Backfocus question. TS 72 Photoline ED and ASI 533 MC

[900SL]

My first test shots with an ASI 533 MC Pro and the 72mm Photoline TS Doublet f6 with a 0.8 reducer gave some really poor stars in one corner. I had a 1.25" L2 filter directly in front of the 533MC and a WO rotator in front of the 0.8 flattener/reducer.

I made some adjustments yesterday evening. Removed the L2, used a 2" L3 in front of the reducer, made sure the image train was solidly screwed together front to back and added a 0.5mm spacer at the camera end 

Guiding was around 0.5 RMS

I have stacked 5 lights, no correction frames , done a basic colour calibration and green noise reduction, and a quick autostretch in SiRiL. Jpeg attached.

Would you consider there is room for improvement here? I don't have any analysis software for checking field flatness etc and I'm struggling to determine whether i need to make further adjustments to backfocus or leave as is.. although I'm seeing a slight amount of 'inwardness' around the edges which I think might mean a touch more backfocus distance is required?

 

 

Edited May 1, 2022 by 900SL

[AstroNebulee]

There's only a very minimal amount of inwardness to the stars around the edges I'd say at had to zoom in quite a lot to see that. Maybe try a thicker shim from the 533 box if you wanted. I can't comment on the field flatness software. From the issues I had, I'd be well happy with that 👍

[alacant]

On 01/05/2022 at 16:52, 900SL said:

I don't have any analysis software

Newer versions of Siril have a useful tilt analyser. Red and blue seem to be the worst culprits. Here's red:

HTH

Edited May 3, 2022 by alacant
pantalla

[newbie alert]

What shim are you using?  I did use a delrin type and had strange looking stars in one corner .. wasn't until o took it out and measured it I found out it was thinner on one edge... So I now use the baader metal ones

[900SL]

I have the aluminium ones, from memory came in a pack of three, 1mm, 0.5mm, 0.3mm

I've added the 0.3mm so I'm currently running a test at 0.8mm, good to hear it looks reasonable, and many thanks for the feedback

Have to say I'm amazed at the SXD2 mount out of the box. Running 0.5 RMS guiding with an all star polar alignment so not accurately PA'd. No spikes, just purring away. Very glad I bought this mount, no issues with ASIAirpro. Hope it continues this way.

 

 

[900SL]

OK quick and dirty 1 hr , 15 x 4min with IDAS NBZ dual narrowband. PA not dialled in 100%, using all sky alignment in AAP.

0 gain, slightly clipped, 180s might be better exposure.   Could only track from 30-45 degrees due to obstructions, so shooting through the dust haze at bortle 8. I think I'll use the 1mm spacer, see if I can get those top right stars better, thanks for the comments, much appreciated

 

Edited May 3, 2022 by 900SL

[900SL]

14 hours ago, alacant said:

Newer versions of Siril have a useful tilt analyser. Red and blue seem to be the worst culprits. Here's red:

HTH

Many thanks for that, Alacant. How do you run that? Is it a command from the image processing tools? Or do you need a script?

 

Edit: Found it  https://www.cloudynights.com/topic/800431-siril-10-rc1-is-out-adding-pixel-math/

Edited May 3, 2022 by 900SL

[alacant]

27 minutes ago, 900SL said:

How do you run that?

[900SL]

Thanks A

 

 

[alacant]

Yes, looking good. Don't be tempted to 'fix' it beyond this. It's already fixed!

Edited May 3, 2022 by alacant

[900SL]

1 minute ago, alacant said:

Yes, looking good. Don't be tempted to fix it beyond this!

The green channel is worse but I'll leave it alone

[newbie alert]

I think you're slightly too far away.. not much but adding another spacer might prove that..

[AstroNebulee]

@900SL I hope you don't mind me jumping onto your thread here. I've always suffered with a lot of tilt on my setup and thought I had it just about sorted, though I don't think it's to bad now.   I was quite fascinated by the function in Siril to check the tilt on the images that I ran mine through too.  In all of my images the top right and bottom right are always the higher of the readings suggesting tilt in that direction.  Now my question is as I have a tilt adjuster fitted to my 294mc pro, which of the three adjustment bolts need adjusting to correct this tilt. I have 3 bolts in total all at 120 degrees apart one at the top and 2 at the bottom. The percentage of tilt over the images ranges for 12 to 19%

Cheers Lee

 

 

Edited May 3, 2022 by AstroNebulee
Addition

[900SL]

Actually I'm glad you posted that as I've been thinking about the same question. I assume if you have radial stars in any corner then it's too close there, and tangential too far. Then the image is inverted in the camera so you need to mirror the field adjustments? 

 

[AstroNebulee]

12 minutes ago, 900SL said:

Actually I'm glad you posted that as I've been thinking about the same question. I assume if you have radial stars in any corner then it's too close there, and tangential too far. Then the image is inverted in the camera so you need to mirror the field adjustments? 

 

I'm not actually sure. Looking at the attachments of mine, am I correct in thinking the top right and bottom left are to close to the sensor as the value is higher and needs to be almost the same as the centre value?

Its a shame that the tilt adjuster doesn't have 4 sets of screws all 90 degree's apart so you can easily alter one corner independently of the rest. 

The stars in my image doesn't look to bad zooming in but the level of tilt suggests differently. 

Edited May 3, 2022 by AstroNebulee
Addition

[alacant]

25 minutes ago, AstroNebulee said:

doesn't look to bad zooming in but the level of tilt suggests differently

Exactly. Look at the images, not the tilt graphic.

Just convince yourself I never mentioned it and you'll be be fine!

Edited May 3, 2022 by alacant

[900SL]

1 hour ago, AstroNebulee said:

I'm not actually sure. Looking at the attachments of mine, am I correct in thinking the top right and bottom left are to close to the sensor as the value is higher and needs to be almost the same as the centre value?

Its a shame that the tilt adjuster doesn't have 4 sets of screws all 90 degree's apart so you can easily alter one corner independently of the rest. 

The stars in my image doesn't look to bad zooming in but the level of tilt suggests differently. 

Three point mounting is better than four.. you cant adjust one corner of a 4 point mounting, the other three will resist it. 

I used to use a Theodolite many moons ago..

 

The basic principle is:

 

Adjust the back two to adjust level across the page

Adjust the front wheel to level out of the page (orthogonal)

 

 

Edited May 3, 2022 by 900SL

[AstroNebulee]

1 hour ago, 900SL said:

Three point mounting is better than four.. you cant adjust one corner of a 4 point mounting, the other three will resist it. 

I used to use a Theodolite many moons ago..

 

The basic principle is:

 

Adjust the back two to adjust level across the page

Adjust the front wheel to level out of the page (orthogonal)

 

 

Thank you that does make sense 👍

Edited May 3, 2022 by AstroNebulee