Cant reach proper focus. Help

[newbie alert]

6 hours ago, Elp said:

Also an image of how you've mounted the camera. I suspect your backspacing isn't set right.

Makes no difference with a small sensor

Scrap that .. read it as a 462,which I assumed being used for planetary..

Edited January 27, 2023 by newbie alert
Changed

[matija]

23 minutes ago, teoria_del_big_bang said:

You still have not said how you are actually focussing.

What software are you using when focussing the ccd ?
Are you using some sort of live view and if so what is the refresh rate of the camera ?

Steve

Hi Steve.

Im focusing by using live view in sharpcap at 200ms and mid to high gain. I rotate the knob manually.

[matija]

20 minutes ago, newbie alert said:

Makes no difference with a small sensor

Scrap that .. read it as a 462,which I assumed being used for planetary..

its a 485 which is a planetary camera but im 48.5mm off

[vlaiv]

There might be a chance that you have a faulty mirror.

That sort of softness that is visible in the image is indicative of spherical aberration.

Spherical aberration with newtonian can either be due to simple coma corrector used - like 2 element sky watcher x0.95 one, or due to poor parabola on the mirror.

This will only show as softness when viewing at high power - like planetary and lunar views and of course - like stars being little balls in high resolution images.

Alternative to that would be that seeing was really poor on particular night and that created bloated stars.

If you want to check if your scope has spherical aberration (you say you did not use coma corrector) - take out of focus image of bright star - on both side of focus.

Try to defocus it only slightly so that you still get rings visible, and do both sides of focus with roughly equal level of defocus.

Here is guide of what you should be looking at:

When you defocus star pattern slightly and have perfect mirror - you should get first row in above table. Note level of defocus - one that still shows a bit of rings. Your star will have "hole" in the middle when you start defocusing as you have central obstruction.

In any case - perfect parabola will show same pattern regardless if you are in / out of focus.

If you get two different images for same level of defocus - one where edge is fuzzy and center is darker and other where edge is better defined and center is brighter - you have some level of spherical aberration present in the system.

Since you have camera - you can also post images here for analysis.

 

[matija]

14 minutes ago, vlaiv said:

There might be a chance that you have a faulty mirror.

That sort of softness that is visible in the image is indicative of spherical aberration.

Spherical aberration with newtonian can either be due to simple coma corrector used - like 2 element sky watcher x0.95 one, or due to poor parabola on the mirror.

This will only show as softness when viewing at high power - like planetary and lunar views and of course - like stars being little balls in high resolution images.

Alternative to that would be that seeing was really poor on particular night and that created bloated stars.

If you want to check if your scope has spherical aberration (you say you did not use coma corrector) - take out of focus image of bright star - on both side of focus.

Try to defocus it only slightly so that you still get rings visible, and do both sides of focus with roughly equal level of defocus.

Here is guide of what you should be looking at:

When you defocus star pattern slightly and have perfect mirror - you should get first row in above table. Note level of defocus - one that still shows a bit of rings. Your star will have "hole" in the middle when you start defocusing as you have central obstruction.

In any case - perfect parabola will show same pattern regardless if you are in / out of focus.

If you get two different images for same level of defocus - one where edge is fuzzy and center is darker and other where edge is better defined and center is brighter - you have some level of spherical aberration present in the system.

Since you have camera - you can also post images here for analysis.

 

And @vlaiv at 2.9um and a 1/1.2" sensor, do you think, id be possible that a part of the problem is the very inaccurate backfocus? I'm missing 48.5mm of it.

[StevieDvd]

@matija and @vlaiv

As per the thread you have on cloudy nights they have suggested this could be an over-sampling issue due to the focal length & pixel size.  That's way out of my comfort range so perhaps best to look into their suggestions before any possible back focus error.

[vlaiv]

40 minutes ago, matija said:

And @vlaiv at 2.9um and a 1/1.2" sensor, do you think, id be possible that a part of the problem is the very inaccurate backfocus? I'm missing 48.5mm of it.

Sensor size does not play major part here, nor back focus - as long as you focused properly, and I'm guessing you have (no sign of defocus stars).

Yes, there is genuine concern about over sampling - but over sampling itself won't produce bloated stars.

What over sampling will do - is to make everything bigger - stars, galaxy - everything in terms of how many pixels are used to cover - but relative sizes of things in image will stay the same.

Here is a comparison between that image and one taken with similar equipment (F/6 8" newtonian and QHY5LIIc camera):

I resampled images to be roughly the same size. Notice difference in sharpness of the galaxy and also relative sizes of objects in two images. Galaxy is roughly of the same size - but stars are x2-x3 larger in left image.

This is either due to very poor seeing or spherical aberration - it is not due to over sampling as it sampling has been reduced to adequate levels.

In fact, I think that focus was as good as could be had under circumstances as there are no defocus artifacts (doughnut shaped stars) that should be obvious at those star sizes if issue was defocus.