Newbie: Could some one diagnose this issue

[Andy56]

Hi All,

 

I have a new WO ZS61 II with the WO Flattener ZS61A. This is mounted on a Star Adventurer. I'm using a un-modded Canon 600D at ISO800, 60secs, with an Optolong CLS filter. I look over the glorious lights of Newbury.

I had about 50% rejects but I think this is periodic error. Also have the Skywatcher 9x50 with the ZWO 120MM for guiding but I did not know how to use it properly and the balance was poor so I took it off. I'm going to mount it on the counter weight bar when an adapter arrives.

I took some images of M42 for test purposes and also it's quite beautiful and easy to find.

Now the imaged seem to show some optical distortion (for the sake of a more precise word) which I thought I would not see with the WO telescope. The WO flattener is set to 5mm which I believe to be correct from other posts.

In the image below the stars at the top middle seem to be elongated but at the bottom they are not. This concerns me because I would have expected the stars to be elongated in the corners if the flattener was not set correctly.

 

Also I had some issues focussing using the Bahtinhov mask and ended up using the live view on the camera so it could be a focus issue as well

Any thoughts or am I being to fussy.

 

Regards

Andy

 

Edited March 10, 2021 by Andy56
Show photo

[symmetal]

It looks like the sensor is not quite perpendicular to the scope optical axis commonly called 'image tilt'. The bottom left is very good focus and as you progress towards the top right the stars get more elongated and point towards the image centre. Field flatteners tend to give this effect, which gets worse the further away from the centre, when the flattener spacing is incorrect, generally when the spacing is not enough, but this depends on the flattener.

The spacing distance looks fine at the bottom left but not at the top right probably due to the tilted sensor. Focuser 'flop' in the imaging train due to gravity can cause this so check everything is tight particularly the camera rotator ring on the front of the flattener. Also with the rotator lock loose, check the nylon grub screws around this rotator ring are tightened the same amount to where you just start to feel resistance and you can rotate the ring smoothly without it flopping about.

The rotator ring is before the flattener elements so being tilted at that point won't affect the flattener spacing itself, but can affect the overall focus through the flattener which can give effects visually similar to spacing errors.

Putting a filter after the flattener requires the flattener spacing distance to be increased by a third of the thickness of the filter. For a 2mm thick filter it's around 0.7mm. Also you can possibly increase the flattener distance further to see if the top right improves without it adversely affecting the bottom left too much.

The 5mm spacing you mentioned is I presume the distance visible on the scale when the adjustment locking ring is tighted against the adjustable section as shown here. It's normally set to 12.7mm on the adjustable section and then the ring tightened against it. Try setting it to around 13.5mm to start with to account for the filter you've used and see if things are better overall.

Alan

Edited March 11, 2021 by symmetal

[Andy56]

Hi Alan,

Many thanks for you great reply.

Now that you've pointed it out, the stars in the bottom left are in focus and top left not so. Also it's more noticeable on the brighter stars at the top. I will take a photo of my set up each time now so I can relate focus to orientation.

I've now check older images with camera lenses and they don't seen to show this as the mechanical coupling is much tighter.

I'll set it up today and check for sloppiness.

I took another look at the flattener and, oops, I forgot about the locking ring (newbie). It is set at approx. 13mm (I struggled to release the locking ring and the adjuster moved a little). I do note that the adjuster is quit sloppy before locking.

I can't find any sloppiness in the rotator on it's own but I'll set it up and try it then.

Now I have to wait for some clear skies to test it again.

I guess pointing it vertical will remove focuser flop so that's worth a try.

I'll check for sloppiness and post some more images when I get another chance i.e. a few minutes of clear skies.

Now to sorting the periodic error.

Many thanks 

Andy

 

PS the Optolong CLS filter is 1mm so the 0.3mm of correction for me.

Edited March 11, 2021 by Andy56
Spelling and PS

[Andy56]

HI,

Another question.

The WO AF61A is adjustable but on their web page they say adjust it to 12.9mm. This assumes a lot ie the T ring adapter will compensate for all flange distances/cameras. Also the diagrams are for the non-adjustable one.

So how is the calculation done?

I remember reading about it some time ago but I can't find it now.

Kind regards

Andy

[Newforestgimp]

4 hours ago, Andy56 said:

HI,

Another question.

The WO AF61A is adjustable but on their web page they say adjust it to 12.9mm. This assumes a lot ie the T ring adapter will compensate for all flange distances/cameras. Also the diagrams are for the non-adjustable one.

So how is the calculation done?

I remember reading about it some time ago but I can't find it now.

Kind regards

Andy

I have the same set up (not the star adventurer) I made sure to buy the WO Canon adapter, it was a much better fit than the generic one i was using. Mine is set to the 12.9mm suggested on the WO site, i did have a filter inline and increased this to 13.7mm but to be honest I've literally just removed the filter as I'm not convinced and gone back to 12.9mm NO FILTER. Will be interested if you manage to get it sorted WITH filter.

