elongated stars issue with Orion 0.8 Reducer

[NorthernLight]

Hi folks,

I acquired an Orion 0.8 Imaging Reducer (Refractor) and used it initially with success on my SW ED80. Lately, however, I noticed that the corner stars appear strangely elongated. There are 2 distinct types of star elongation that I have found in my images (in order of appearance):

- elongated following roughly the sensor diagonal and all parallel, shape is like a comet (bright big part followed by a thinning tail).

- elongated in a circular fashion with the effect increasing away from the center of the image towards the outer edges. the arcs are longest in the corners.

I usually do not take the reducer out and leave the reducer-T-Ring combination attached to the scope. So I wonder, whether there is stress in the system that manifests itself in worsening perpendicularity or whether there is an issue with how far the reducer needs to be inserted into the focuser. Has somebody seen this happening before or can deduct a solution from the description?

PS: I would not want to exclude anything prematurely but my guiding graph stays constantly below 1.5", so I do not think this is a tracking issue.

Cheers,

[gregbradley]

Sounds like tilt. If the corners were good originally and now have developed elongations in the corners without changing anything from before it sounds like tilt.

You need to get the imaging train square again.

That is best done by packing out the corners to get it square with thin packing material. I used brass shim from a metal supplier to do it.

First though you need to ascertain which corner of the image relates to which corner of your camera otherwise its a lot harder to adjust for this.

I take a short photo with a dimmed torch lighting up the top or the bottom and see where the light is in the resulting image. Is it the same or reversed ie shine a torch at the top of the scope but it shows on the bottom of the image = top and bottom are reversed. I always orient from standing behind the camera looking at it in front.

Do the same for left and right.

For a mirrored scope its usually reversed top and bottom. For a refractor I am not sure.

Now pack out the corner that is worst and take another quick photo and examine.

Back and forth until it is corrected. It can take a while unfortunately.

Once you nail it take notes about what was needed for future reference in case you remove the camera etc.

Judging by your images I'd say you are a fair way away from being square. It will need probably .5mm packing or so, perhaps more.

Greg.

[Atmos]

I’d agree with Greg, it sounds like tilt. Depending on the camera orientation and the offending corners you may also want to try tightening and loosening the grub screw for drawtube stiffness as I’ve found in the past that this can change tilt as it physically lifts the drawtube.

[xelasnave]

Hi Greg
Although I did not start this thread I would like to say thank you for your post, I knew that one could shim but your guidance on what to do I did not know about.
Alex

[NorthernLight]

Hi guys,

Thanks a lot for that tip Greg! I'll give this ago. Any chance you could point me to photos of the final 'packed out' -product? Cause I haven't really got a clue what to do or where to start.

Any idea why the type of aberration has changed over time?

Cheers,