68 replies to this topic

[Yomamma]

I've got a Meade 12" ACF SCT scope and have noticed that I am getting flared stars.  They flare from the center of the image outward. Consensus is that it looks like coma but ACF scopes are not supposed to have this.

 

I have a Meade 8" ACF SCT and it has absolutely no coma and I can even image with it and I expected this scope to be the same.

 

Here is a sample image.  The flares are pretty bad lol.

 

 

 

The coma is so bad I wonder if they mislabeled my scope as an ACF model when it obviously is not.

 

Should a Meade ACF SCT have coma like this?

Edited by Yomamma, 21 June 2025 - 06:15 AM.

[quilty]

No. This is much worse than non ACF.

As there're not too many culprits possible I think someone swapped mirrors between f/8 and f/10

Edited by quilty, 21 June 2025 - 06:56 AM.

[Mbenj2405]

How is the collimation?

[Yomamma]

Collimation looks good.  I checked it when I first focused it and it looked spot on.

[12BH7]

How does it look visually? Any coma with just an EP and diagonal?

[Yomamma]

How does it look visually? Any coma with just an EP and diagonal?

I have only checked the collimation with the eye piece.  Collimation looks good, and the coma seems to be evenly spread with no side seeming to have a heavier balance of leaning that might indicate collimation having an affect. The coma error has all spikes being directed outward.

[JimTheEngineer]

Mine just started doing this 2 months ago. In my case I think something shifted. So I started with star collimation. No joy.

I then noticed bad camera tilt and had inconsistent results using ASTAP, so I fixed this using a laser from Astro Precision. Still no joy.

This “coma” seems to indicate a center location off to one side. I think I had inverse on the ASTAP aberration inspector using the HFR diagram and not really sure which way was left/right on the P1 camera.

Used a laser to check secondary center alignment, and it looks spot on without any adjustments being made (checked laser collimation before start and at 87ft, no more than 2-4mm max drift - 15-30arcsecs). Then using a holographic circle, adjusted the secondary tilt to align the secondary shadow to align with the circles. The primary mirror edge looked close (can’t really adjust the primary though). Thought I had this solved. Stars were horrible.

I also racked the focuser in and out to see if there was any mirror movement. None noticed in a polar position and counterweights down. Will try on each side of meridian, but I don’t think anything is loose, all these results have the lowest HFR towards the same sensor position, so I’m not expecting mirror shift.

I have tried two different cameras (Poseidon and 2600MC) and one with a Lepus reducer and one without. No joy.

Still trying to figure this out. I don’t expect to get rid of the “flaring” on bright stars. I just want it back the equal flaring on all sides that I had before.

Attached Thumbnails

• 
• 

Edited by JimTheEngineer, 21 June 2025 - 04:08 PM.

[davidc135]

I have only checked the collimation with the eye piece.  Collimation looks good, and the coma seems to be evenly spread with no side seeming to have a heavier balance of leaning that might indicate collimation having an affect. The coma error has all spikes being directed outward.

It looks like a reflection artefact that mimics coma but I don't know how. There is a separation between the circular main star image and the artefact whilst the remaining stars which are too dim to generate an artefact look fine.

 

I suppose that the scope has no warrantee?

 

David

Edited by davidc135, 21 June 2025 - 02:28 PM.

[Yomamma]

I have a Meade 8" ACF SCT on which I use the same image train and I get nice stars on this scopes images  The 8" collimation and the 12" collimation look identical, both are good.  If collimation were off the stars would lean in one direction and here they do not.  The stars look round they just have that flare that emantes from the center of the frame and goes out. The flare is uniform all the way around the frame and always points outward.

[JimTheEngineer]

After laser collimation (extra-focal, in focus, intra-focal)

Attached Thumbnails

• 
• 
• 

Edited by JimTheEngineer, 21 June 2025 - 04:25 PM.

[JimTheEngineer]

Then I re-collimated on the stars and got this. 
 

Attached Thumbnails

• 
• 
• 

[JimTheEngineer]

This was the laser image after star collimation. I also have an HFD diagram at f/5 and native f/8.

Attached Thumbnails

• 
• 
• 

[RichA]

Not sure how the secondary attachment works on those scopes, but is it possible someone reversed the corrector plate?

[quilty]

that's easy to perform. Another possibility. I doubt though this would make such a difference.
In theory, the effect of a reversed Schmidt plate should not be visible. Can't imagine how

Edited by quilty, 22 June 2025 - 02:00 AM.

[davidc135]

Correction can be put on either side of the plate or divided between the two without making any difference optically, in theory. Maybe in practice there could be some astig due to figuring irregularities being re-orientated.

 

David

[davidc135]

I have a Meade 8" ACF SCT on which I use the same image train and I get nice stars on this scopes images  The 8" collimation and the 12" collimation look identical, both are good.  If collimation were off the stars would lean in one direction and here they do not.  The stars look round they just have that flare that emantes from the center of the frame and goes out. The flare is uniform all the way around the frame and always points outward.

