C8 back focus question

[stargazermanc]

After reading everyone's advice, I have decided to abandon taking a test stack without the reducer as I don't think I have a mirror issue and I think my collimation is good, I think my issue is the back focus distance which in my case is not the 105mm as stated in multiple sources but probably longer...

I need to take some new test images as I have messed about with that many combinations now that I have forgotten where I got up to. So on the next clear night, I will setup with 105mm back focus, take a test stack of Capella, review and take it from there.

[stargazermanc]

So last night I managed to do some testing before the clouds rolled in. I managed to take 10 x 60sec exposure of Capella at 3 different back focus lengths (105 mm, 109.5 mm & 126 mm) based on the spacers I had available, each time I refocused using the ZWO EAF. Each image is the stacked output and stretched.

There is definitely something not right, I am going to recheck my collimation but what do people think?

[Same old newbie alert]

I'd agree that collimation could do with a slight adjustment.. how are you doing this just visually?

 

[stargazermanc]

No. I'm using my camera and defocus inwards and outwards to ensure its good in both directions. 

Which backfocus would you say is the closest?

[jacko61]

At face value, the third photo looks to be closer to the correct back focus than the other two. Looking at the first one, the stars on the left of the pic all seem to be rushing away from the centre which is what we'd expect if the sensor is too close.  However, the stars down the right hand side don't seem to show anywhere near as much distortion.  It's possible that this is due to poor collimation but it could also be caused by some tilt somewhere in the imaging train. While the third image seems to have better back focus, the focus overall is poor. What are you using to focus?  If you're using a bahtinov mask, I've read it's best to focus on a star in one of the thirds rather than a central object. 

Graeme

 

[stargazermanc]

34 minutes ago, jacko61 said:

At face value, the third photo looks to be closer to the correct back focus than the other two. Looking at the first one, the stars on the left of the pic all seem to be rushing away from the centre which is what we'd expect if the sensor is too close.  However, the stars down the right hand side don't seem to show anywhere near as much distortion.  It's possible that this is due to poor collimation but it could also be caused by some tilt somewhere in the imaging train. While the third image seems to have better back focus, the focus overall is poor. What are you using to focus?  If you're using a bahtinov mask, I've read it's best to focus on a star in one of the thirds rather than a central object. 

Graeme

 

Hi the focus is handled by the eaf and asiair pro. I am going to check the collimation again on the next clear night

Edited February 22, 2022 by stargazermanc

[Same old newbie alert]

2 hours ago, stargazermanc said:

No. I'm using my camera and defocus inwards and outwards to ensure its good in both directions. 

Which backfocus would you say is the closest?

I always approach focus from one direction, same with collimation I approach from the same way, go through focus and get my donut..The reason I do this is because on my scope my donut is different either side of focus,as you can see... So I focus in the + direction on my XLT version, go through focus and collimate from there

[stargazermanc]

1 hour ago, Same old newbie alert said:

I always approach focus from one direction, same with collimation I approach from the same way, go through focus and get my donut..The reason I do this is because on my scope my donut is different either side of focus,as you can see... So I focus in the + direction on my XLT version, go through focus and collimate from there

 

 

Thanks for the tip.

[catburglar]

I’ve just platesolved the images on astrometry.net:

the first image at 105mm spacing has a pixel scale of 0.596arcsec per pixel- which gives an effective focal length of around 1301mm.

the last image at 129mm has a scale of 0.638 arcsec per pixel which gives an effective focal length of 1215mm.

So the first one is closest to the spec of 0.63 reduction factor.

 

The other thing that’s odd with those images is that the coma appears to be pointing in the same direction (almost horizontally) across the entire image- if it was simply a spacing issue I’d expect the coma to be more radially symmetrical. But I’m sorry I don’t know what else might explain it. Could it be tilt- it's certainly much worse on the left side of the frame?

Edited February 22, 2022 by catburglar
Updated ? tilt?

[catburglar]

I platesolved the middle image at 109.5mm spacing- it works out at 0.603 arcsec per pixel and an effective focal length of 1286mm which is almost exactly on spec for a 2032mm fl and 0.63 reducer- which should give an effective focal length of 1280.

Still not sure what's causing the dodgy star shapes.

[stargazermanc]

4 minutes ago, catburglar said:

I platesolved the middle image at 109.5mm spacing- it works out at 0.603 arcsec per pixel and an effective focal length of 1286mm which is almost exactly on spec for a 2032mm fl and 0.63 reducer- which should give an effective focal length of 1280.

Still not sure what's causing the dodgy star shapes.

So you think the second image is the more accurate back focus distance?

I am going to check the collimation. Once I have sorted that and got as close as I can on the backfocus then I can start looking at the tilt.

Edited February 22, 2022 by stargazermanc

[catburglar]

2 minutes ago, stargazermanc said:

So you think the second image is the more accurate back focus distance?

I think so:

The C8 has a specified focal length of 2032mm so for a 0.63 reducer you'd expect to be somewhere around 2032*0.63=1280mm

The specs of the camera are 3.76micron pixels and 23.5mm x 17.5mm dimension for the imaging chip.

From the platesolved image (rearranging the formula pixel_scale=pixel_size*206.265/focal_length) gives focal_length=3.76*206.265/0.603=1286mm.

So that's pretty close to the target EFL.

 

[michael8554]

I would stop trying to get 1280mm FL, and concentrate on getting the best Back Spacing, based on images.

Image 1  Best right hand side, Image 2 worse, image 3 worst  right hand side.

Image 1  Poor left hand side,  Image 2 worse, image 3 best left hand side.

So image 1 (105mm back spacing) could be the best if the tilt were corrected.

I'd say you have varying Tilt, or the primary mirror is very sloppy.

What magnification are you using for those Airy disks ?

You need around x400 for the final tweaks.

Michael

[stargazermanc]

5 minutes ago, michael8554 said:

 

What magnification are you using for those Airy disks ?

You need around x400 for the final tweaks.

Michael

None, it is being done using a camera as I said in a previous reply. 

In terms of the tilt, I don't think this is the camera side as I use it regularly on my redcat and zs73 with zero tilt. It could be my mirror which has been suggested on another forum.

[michael8554]

19 minutes ago, stargazermanc said:

None, it is being done using a camera as I said in a previous reply. 

It's Catch 22 that you need high-mag to accurately collimate, which you don't get with the camera, but do with a high-power eyepiece.

But risk the camera setup not having a similar collimation, as in your case, due to tilt.

Perhaps you need a Zero Shift Focuser, so that the Primary doesn't have to be moved.

Michael

 

[stargazermanc]

So this evening I managed to set up and the collimation was indeed out so I spent some considerable time making alterations at different focus and as you can see from the pictures, the collimation is as good as I can get it with a camera versus a high powered eyepiece.

I checked several times going out of focus and back in so pretty confident that it is now sorted. Unfortunately the clouds rolled in so I didn’t have time to take any proper test stacks but from the test shot I did get, it does look like a tilt issue which could be my next headache.

[Same old newbie alert]

Collimation looks good, so I'd rule that out..

Tilt is very possible, especially with a 2600,there are several thread on it on here as well as you tube