I'd also like to know what figure to adjust to if adding a filter drawer in this setup ? 

Regards

Andy

 

Edited March 11, 2021 by Newforestgimp

[Andy56]

Hi,

I've just set the rig up and noticed that there is slack between the t-ring adaptor and the camera flange. All other joints/couplings seem OK.

If it clears up soon I'll try a piece of paper in the bottom and to and see if the results change. If this fixes the problems I'll try the WO version it's £7 more.

It's supposed to be clear from about 8 onwards but there will be a breeze.

Regards

Andy

Edited March 11, 2021 by Andy56

[symmetal]

Hi Andy,

The back focus distance for the 61A is 67.7mm as shown at the end of the WO info page. This is with the FF adjustable section set to zero. Canon back focus is 44mm, the T adaptor is 11mm thick making 55mm. So you need to add the difference, 67.7 - 55.0 via the adjustable section on the flattener. This is 12.7mm. Close to the 12.9mm specified setting on the pictures. This difference may be to help account for possible filters present.

As you say taking a test image pointing vertically should help indicate if droop is causing the problem. As you've found not all T-ring adapters are exactly the same so changing it may help eliminate some looseness.

Correcting tilt is one of the more frustrating challenges to solve. 😬 Good luck.

Alan

Edited March 11, 2021 by symmetal

[Andy56]

Andy: On finding my T-ring is loose I've ordered the WO version.

Alan:

Thanks for the info. I couldn't work out whether the diagram was correct because it didn't seem to be for the adjustable version. Also I wasn't sure if it was A or B.

So with a 1mm filter (my T-ring adaptor is 9.77 Astro Essentials):  44 - 9.77 - 0.3 = 13.63mm

1 hour ago, symmetal said:

Correcting tilt is one of the more frustrating challenges to solve.

I haven't started on the Star Adventurer yet. I rejected 50% of 60sec subs the other night. I made a video of all the subs and it has periodic error. Not sure how much the guider will compensate for, when I can mount it sensibly. But lets get the telescope fixed first.

Regards and thanks

Andy

 

 

Edited March 11, 2021 by Andy56
Typo

[symmetal]

Yes the dimensions diagram is for the non-adjustable Z61 but it looks like the distance from the M48 rear attachment seems to be the same as the adjustable one, perhaps that accounts for the 0.2mm difference in the diagram and the photos. Distance B is from the M54 thread inside the non-adjustable flattener which is accessible by removing the M48 rear plate.

You need to increase the FF spacing distance if you add filters so your 67.7 flattener spacing would increase to 68.0mm, which happens to be very close to the 67.9 figure used in the photo where 12.9mm is shown on the adjustment in the photo. I think your equation may be assuming this but is not quite right as shown. 🙂

With your narrower T-ring adapter from the more common 11mm thickness, your camera + T adapter is 44 + 9.77mm = 53.77mm, so your setting on the FF adjustment should be 68 - 53.77 = 14.23mm. In reality a spacing error of 0.5mm or so shouldn't make a noticeable difference visually. For full frame sensors it's more critical of course. Try setting it to just 14.0 on the adjustment and see if the star shapes improve top right (assuming there is still some tilt) and the bottom left stars don't get worse.

Alan

[Andy56]

HI,

I did the calculation on a spread sheet and did not copy them over correctly. Apologies. Should have been 67.7 - 44 - 9.77 - 0.3 = 13.63mm

I've set the flattener to 13.63mm (or as near as I could get it using the marking on the flattener.

So I checked the focus train and found the camera to t-ring was not good. I have wedged a single sheet of paper between the T-ring adapter and the camera at the bottom so it effectively lifting the back of the camera.

I've also made sure the balance and focus were correct and  I have achieved this :

which to me is much better. (CR2 file below). The only elongation I'm getting now is along the RA travel b ecause my Star Adventurer has significant periodic error. I took someone suggestion and left the adventurer on to 48hrs at x12. The results are better but it could also be better balancing.

So when the WO t-ring adapter arrives I'm hoping I won't need the paper.

Also its good seeing tonight with the breeze.

Kind regards

Andy

 

 

L_3545_ISO800_60s__17C.CR2

[symmetal]

Hi Andy,

Yes those stars are much better so you've found the culprit causing the tilt which is good. Hopefully the WO T-Ring should be a snugger fit. 🙂

4 hours ago, Andy56 said:

I did the calculation on a spread sheet and did not copy them over correctly. Apologies. Should have been 67.7 - 44 - 9.77 - 0.3 = 13.63mm

This should be 67.7 - 44 - 9.77 + 0.3 = 14.23mm as the filter increases the specified FF spacing distance as it modifies the actual light path like this

 (Picture from CN forum)

This has caused much confusion in the past on many forums, including SGL, as some camera manufacturers have said that adding filters effectively reduces the back focus distance, but they are referring to the camera back focus from the lens flange to the sensor and not the FF spacing distance which confusingly is sometimes called the FF back focus.

