Eggy Stars and M78

[Roy_E]

It seems to me that as soon as I solve one problem another appears. This time its eggy stars mostly showing in the top right hand side of the image, quite obvious in the image of M78. I loaded the autosave file from DSS into CCDInspector and the 3D display shows severe distortion corresponding to the eggy stars. Trouble is I don't know what the likely cause is - camera tilt perhaps - or how to correct it. Any help would be appreciated - its getting on my nerves.

Thanks in advance,

Roy

[jnc71106]

some more info on the scope camera etc might help the members answer your question. it looks like field curvature

are you using a field flattener ?

[Roy_E]

Sorry about that. I've changed my login and forgot to transfer details of my kit etc.

Scope: 200PDS

Camera: EOS 1100D self modded and fitted with a Baader MPCC

9X50 finder and QHY5 guiding

Image: stack of ten 420 sec exposures @ ISO 1600 in DSS - no darks or flats

[Jessun]

Looks like a tilt made the effects of the curvature worse - the bad corners shouldn't have to be that bad if the the chip was orthogonal.

Just the usual 2p.

/Jesper

[Roy_E]

Should have said that star distortion is worse in the top left hand corner. Thanks for the comment, but I was hoping for help towards a solution. Colimation seems to be fine, but if it is camera tilt how is it identified and corrected - trial and error?

Roy

[ollypenrice]

I'm sure tilt is making a naturally curved field worse, as Jesper says.

Check for play in your focuser draw tube. Tighten any adjustment on the stiffness of the movemet supports. If using push fit, ensure that you push the nosepiece all the way in up to the flange. Try packing with thin aluminium tape around the nosepiece as well.

Then you need to try inserting little shims between fouser and OTA to see what makes it better or worse.

It's trial and error, I'm afraid.

Olly

[Roy_E]

Thanks Olly, I had a feeling that it might come down to trial and error, which would be fine if I managed more than one clear night per month!

Roy

[Daniel-K]

make sure all the screws are tight on the focuser aswell, i had eggy stars when i got my quattro even though i swore my collimation was right, mirror pinching my be an issue as well or a tilt in the secondary.

[Tim]

As mentioned above, the problem is that the camera focal plane is tilted.

What exactly is doing this needs investigating. It may well be that the focuser draw tube or something is slopping, but to be honest you need to go further back to basics than that and ensure the scope itself is corrrectly aligned before you even attach a camera to it, ie you need to collimate the image train in the scope, including focuser, primary and secondary mirrors.

To me it looks like the scope isn't collimated properly to begin with. You will still get stretched stars towards the corners of an image, but they shoud be equally spaced around the edges of the image, radiating from a central point.

There's loads of thread on proper collimating of newts on here, and some videos on the wibbly wobbly too

Cheers

Tim

[rfdesigner]

When you collimate, try hanging the camera off the end of the focusser on a string/strap, and see how much the collimation moves with/without the camera.

Newts can suffer from lots of weight at the focusser as the tubes can flex unless additional support is added or the tube is naturally very stiff

Derek

edit: look at what OO do to their AG scopes to cope with heavy camera assemblies (note: it's carbon fibre super stiff tube)

you'll need to pan down a bit

http://www.orionopti...eand(ota)t.html