Any suggestions: trouble achieving sharp focus with Edge HD

andrea tasselli · Mar 9, 2026 at 10:04 AM

If you can reach focus on a star and move beyond it then you should be able to get a decent focus. Please share a pic of your set-up for planetary imaging.

Hi Andrea, focussing on a star is also a problem, although the Bahtinov was ok (I think). Stars keep blurry and in the ASIAIR starsize was not below 8.

This is the setup I used yesterday. Do you mean a photo like this? The image of Jupiter was taken with this setup (120s video, stacked with AutoStakkert).

The ZWO ASI 678Pro is connected to the Edge with the standard 1,25” nosepiece, no extra spacers were needed. I also use a rotator.For one video al also used a 2x Barlow, same problem with the focus.

Thank you for your respons.

📷 IMG_2541.jpeg📷 IMG_2543.jpeg

Good summary of how to test and align the optics of an SCT here. In your case the adjustment process will be slightly different because it’s an Edge. But I’ve done this entire procedure except for the “spinning on a jig” part, and I’m a complete idiot. So it should be easy.

https://www.wilmslowastro.com/tips/c14_optics_alignment.html

andrea tasselli · Mar 9, 2026 at 10:48 AM

Are you taking the scope from inside your home to the back-garden by any chance and if so how much are you waiting for cool down.

Yes, I take I from inside my home to the garden. But I always wait for at least one hour. Yesterday even longer: the Edge was outside around noon and the imaging was ten hours later. I also use a dewheater or a dew shield. Yesterday I used the dewheater and connected is two hours before shooting.

Maybe just poor seeing? Looks like it from the ‘donut’ image. The shorter focal length refractor wouldn’t be affected as much.

Cheers,

Scott

Paul Larkin · Mar 9, 2026 at 01:28 PM

Hi, Alexander.

John mentions checking that the corrector is correctly positioned. For ages, I was able to get great doughnuts collimating my Edge HD8 but still had misshapen stars.

When I removed the ring around the edge of the corrector, I noticed that there were indeed markings, and they were not lined up. No idea why. Anyway, I realigned the corrector, collimated again and now have nice round stars.

Yours is a focus issue rather than misshapen stars and may have a completely different cause (or causes) but I suspect it’s worth checking.

Cheers.

Paul

Thanks, seems like a simple check. Will definitely do this one!

Scott Badger · Mar 9, 2026 at 12:38 PM

Maybe just poor seeing? Looks like it from the ‘donut’ image. The shorter focal length refractor wouldn’t be affected as much.

Cheers,

Scott

Hi Scott, that’s also a possibility. The pictures I took of Jupiter with my 480mm refractor were much better in terms of focus and sharpness. Seeing conditions are indeed not optimal where I live (Netherlands, near German border).

Will do some checks as suggested by others and see what I can do.

Cheers!

Alexander Bouwman · Mar 9, 2026, 01:42 PM

John Tucker · Mar 9, 2026 at 12:11 PM

Alexander Bouwman · Mar 9, 2026, 11:57 AM

John Tucker · Mar 9, 2026 at 11:32 AM

Good summary of how to test and align the optics of an SCT here. In your case the adjustment process will be slightly different because it’s an Edge. But I’ve done this entire procedure except for the “spinning on a jig” part, and I’m a complete idiot. So it should be easy.

https://www.wilmslowastro.com/tips/c14_optics_alignment.html

Thanks, I will take a look! Seems like a serious procedure, will see if I feel comfortable enough to do this. Maybe as a last resort, if the problem stays.

I guess my other suggestion would be to check your collimation a little more rigorously. Does the donut stay symmetrical when as you get closer to focus? When out of focus do the non-centered donuts all “lean” toward the center of the field of view, or do they aim toward some other spot?

I don’t know how true this is of the Edge, but for the lower end SCTs there is usually a preferred arrangement of the secondary mirror and of the corrector plate relative to the tube axis. (Actually it’s a preferred relative rotational arrangment of the primary and secondary mirrors).

If you simply remove the retaining ring that holds the secondary in place and examine the corrector and secondary mirror, you should find some sort of mark on each. Note the current rotational arrangement and then try aligning the marks on the secondary and corrector plate with any mark you see on the tube. If there is no mark on the tube, try aligning the marks with the focuser knob.