In your case with the thin filters, the 0.6mm difference in FF spacing calculated is probably not enough to cause any star issues.  Often the actual FF distance to get the best star shapes can vary by a millimetre or so from that specified, so if they aren't good at the calculated distance try varying the distance by a bit. 

Alan

[Andy56]

Many thanks for the info.

I've re-set it using your calcs and checked it with a digital calliper (14.53mm) so I'll use it like this and try +/- a couple of mm to see what's best.

I managed 25 subs (5 darks at ISO800) last night and after stacking in Sequator and a quick stretch using Gimp I got:

 

 

There are a few positional issues; a few twigs from my neighbours hedge causing diffraction, and light pollution from the town due south but much better.

I think it's over exposed and there are tracking issues and the dark frames haven't got rid of all of the hot pixels.

At least I don't get the chromatic aberration from the vintage lenses I been using.

Many thanks 

Andy

 

Edited March 12, 2021 by Andy56
Grammar

[Newforestgimp]

8 hours ago, symmetal said:

Hi Andy,

Yes those stars are much better so you've found the culprit causing the tilt which is good. Hopefully the WO T-Ring should be a snugger fit. 🙂

This should be 67.7 - 44 - 9.77 + 0.3 = 14.23mm as the filter increases the specified FF spacing distance as it modifies the actual light path like this

 (Picture from CN forum)

This has caused much confusion in the past on many forums, including SGL, as some camera manufacturers have said that adding filters effectively reduces the back focus distance, but they are referring to the camera back focus from the lens flange to the sensor and not the FF spacing distance which confusingly is sometimes called the FF back focus.

In your case with the thin filters, the 0.6mm difference in FF spacing calculated is probably not enough to cause any star issues.  Often the actual FF distance to get the best star shapes can vary by a millimetre or so from that specified, so if they aren't good at the calculated distance try varying the distance by a bit. 

Alan

Just to expand on this Alan, what happens if from the calculation you end up with a negative value ?

I've been considering a Filter drawer into this image train, so is it....
67.7 - 44 - 11 - 17(filter drawer) + 0.3  = -4 ???
does this mean I cant add a filter drawer into the train ?

Andy H

[fifeskies]

There are extra thin low profile "T" adapters that can help you gain enough space to fit in a filter drawer.

They claim to only need about 1mm rather than the usual 11mm or so.

 

[symmetal]

4 hours ago, Andy56 said:

Many thanks for the info.

I've re-set it using your calcs and checked it with a digital calliper (14.53mm) so I'll use it like this and try +/- a couple of mm to see what's best.

I managed 25 subs (5 darks at ISO800) last night and after stacking in Sequator and a quick stretch using Gimp I got:

There are a few positional issues; a few twigs from my neighbours hedge causing diffraction, and light pollution from the town due south but much better.

I think it's over exposed and there are tracking issues and the dark frames haven't got rid of all of the hot pixels.

At least I don't get the chromatic aberration from the vintage lenses I been using.

Many thanks 

Andy

 

That's looking pretty good Andy. The bottom right is actually showing slightly more star elongation than the rest of the frame, compared to your previous image so possibly your paper tilt adjustment has shifted a bit or it's better with your previous 13.63mm setting. When you get the WO flattener everything should be more stable. 🙂

3 hours ago, Newforestgimp said:

Just to expand on this Alan, what happens if from the calculation you end up with a negative value ?

I've been considering a Filter drawer into this image train, so is it....
67.7 - 44 - 11 - 17(filter drawer) + 0.3  = -4 ???
does this mean I cant add a filter drawer into the train ?

Andy H

 Hi Andy H,

Yes, as you've calculated above, the filter drawer makes the spacing difference too great even at 0.0 on the flattener adjustment. As fifeskies mentioned your best option is the narrow T-ring adapter which should allow you to use the filter wheel.

Alan

[Andy56]

6 hours ago, symmetal said:

The bottom right is actually showing slightly more star elongation than the rest of the frame

This is good. I means it is very likely to be the t-ring and not the telescope/flattener.

 

So I've processed M42 as well as I can with my experience:

 

Overall I'm impressed. I think with more subs it would be less grainy. 

Cheers

Andy

[symmetal]

41 minutes ago, Andy56 said:

This is good. I means it is very likely to be the t-ring and not the telescope/flattener.

Yes, that's right. In my last post I meant to say it should be better when you get the WO T-Ring, rather than the WO flattener. 😁

More subs is the only cure for noisy backgrounds. 🙂 You've crushed the background a bit too much on your last image. The previous one is better in that respect.

Alan

[Andy56]

Hi Alan,

By crushed I assume you mean it's too dark.

I've learned a lot from tutorials today.

I'll have another go so I can remember  the methods.

Cheers

Andy

[symmetal]

Yes, that's right. The night sky isn't black and the consensus is that the sky background should be 23,23,23 on the standard RGB readout of your photo editing software, displayed as 8 bit values from 0 to 255. You lose subtle detail when it's too dark though if your image is very noisy making it a bit darker can make it look better as some of the noise is less noticeable.

Alan