GSO 8" - Triangular Stars....

[RichieJarvis]

Hi All,

I've got (another) equipment problem I hope I can get your advice on.

I loosened the collimation screws on the Primary mirror and made sure they weren't too tight, and then attempted to image M81. Attached is the result.

If you look closely at the stars, you will see that they triangular in shape across the entire FoV of the SXV-H9. I used my MPCC inline with this (approx) 56mm away from the chip - or as near as I could get it.

The collimation was not pin-point perfect, but the doughnut both inside and outside of focus was pretty near the centre when this was imaged - I tested the doughnut using the SXV-H9, which seems to be alot less forgiving than my eye when I test collimation!

To me, it looks like light is flaring to one side (the right of each triangle), but I am at a loss as to what might be causing this in a Newt.

Can anyone offer any advice/places to start looking?

My first thought is to try it without the MPCC in-place and see if that is the problem, as the same imaging train on my AP102ED (minus MPCC) showed no such issues - could it be the MPCC? I do have to really tighten the brass compression ring to get the SXV-H9 to stay fixed in place (see the other thread about my problems with that part)

Any takers?

Cheers, and thanks in advance,

Richie

[GazOC]

Pinched secondary?

[John]

Pinched secondary?

That rings a bell with me as well Gaz. I'm not an imager but I have read that the secondary mount of GSO scopes (including Meade Lightbridges which use GSO optics) can "pinch" the mirror a bit sometimes. If it's like mine, the secondary sits fully inside a plastic holder wheras the Skywatcher secondaries are stuck onto a diagonal mount. I think the cure is to remove the secondary, trim the plastic holder a bit then re-mount the secondary.

John

[barkis]

I don't suppose the main mirror is pinched. Does the cell have lateral adjusters that may have been over tightened. The secondary pinching seems more like the culprit though.

Ron.

[FLO]

It does look like pinched optics, the secondary seems to be the favourite suspect but check also the primary.

... haven't we discussed this somewhere else :scratch:

[barkis]

Yep! A case of Deja Vous.

Ron.

[RichieJarvis]

Erm, yes - but the difference was that that was an image posting, whereas this is a cry for help!

I redid the primary, and made sure that the lock screws were not over-tight, so I dont think its that - which leads me to the secondary.

The problem is that the mirror is stuck to a piece of plastic, and there is only one (loose) mounting on it, so I don't understand how it can be pinched!

Yours confused,

Richie

[Trevor]

Hi Richie

Could it possibly be some flexure in the imaging train? It wont take very much, re-collimate the scope BUT leave attached all the imaging equipment to the focuser, hang the imaging train from the focuser with some chain or strong string while you collimate.

See here.

http://www.astroimage.info/images/equipment/newtonian_collmination_test.htm

I guess the next problem will be working out the orientation of the camera associated with where to hang the weight of the camera from on the focuser.

Might be worth a go.

Trev.

[Eddie]

Looks like it's out of collimation, that last little tweak can be critical on a fast scope. I have had such an effect on the GSO, and it was the collimation. I have put your image through CCD inspector and it also says its the collimation.

HTH,

Eddie