel (add appropriate spacing back to 55mm).. and even the FF/FR (to check an image straight from scope only)

It seems such a common route you've taken with the ZWO image chain and SW FF/FR... in theory what you did first should have been fine (maybe tiny tweaks later)

Edited July 5, 2020 by geeklee

[MarkAR]

That's weird, it does actually look worse.

It might help taking shots of the same target at same length of exposure. You'll be able to tell more accurately how much each change affects things by the length of star elongation.

[Adam1234]

55 minutes ago, david_taurus83 said:

I've seen both kinds of aberration either side of optimum.. That chart is useless imo. Your clearly too far away and you still have stars pointing inward.

You need small adjustments. Try it the other way first. 16.5 + 21 + 11 + camera. No filterwheel. See what its like at 55mm.

 

Hi, I've tried it without the filter wheel and used the 16.5 + 21 + 11 + camera to make 55mm, it doesn't look much better

 

[Adam1234]

This is without the FF/FR (taken previously before I got the FF/FR)

It actually looks better without

[Adam1234]

9 minutes ago, MarkAR said:

That's weird, it does actually look worse.

It might help taking shots of the same target at same length of exposure. You'll be able to tell more accurately how much each change affects things by the length of star elongation.

I was going to take shots of the crescent nebula same as the first image, but it wasn't quite above the hedge in my garden yet tonight 

[MarkAR]

Interesting, without the FF/FR there is a slight blur inwards, with the FF/FR Crescent image there is a very weird change.

The centre 3rd looks ok, the intermediate 3rd looks like there is a circular distortion as if it's too far out and the outer 3rd looks like it's too close with the blur radiating outwards.

Wild guess is that you have the wrong FF/FR or it's duff.

[Adam1234]

6 hours ago, MarkAR said:

Interesting, without the FF/FR there is a slight blur inwards, with the FF/FR Crescent image there is a very weird change.

The centre 3rd looks ok, the intermediate 3rd looks like there is a circular distortion as if it's too far out and the outer 3rd looks like it's too close with the blur radiating outwards.

Wild guess is that you have the wrong FF/FR or it's duff.

I don't think it's the wrong one as it's the one optimised for the SW Evostar ED80 Pro, which I have.  

I sure hope it's not a duff, I was waiting 3 months for this to arrive 🙈.

Is there anything I could be overlooking?

[david_taurus83]

If you've bought new, it needs to go back. Ive not owned one but all the example pics I've seen show good correction with this flattener even with a DSLR. Adding the reducer should certainly not make it worse which is the case here.

[Adam1234]

23 minutes ago, david_taurus83 said:

If you've bought new, it needs to go back. Ive not owned one but all the example pics I've seen show good correction with this flattener even with a DSLR. Adding the reducer should certainly not make it worse which is the case here.

Thanks David, I bought it from FLO so I'll speak to those guys. One other thing I will try is see what images with my DSLR come out like

[Adam1234]

Is there anything else I can try before I speak to FLO about a replacement? Just in case there's something I'm missing 

[MarkAR]

I take it it can not be put on back to front so my only idea now is does it rattle when shaken?

[Adam1234]

7 hours ago, MarkAR said:

I take it it can not be put on back to front so my only idea now is does it rattle when shaken?

Just checked, it can't be put on back to front (that would have been embarrassing), nor does it rattle. 

Next things I'm going to try are decrease the spacing to below 55mm/56mm by replacing the 11mm extender with an 8mm extender to give 53mm, then maybe add some thin spacers to get close just under 55mm, then maybe try it with my dslr. Maybe also try at 57mm. 

Failing that, contact FLO I guess

[david_taurus83]

Or see if someone will lend you a known good reducer and try that to see if it is in fact your one thats the problem.

[Adam1234]

44 minutes ago, david_taurus83 said:

Or see if someone will lend you a known good reducer and try that to see if it is in fact your one thats the problem.

@geeklee has very kindly offered to lend me his actually so I may take up that offer 

Edited July 7, 2020 by Adam1234

[retret66]

Definitely a bad copy and have that replaced. IT could be a factory defect where they slap in a ED80 ring on a ED120. There are three versions of these. ED80,ED100 and ED120.. The only logical explanation is what you have could be for ED120.

Edited July 8, 2020 by retret66

[Adam1234]

5 hours ago, retret66 said:

Definitely a bad copy and have that replaced. IT could be a factory defect where they slap in a ED80 ring on a ED120. There are three versions of these. ED80,ED100 and ED120.. The only logical explanation is what you have could be for ED120.

Well that's reassuring - I emailed FLO late last night and got a reply this morning, they have had a few others email with the same issue so it seems to be a batch issue and have informed Sky-Watcher. They are going to replace it when the next batch arrives towards the end of the month.

At least I can use this time to have a practice with the ASI1600mm pro and refine my exposure times, assuming I get at least a couple clear skies 🤔 If no clear skies I'll just go fishing instead 🎣

Edited July 8, 2020 by Adam1234