Focus only in center of image, borders are blurry

[graemlourens]

Hi guys.

I have been doing a lot of dry-runs for testing purposes (flats, darks & so on), and i stumbled across something that looks fishy.

Attached is a single non-edited, non-stacked image taken for testing flats (just created a different thread earlier)

If you zoom into the top part of the letter 'A' you will see that everything is pretty much well focused.

If you go to the right side of the 'H' oder left side of the 'T' you will see that it seems that the focus is off.

I checked colimation, it seems ok to me, but hard to tell if there is maybe a slight variance ( checked with a 8mm eyepiece yesterday on altair, but sight conditions were terrible).

Generally i guess i'm asking, could this have to do with colimation or is there another reason why this would happen?

Thx for any hints, Graem

[NickK]

There's a couple of things about telescopes..

Field curvature - if you look carefully at the horizontal line it doesn't look straight but bends down at the edges.

Spot size - the spot size for each point of the lens is different as you move from the centre (smallest spot) to the outer (the spots tend to warp). If the camera has small pixels and the outer edge spot size is larger than the sensor pixel size you may see a slight blurring.

[ronin]

Sharper centre then edges tends to be normal, the focal plane is not flat, it is curved and a DSLR has a large sensor so it shows it up worst.

Next the A is on axis, the H and the T are therefore off axis.

The reducer may be making it worse, shorter focal length means more curvature and the reducer can well be adding to the general optical problems.

Possible problems are you may need a flattener and you may need a coma corrector. As a final point the Celestron Nexstar Evolution 6" I can see is on an Alt/Az mount and that will not do long exposure imaging well. How it moves and how the sky moves do not match up.

[michael.h.f.wilkinson]

I am not sure if you are using a focal reducer here, but that limits the image circle of the C6 (likewise on my C8, although that is a bit wider). Apart from vignetting you  What you may be seeing is the drop-off in quality towards the edge of the image circle.

[michael.h.f.wilkinson]

BTW, the reducer also flattens the field (or at least mine does) so field curvature tends to be a problem only when the distance between reducer and focal plane is wrong.

[graemlourens]

@Ronin, The Alt/Az problem i'm aware of, as i just got into astronomy few months ago, this will be my testsetup for the next few years, i can get decent 40-60 second exposures, and i'll keep to the bright objects for now

@Michael, yes i am using the celestron f6.3 reducer/flattener, but i also still have currently an additional 2" extension tube attached, i will remove it and make comparison shots, also removing the reducer and see the difference.

I didn't think of the space between the flattener and the chip as a problem, is there a general rule here (closer better) or does that totally depend on lots of variables?

Thx for your thoughts, its really helping me learn the basics.

Kind regards, Graem

[michael.h.f.wilkinson]

I always used the standard Celestron T-mount adaptor or off-xis guider behind the reducer. Both should get the right distance to the film plane or chip when using a (D)SLR

[graemlourens]

ok i'll try the difference tonight.

I was using the 2" back as was testing afocal with the hyperions directly connected to the dslr via 2" nosepiece.

Will inform you about the testresults tonight.

Regards, Graem

[ollypenrice]

Even with the flattener the true flat field of the non-Edge or ACF SCTs is quite limited, certainly not enough to cover this chip perfectly. The chip distance should also be respected. From memory I think it's 105mm but don't trust my memory (because I never do!)

Olly

[carl20171]

I dont know if it would help any but my Celestron 11 HD to Celestron Nightscape 8300 is 146 mm from back of threads to camera sensor  5.75 in

[graemlourens]

hey guys.

I did a quick testing yesterday, and first thing is i removed the Focal reducer, and mounted the normal T-Mount, and it has the same problems of very blurry edges. (so nothing to do with focal reducer)

Second what i found is that i have a much greater FOV if i put the camera further away from the rear. I didn't know this had an effect.

I will shoot the exact same szenery with all different combinations, and compare the blurry edges, and the fov.

Will post the results later.

Regards, Graem

[Psychobilly]

IIRC Buried away in the reducer manual (sheet of paper) there was info about focusing at the center of the FOV then making a small adjustment to the focus for "optimum" focus across the whole FOV rather than the best possible focus at any one point in it...

I always try and focus on a thirds intersection point...  the four imaginary points in the FOV where the horizontal and vertical thirds lines  used for "composition" cross...

I will try and find the info about focus adjustment...

Here you go...

Peter...

[graemlourens]

Hi Peter.

Thank you, i actually did read that when i purchased the FR but I didn't remember that one.

It makes sense, so in that case 'everything will be a little out of focus.

I guess it depends on the size of your target, if it fills your FOV then that makes sense, otherwise not.

Focusing on the one of the 4 crosspoints is also a very good hint, will keep that one in mind!

Regards, Graem