Cant reach proper focus. Help

[matija]

Hello.

I own a 250pds (1200fl (f4.7)) and an Asi 485mc (2.9um pixels) and for some reason I cant get it to focus. I am collimated with a laser and the atmosphere was not too bad. I also have enough room with the focuser to move it past proper focus and far out. Though I still cant get the image to look sharp. I get that I am a little oversampled but the stars are huge and the object is fuzzy. The scope was also properly thermally acclimated so I really can’t think of anything that could be causing this.

Thanks!

[ONIKKINEN]

Can you post a screenshot or a sample JPEG of what the images look like? Since you can move past focus it just sounds like the stars are unexpectedly large which may not be focus related.

With those pixels you are probably looking at Binning x3 to be nicely sampled, or even x4 if your seeing conditions are bad. At x1 you have 0.5'' per pixel, which will be nicely sampled if you are on a mountaintop in a desert and you have a guide error of 0.0 and actually still probably not good enough for that.

[Elp]

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

[matija]

34 minutes ago, ONIKKINEN said:

Can you post a screenshot or a sample JPEG of what the images look like? Since you can move past focus it just sounds like the stars are unexpectedly large which may not be focus related.

With those pixels you are probably looking at Binning x3 to be nicely sampled, or even x4 if your seeing conditions are bad. At x1 you have 0.5'' per pixel, which will be nicely sampled if you are on a mountaintop in a desert and you have a guide error of 0.0 and actually still probably not good enough for that.

@ONIKKINEN I am using a cmos camera and as such it doesn't have hardware bin so binning wouldn't help at all. Here is an image.

[matija]

15 minutes ago, Elp said:

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

How can I check that?

[Elp]

Just now, matija said:

How can I check that?

You can take an image of it all set up with the focuser.

When I used a 130pds with a gso coma corrector I had to add 20mm spacer rings between the corrector and the 224mc I was using to get sharp focus after racking the focuser to the correct point.

Also on that image above what was the exposure duration, is it a single photo? If atmospheric seeing is bad your stars and target will also look bad if the image was taken at that time, similarly if your target is low on the horizon due to imaging through more atmosphere.

[johnturley]

3 minutes ago, matija said:

@ONIKKINEN I am using a cmos camera and as such it doesn't have hardware bin so binning wouldn't help at all. Here is an image.

Looking at the above image, it appears to be out of focus, but at least the star images seem to be quite symmetrical discs indicating not a collimation problem

John 

[matija]

44 minutes ago, johnturley said:

Looking at the above image, it appears to be out of focus, but at least the star images seem to be quite symmetrical discs indicating not a collimation problem

John 

But I cant get any better of a focus. Is there something else I can try?

 

Matija

 

[teoria_del_big_bang]

So are you saying that if you start with your focusser racked fully in and then start adjusting it outwards the stars get smaller and smaller as it approaches what you think is the correct focus position and the stars only get as small as the mage you have posted, then if you continue to wind the focusser outwards the stars get bigger anagin ?

Exactly what is the method you are using to obtain focus ?

Steve

Edited January 27, 2023 by teoria_del_big_bang

[StevieDvd]

Have you anything fitted in between the camera and focuser, such as a flattener/reducer or coma corrector and/or any extensions fitted to the end of the focuser tube?

Have you focused with any other camera or just visual/eyepieces?

If you have nothing in the light path that affects the focus position then perhaps there is an extension tube to be added/removed.

The PDS versions of the scopes are shortened versions of the standard scope to allow DSLR usage, they typically have a 55mm back focus. So when using a different camera such as the 485MC which as a 17.5mm back focus you may need spacers to add another 37.5mm

But that really depends on whether you don't have enough back fockus or inward focus.

Inward means you probably need to remove an extenstion tube. Outer means you may need more spacers. Difficult to tell without more info or a picture of the focuser with camera added and where best focus is (furthest in or furthest out).

[ONIKKINEN]

1 hour ago, matija said:

@ONIKKINEN I am using a cmos camera and as such it doesn't have hardware bin so binning wouldn't help at all. Here is an image.

The image looks a little bit out of focus, but it would still be  extremely  oversampled even if it was in focus. Can you explain how exactly binning would not help with that and what does hardware binning have to do with that?

[Stefan73]

Have you tried with a focusing mask?  Might be easier to be sure you're at the right "in focus" position with that and that you're really passing the in focus position when you think you are.

[AstroMuni]

2 hours ago, matija said:

I own a 250pds (1200fl (f4.7)) and an Asi 485mc (2.9um pixels) and for some reason I cant get it to focus.

On the 130pds you cant get focus unless you add the extension tube onto the focuser. Not sure if you would have similar situation? Does the OTA come with an extension tube?

Check where you get focus using the eyepiece and then you can work out how much to move the focuser inwards (to cover for the distance to sensor on your camera)

Edited January 27, 2023 by AstroMuni

[matija]

47 minutes ago, StevieDvd said:

Have you anything fitted in between the camera and focuser, such as a flattener/reducer or coma corrector and/or any extensions fitted to the end of the focuser tube?

Have you focused with any other camera or just visual/eyepieces?

If you have nothing in the light path that affects the focus position then perhaps there is an extension tube to be added/removed.

The PDS versions of the scopes are shortened versions of the standard scope to allow DSLR usage, they typically have a 55mm back focus. So when using a different camera such as the 485MC which as a 17.5mm back focus you may need spacers to add another 37.5mm

But that really depends on whether you don't have enough back fockus or inward focus.

Inward means you probably need to remove an extenstion tube. Outer means you may need more spacers. Difficult to tell without more info or a picture of the focuser with camera added and where best focus is (furthest in or furthest out).