Anytime you partially or fully disassemble an SCT, take pictures with your phone and make marks with a pencil or sharpie that ensure you can put it back together exactly as you found it, including the rotational angle of the parts in the tube.

Thanks, will try this one. And I will take pictures before changing anything, good one!

The secondary is inside a cell that fits in a hole in the middle of the corrector. It is possible to rotate the secondary cell within that hole while holding the corrector position constant.

So there are two issues:

• Is the rotational position of the corrector relative to the tube (and thus the primary mirror) correct?

• Is the rotational position of the secondary mirror relative to the tube (and thus the primary mirror) correct?

These can be adjusted independently. My understanding is that of the two, the rotational orientation of the secondary mirror is much more important.

Be very careful to mark the secondary, corrector, and tube so that you can put it back together the way you found it in case you can’t find the markings I described.

It is also possible to test for this by trial and error if you can’t find the markings. Rotate the secondary 30 degrees or so and put the OTA back together. Recollimate. See if things improve or get worse. Rinse and repeat. Its tedious but possible.

While you have the corrector assembly removed from the tube, you might put a laser in visual back and lay a meter stick across the open end of the tube to assess whether the baffle tube is aligned with the axis of the optical tube. There are some other tests you can do while the corrector is removed that are described in the link I posted above. They are relatively easy to do and don’t involve the more difficult and complex operation of removing the primary mirror.

If upon removing the retaining ring you find that the corrector plate is stuck and difficult to remove, you can warm the points of contact with the optical tube with a blow dryer.

John Tucker · Mar 9, 2026 at 02:45 PM

Alexander Bouwman · Mar 9, 2026, 01:42 PM

John Tucker · Mar 9, 2026 at 12:11 PM

Alexander Bouwman · Mar 9, 2026, 11:57 AM

John Tucker · Mar 9, 2026 at 11:32 AM

Good summary of how to test and align the optics of an SCT here. In your case the adjustment process will be slightly different because it’s an Edge. But I’ve done this entire procedure except for the “spinning on a jig” part, and I’m a complete idiot. So it should be easy.

https://www.wilmslowastro.com/tips/c14_optics_alignment.html

Thanks, I will take a look! Seems like a serious procedure, will see if I feel comfortable enough to do this. Maybe as a last resort, if the problem stays.

I guess my other suggestion would be to check your collimation a little more rigorously. Does the donut stay symmetrical when as you get closer to focus? When out of focus do the non-centered donuts all “lean” toward the center of the field of view, or do they aim toward some other spot?

I don’t know how true this is of the Edge, but for the lower end SCTs there is usually a preferred arrangement of the secondary mirror and of the corrector plate relative to the tube axis. (Actually it’s a preferred relative rotational arrangment of the primary and secondary mirrors).

If you simply remove the retaining ring that holds the secondary in place and examine the corrector and secondary mirror, you should find some sort of mark on each. Note the current rotational arrangement and then try aligning the marks on the secondary and corrector plate with any mark you see on the tube. If there is no mark on the tube, try aligning the marks with the focuser knob.

Anytime you partially or fully disassemble an SCT, take pictures with your phone and make marks with a pencil or sharpie that ensure you can put it back together exactly as you found it, including the rotational angle of the parts in the tube.

Thanks, will try this one. And I will take pictures before changing anything, good one!

The secondary is inside a cell that fits in a hole in the middle of the corrector. It is possible to rotate the secondary cell within that hole while holding the corrector position constant.

So there are two issues:

• Is the rotational position of the corrector relative to the tube (and thus the primary mirror) correct?

• Is the rotational position of the secondary mirror relative to the tube (and thus the primary mirror) correct?

These can be adjusted independently. My understanding is that of the two, the rotational orientation of the secondary mirror is much more important.

Be very careful to mark the secondary, corrector, and tube so that you can put it back together the way you found it in case you can’t find the markings I described.

It is also possible to test for this by trial and error if you can’t find the markings. Rotate the secondary 30 degrees or so and put the OTA back together. Recollimate. See if things improve or get worse. Rinse and repeat. Its tedious but possible.