You could try masking off the innermost 1 or 2'' zone of the corrector plate in the hope that it intercepts the faulty rays without affecting the light gathering of the scope too much.

 

David

[tturtle]

I don’t think any SCT should look like this, ACF or not. A wild guess is that ACF optics and non ACF optics have somehow been mixed. Was this scope part of the Meade close out inventory that went to High Point Scientific?

Edited by tturtle, 22 June 2025 - 05:55 AM.

[Yomamma]

I don’t think any SCT should look like this, ACF or not. A wild guess is that ACF optics and non ACF optics have somehow been mixed. Was this scope part of the Meade close out inventory that went to High Point Scientific?

Yes it was.  And I had the same thought that you do.  Its like they used whatever parts were available.  I expected this to have no coma just like my 8" ACF.  The collimation is good on the 12" so that is not the problem I have checked that and anyway if there was coma you would see it in the stars leaning in one direction.  These stars all flare outward to the edge.

[davidc135]

Coma increases in proportion to the distance from the centre of field whereas these only vary with the brightness of the star. If it was coma you'd see some distortion of the other minor stars especially towards the edge.

 

Speculatively, what if the inner zone of the corrector plate was being stressed by over-tightness which caused a 360^o flare but something such as the secondary baffle was obstructing all but a 90-120^o section of it. There is also a gap between the main star image and its flare.

 

David

Edited by davidc135, 22 June 2025 - 07:00 AM.

[JimTheEngineer]

I tried a small 1/4” mask around the secondary and it looked like the stars were maybe slightly smaller but the coma was still there. I’ll try a much thicker one this evening.

You could try masking off the innermost 1 or 2'' zone of the corrector plate in the hope that it intercepts the faulty rays without affecting the light gathering of the scope too much.

David

Edited by JimTheEngineer, 22 June 2025 - 06:56 AM.

[davidc135]

I tried a small 1/4” mask around the secondary and it looked like the stars were maybe slightly smaller but the coma was still there. I’ll try a much thicker one this evening.
 

I think that you really do have coma due to misalignment and, if so, masks aren't going to help much.  David

[JimTheEngineer]

Remember the frame shown of Dumbell was a processed image. BlurX can do some strange things. Maybe a single unprocessed frame would be better?

Coma increases in proportion to the distance from the centre of field whereas these only vary with the brightness of the star. If it was coma you'd see some distortion of the other minor stars especially towards the edge.

Speculatively, what if the inner zone of the corrector plate was being stressed by over-tightness which caused a 360^o flare but something such as the secondary baffle was obstructing all but a 90-120^o section of it. There is also a gap between the main star image and its flare.

David

Edited by JimTheEngineer, 22 June 2025 - 07:43 AM.

[Yomamma]

Remember the frame shown of Dumbell was a processed image. BlurX can do some strange things. Maybe a single unprocessed frame would be better?

 

Coma increases in proportion to the distance from the centre of field whereas these only vary with the brightness of the star. If it was coma you'd see some distortion of the other minor stars especially towards the edge.

 

Speculatively, what if the inner zone of the corrector plate was being stressed by over-tightness which caused a 360^o flare but something such as the secondary baffle was obstructing all but a 90-120^o section of it. There is also a gap between the main star image and its flare.

 

David

Thanks David.

 

You can actually see the distortion in the smaller stars when you zoom way in .  They mimic the larger stars although there is no flare just a slight bulge in the outward direction towards the closest edge.

 

Given the pattern of the coma I had the same thought as you.  It looks as though something is providing pressure in the center that is causing the stress on the perimeter but the three adjustment screws are on the perimeter and there is nothing in the middle to push down.  I also wondered if all three collimation screws are torqued above spec to the same degree, maybe giving this type of distortion. Even with the collimation being pretty much spot on if the screws are overtorqued would that affect the collimation at all? Maybe not. If the orientation of the secondary is correct that is all that matters for collimation and this one is correct.

[JimTheEngineer]

Here is a close up of a star just to the right of my “center” on the left side of the sensor. The second one is same, but further from the center on the right. These are single 5s frames no processing. The first one is almost a normal bright star. It’s the second one that is a definite problem.

Attached Thumbnails

• 
• 

Edited by JimTheEngineer, 22 June 2025 - 09:20 AM.

[tturtle]

I don’t think any of the suggestions in this thread are going to account for the severity of the problems you are seeing. If it were me I’d be on the phone to highpoint looking for a full refund and probably not even bother to send it back. They are a good company so I’m sure they will work with you. 

Yes it was.  And I had the same thought that you do.  Its like they used whatever parts were available.  I expected this to have no coma just like my 8" ACF.  The collimation is good on the 12" so that is not the problem I have checked that and anyway if there was coma you would see it in the stars leaning in one direction.  These stars all flare outward to the edge.