For this image I didn't have any ccs or reducers. I was able to get what looked like perfect focus with an eyepiece (sky panorama 23mm 82deg) after adding the included extension tube.

 So does that mean, that my sensor has to be 37.5mm away from the end of the focuser or..?

I don't see how adding even more space between the cam and scope would make it easier to focus, considering, I can already overshoot and undershoot focus without any tubes.

 

Thanks!

Matija

[jfrijhoff]

41 minutes ago, matija said:

For this image I didn't have any ccs or reducers. I was able to get what looked like perfect focus with an eyepiece (sky panorama 23mm 82deg) after adding the included extension tube.

 So does that mean, that my sensor has to be 37.5mm away from the end of the focuser or..?

I don't see how adding even more space between the cam and scope would make it easier to focus, considering, I can already overshoot and undershoot focus without any tubes.

 

Thanks!

Matija

So you based correct focus using the eyepiece, and not with the camera? I’m confused.

[StevieDvd]

With the eyepiece and the extension tube did you also have a diagonal?

If you added a diagonal & extender then effectively that increased the focus point back.  Without a diagonal/extender, to use a DSLR for example, it would need an adapter to fit onto the end of the focuser and that with the DSLR is usually 55mm.

But if, as you say, the focus can go either side of the example image (stars increase in size when focusing either side of that position) then that should be the focus point but  for some reason your camera ccd is not in the right spot.

How are you connecting the camera and are there any filters attached to it?

 

[matija]

@StevieDvd I did not have a diagonal. also, with my scope and camera (like you said) configuration I need 54mm between the focuser plane and the camera body. When Imaging this image I had 0. But the thing is, that I was still able to undershoot and overshoot the focus, meaning, I turn the focus knob, the focus keeps getting better until a certain point and then its starts going in the other direction. I technically can focus, its just that the focus that I get is soft/blurry. Could the missing space between the focuser plane and the camera be causing such a phenomena?

 

Thanks!

[Elp]

Is it hot or cold where you are when you took this image? How long was the scope outside before you took it?

[matija]

10 minutes ago, Elp said:

Is it hot or cold where you are when you took this image? How long was the scope outside before you took it?

Its cold where I am and the scope was out for 4 hours before imaging. one more thing, with my scope and camera configuration I need 54mm between the focuser plane and the camera body. When Imaging this image I had 0. But the thing is, that I was still able to undershoot and overshoot the focus, meaning, I turn the focus knob, the focus keeps getting better until a certain point and then its starts going in the other direction. I technically can focus, its just that the focus that I get is soft/blurry. Could the missing space between the focuser plane and the camera be causing such a phenomena?

 

Thanks!

[StevieDvd]

Okay this is my logic, and I am prepared to be corrected by a proper astroimager.

I have a WO Z61 with a flattener and an ASI 585MC fitted. I had to add spacers to the end of the focuser so the image plane lined up with the sensor of the camera.

See the WO diagram here the diagrams are towards the end of the page. As you move the focuser in/out the image plane stays the same distance from the end of the focuser. Your ccd needs to be on that image plane, it's not the distance between the scope and camera that's causing the error but the fact the camera is always off the image plane.

I understand a little but had always thought the focuser made up distance unless you had a reducer in the chain before I used a flattener myself.

Of course the experts reading this will, hopefully, correct any mis-conception I have - but it seems to fit the facts.

[Elp]

As long as you don't alter the optical train, the focus plane should remain at the same point. Adding or removing backspacing will show visual changes to the shapes of the stars at the edge of the field if you've got a decent size sensor.

Is your image above cropped? If so what does the full size look like? Also with the 485mc you need to use a luminence or IR cut filter to get the sharpest image as it has an AR window only, not an IR cut. I often shoot RGB unfiltered, then do a luminence run.

[matija]

20 minutes ago, StevieDvd said:

Okay this is my logic, and I am prepared to be corrected by a proper astroimager.

I have a WO Z61 with a flattener and an ASI 585MC fitted. I had to add spacers to the end of the focuser so the image plane lined up with the sensor of the camera.

See the WO diagram here the diagrams are towards the end of the page. As you move the focuser in/out the image plane stays the same distance from the end of the focuser. Your ccd needs to be on that image plane, it's not the distance between the scope and camera that's causing the error but the fact the camera is always off the image plane.

I understand a little but had always thought the focuser made up distance unless you had a reducer in the chain before I used a flattener myself.

Of course the experts reading this will, hopefully, correct any mis-conception I have - but it seems to fit the facts.

Thank you very much. I suppose missing those 48.5mm really messed up my images. Ill try gain with the extension tube, when it clears up.

[matija]

12 minutes ago, Elp said:

As long as you don't alter the optical train, the focus plane should remain at the same point. Adding or removing backspacing will show visual changes to the shapes of the stars at the edge of the field if you've got a decent size sensor.

Is your image above cropped? If so what does the full size look like? Also with the 485mc you need to use a luminence or IR cut filter to get the sharpest image as it has an AR window only, not an IR cut. I often shoot RGB unfiltered, then do a luminence run.

The image is uncropped. I am missing 48.5mm of backfocus tho. That sounds like a big deal to me?I do use a lum filter. The baader 1.25".

[teoria_del_big_bang]

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

[Elp]

41 minutes ago, matija said:

The image is uncropped. I am missing 48.5mm of backfocus tho. That sounds like a big deal to me?I do use a lum filter. The baader 1.25".

If you're off 48.5mm I doubt you'd even see an image on camera. Best to show how you've set it up and someone can comment on where you measure from.