While you have the corrector assembly removed from the tube, you might put a laser in visual back and lay a meter stick across the open end of the tube to assess whether the baffle tube is aligned with the axis of the optical tube. There are some other tests you can do while the corrector is removed that are described in the link I posted above. They are relatively easy to do and don’t involve the more difficult and complex operation of removing the primary mirror.

If upon removing the retaining ring you find that the corrector plate is stuck and difficult to remove, you can warm the points of contact with the optical tube with a blow dryer.

Hi John, thank you so much for this comprehensive answer. I will read it closely and see what I can do - and what feels comfortable. Reading other reactions, it can also be a combination of factors: slight misalignment, poor seeing and internal thermal issues. Let’s find out.

Clear skies!

Alexander Bouwman · Mar 9, 2026, 03:43 PM

John Tucker · Mar 9, 2026 at 02:45 PM

Alexander Bouwman · Mar 9, 2026, 01:42 PM

John Tucker · Mar 9, 2026 at 12:11 PM

Alexander Bouwman · Mar 9, 2026, 11:57 AM

John Tucker · Mar 9, 2026 at 11:32 AM

Good summary of how to test and align the optics of an SCT here. In your case the adjustment process will be slightly different because it’s an Edge. But I’ve done this entire procedure except for the “spinning on a jig” part, and I’m a complete idiot. So it should be easy.

https://www.wilmslowastro.com/tips/c14_optics_alignment.html

Thanks, I will take a look! Seems like a serious procedure, will see if I feel comfortable enough to do this. Maybe as a last resort, if the problem stays.

I guess my other suggestion would be to check your collimation a little more rigorously. Does the donut stay symmetrical when as you get closer to focus? When out of focus do the non-centered donuts all “lean” toward the center of the field of view, or do they aim toward some other spot?

I don’t know how true this is of the Edge, but for the lower end SCTs there is usually a preferred arrangement of the secondary mirror and of the corrector plate relative to the tube axis. (Actually it’s a preferred relative rotational arrangment of the primary and secondary mirrors).

If you simply remove the retaining ring that holds the secondary in place and examine the corrector and secondary mirror, you should find some sort of mark on each. Note the current rotational arrangement and then try aligning the marks on the secondary and corrector plate with any mark you see on the tube. If there is no mark on the tube, try aligning the marks with the focuser knob.

Anytime you partially or fully disassemble an SCT, take pictures with your phone and make marks with a pencil or sharpie that ensure you can put it back together exactly as you found it, including the rotational angle of the parts in the tube.

Thanks, will try this one. And I will take pictures before changing anything, good one!

The secondary is inside a cell that fits in a hole in the middle of the corrector. It is possible to rotate the secondary cell within that hole while holding the corrector position constant.

So there are two issues:

• Is the rotational position of the corrector relative to the tube (and thus the primary mirror) correct?

• Is the rotational position of the secondary mirror relative to the tube (and thus the primary mirror) correct?

These can be adjusted independently. My understanding is that of the two, the rotational orientation of the secondary mirror is much more important.

Be very careful to mark the secondary, corrector, and tube so that you can put it back together the way you found it in case you can’t find the markings I described.

It is also possible to test for this by trial and error if you can’t find the markings. Rotate the secondary 30 degrees or so and put the OTA back together. Recollimate. See if things improve or get worse. Rinse and repeat. Its tedious but possible.

While you have the corrector assembly removed from the tube, you might put a laser in visual back and lay a meter stick across the open end of the tube to assess whether the baffle tube is aligned with the axis of the optical tube. There are some other tests you can do while the corrector is removed that are described in the link I posted above. They are relatively easy to do and don’t involve the more difficult and complex operation of removing the primary mirror.

If upon removing the retaining ring you find that the corrector plate is stuck and difficult to remove, you can warm the points of contact with the optical tube with a blow dryer.

Hi John, thank you so much for this comprehensive answer. I will read it closely and see what I can do - and what feels comfortable. Reading other reactions, it can also be a combination of factors: slight misalignment, poor seeing and internal thermal issues. Let’s find out.

Clear skies!

One thing that might be helpful for assessing this is to get an average star size measurement (FWHM) before you touch anything to get a quantitative picture of where you are. This can be used as a baseline for before/after comparisons if you change anything. If you aren’t using acquisition software that does this, you can always run a sub or two through Deep Sky Stacker. It will generate an average FWHM for each sub and add it to the table.