13 replies to this topic

[matija123]

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. Here is also an example of a single 1 min sub.

Thanks!

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[idclimber]

You have an image scale of 0.5 arc seconds per pixel. You are most likely seeing limited unless you happen to be on the top of the mountains in Chili. 

 

You may be able to improve the focus if you are manually doing it now with a mask. A focus motor and a good focusing program will be more accurate. You may also be able to improve collimation if you refine it through the camera. This is typically the best way with my SCT.

[matija123]

You have an image scale of 0.5 arc seconds per pixel. You are most likely seeing limited unless you happen to be on the top of the mountains in Chili. 

 

You may be able to improve the focus if you are manually doing it now with a mask. A focus motor and a good focusing program will be more accurate. You may also be able to improve collimation if you refine it through the camera. This is typically the best way with my SCT.

Hi.

 

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!

[unimatrix0]

Hi.

 

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!

It's called oversampling. It has nothing to do with your focus, as idclimber already explained. 

Your stars won't become a pinpoint, because at 1200mm focal length and your sensor's pixel pitch makes the stars take up a lot more pixels than at lesser focal length. 

Your stars will always be bloating looking at this focal length and camera.  Only thing you can do is either drop down on the focal length; use a camera with larger pixel pitch; or bin the exposures.

[idclimber]

Here is a raw uncalibrated sub from last spring. The focal length is 2134mm with a pixel that is 3.74um. This gives me an image scale of 0.37"/px, a bit less than yours. I spent a bunch of time getting this scope to image well. I have an external focuser. I have done very extensive collimation through the camera. I spent a bunch of time optimizing guiding through an OAG, A bunch of time analyzing my focus curves and optimizing the settings. In all several thousand dollars and a many months of work. When seeing is good I can get decent subs, but they are still blurry compared to my refractors. 

 

[matija123]

unimatrix0 , then why was I able to get this image with a (0.3"/pix) system. It was even uncollimated as you can see. These are 5s shots stacked.

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Edited by matija123, 27 January 2023 - 03:12 PM.

[Spaceman 56]

It's called oversampling. It has nothing to do with your focus, as idclimber already explained. 

Your stars won't become a pinpoint, because at 1200mm focal length and your sensor's pixel pitch makes the stars take up a lot more pixels than at lesser focal length. 

Your stars will always be bloating looking at this focal length and camera.  Only thing you can do is either drop down on the focal length; use a camera with larger pixel pitch; or bin the exposures.

are you sure unimatrix ?

[Spaceman 56]

Hi.

 

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!

if the stars get smaller as you come to focus ( manually ) and then get bigger once you go past, then I don't think its a focus problem.

 

once they are as small as they can be, thats it. your focused.

 

I would be looking at optical problems, DEW on the lens, collimation.....

[PhilHoyle]

You don't say whether you are using a bahtinov mask. Unless you are using an autofocusing system, that is essential. If you go past focus and can't hit the sweet spot, you may need to get a focuser with fast and fine knobs. You have a somewhat fast system (low f ratio). The lower the f ratio, the more critical getting exactly the right focus is.

[fewayne]

You can try imaging a very bright target with really short exposures. For example, get the very best focus you can attain and then shoot a minute of video of, say, Jupiter and run it through AutoStakkert. If the worst frames are this blurry and the best are much sharper, then you're fighting seeing issues. A one-minute exposure is way, way longer than typical seeing fluctuations, but if you take a bunch of images a fraction of a second long and you get some that are sharper, that should tell the tale.

 

If all of your video frames are much better than the 1-minute exposure (shoot one on the same target for comparison), then it might be tracking or guiding. Normally we see those as elongated stars, but it's also possible to have errors in RA and DEC that balance out enough to have stars that are reasonably round, but blurry.

 

You might also compare the results of lucky-imaging a bright star (not a planet) at the zenith with one near the horizon. At 0.5"/px, I would be astonished if there wasn't a visible difference, since you're looking through very different amounts of atmosphere, but it might give you a yardstick for seeing effects at your location.

As idclimber notes, sub-pixel imaging scales are pretty advanced, everything has to be spot-on to get reasonable results. Hoo-boy, I know that only too well.

[smiller]

I can sometimes (not often) get good focus with reasonably sharp stars at 1125mm focal length at 0.69” per pixel, but if seeing is really poor, I’ll get subs like what you showed and when I’m used to seeing sharper stars, it kind of feels like I can’t focus.

Edited by smiller, 27 January 2023 - 06:57 PM.

[nitegeezer]

unimatrix0 , then why was I able to get this image with a (0.3"/pix) system. It was even uncollimated as you can see. These are 5s shots stacked.

You are comparing a 5s exposure to a 1min, and that all depends on guiding and seeing. Shorter subs are less sensitive to errors, this is why alt/az shots can be clean when short. It takes excellent seeing, guiding, and PA to take long exposures when oversampled to avoid bloating.

[unimatrix0]

are you sure unimatrix ?

I'm sure. Here is an image at 1370 mm  focal length with 3.76qm pixel array (533mc pro) . Collimation perfect, focusing perfect, guiding is good. 

  Still oversampled, a lot. 

 

The OP using a 2.9qm .  I mean, the only other thing I can think of is his scope not cooled down to ambient, he's got a mirror moving issue or collimation changes when he slews. 

 

Edited by unimatrix0, 27 January 2023 - 05:54 PM.

[revans]

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. Here is also an example of a single 1 min sub.
Thanks!

1200mm FL for me is just about where imaging starts to get more difficult because of oversampling for my sub-average seeing.  When I get at 2100mm to 2700mm with my 12 inch Mak-Cas then I'm about 0.29 to 0.37 arc sec/pixel with my rig.  Focus is miserable and I'm way oversampled for my typical 3 to 5 arc second seeing conditions.  So when I go this route, I have binned x4.  Results are still fairly poor and an example is here
 
Using my C9.25 at f6.3 gives a 1480mm focal length and that is about as high as I want to be.  Then I'm at about 0.52 arc sec/pixel resolution for my rig.  But I don't bin with this scope and a 533MC camera.  Results are more or less OK.  An example is here
 
I get my best galaxy results (not counting Andromeda) with my 120mm f7 refractor and C6 at f6.3, with focal lengths of 840mm and 945mm.  I get better images in my seeing conditions by using these two scopes and cropping the results as opposed to using a higher focal length scope.  With the refractor I'm at 0.92 arc sec/pixel and with the C6, I'm at 0.82 arc sec/pixel.  An example with the refractor is here and with the C6 here
 
For larger targets I usually use an 80mm or 106mm refractor at f5 except for very large targets, when I might use a camera lens with a cooled astrocam.
 
PS - one thing I forgot to mention is that with some stacking programs like Astropixel processor, you can combine sessions using different telescopes and cameras. Below is a 10.5 hr exposure of the Iris Nebula divided between my C9.25 and FSQ106.  This is another way of evening out issues with difficult scopes and focal lengths etc.

 
Rick