Oval stars

[Corries]

I have an Explore Scientific ED 127 Apochromatic Refractor and all the images I take show the following characteristics.  While all the stars on the left side of the image are nice and circular the stars on the right have a definite tendency towards the oval.  Would this be caused by a problem with the scope?  Does anyone know how it can be corrected?

[TobiasLindemann]

This is caused by a tilt of the camera, or flattener. Can you post an example? You can try to make a photo without flattener. If the image is symmetric  it is most likely caused by the flattener.

[Corries]

I don’t use a flattener.  I included a flattener when I used a Canon 5D camera.  Now I am using a SBIG ST-8300 camera.  Do I need to use a flattener with this camera also?  I have attached an example.

[Zocky]

Can you please stretch an image a little more, so that more stars become visible.
It looks that all of the stars are slightly elongated. How much payload can your focuser carry? It looks like the camera is "hanging" in focuser.

[Corries]

I'm not sure how much payload the focuser can carry but I have posted the stretched image.

[TobiasLindemann]

Hi,
can you please post a stretched image with a better resolution. You don´t see much on this little picture.
Yes, you need a flattener with the ST8300.

Tobias

[Corries]

Hi Tobias,

This image is better. 

Thanks for your help.

[TobiasLindemann]

Hi,
this is maybe a stupid question, but are you sure this is is made without a flattener? It looks a little bit to good for me.
But nevertheless it looks like a tilt somewhere. You can try to rotate the Telescope in its clamps to test if the camera is to heavy for the focuser. When the oval stars are at the same position, the problem is most likely a general tilt. When the position changes, the focuser, or something else is too weak, or loose.

Greetings
Tobias
 

[Corries]

Hi Tobias,

The image was made up of 10 each of red, green, blue and luminescence of 60 seconds exposure and then processed in PixInsight as per Harry Page’s video tutorials.  But definitely no flattener used.  I have now inserted a flattener and the camera seems to sit in the focuser better than before. 

I am hoping this has cured the problem.  When I get some clear skies I will test it and get back to you.

Thanks for your help.

[Corries]

I have now used the flattener hoping it has cured my oval star problem but I'm afraid not.  Could the oval stars be caused by the telescope needing collimating?

[Geoff]

Two things occur to me:
1.  The fact that the oval stars are on one side only suggests focuser tilt or objective tilt (miscollimation).
2. The direction of elongation changes from the top corner to the bottom corner.  This seems to indicate field curvature.

I would suggest checking and fixing the problems caused by Point 1.  When this has been done, any elongation should be radially out from the centre and should be very similar in all 4 corners.
If the image is still not acceptable due to field curvature, then a field flattener will be needed. 
Geoff

[Corries]

Thanks Geoff.  I will look into that.

Terry

[troypiggo]

Could also be the star you focused on?  I've found that if I focus on one star on the left edge of the image (perhaps because it's bright and easy to use, or FocusMax was autodetecting it), I can get something like what your image shows.  If I focus on a star around the middle third of image, usually stars pretty round.

I've recently started using Sequence Generator Pro.  It's autofocus feature uses the HFD of all stars in the image (if you choose), so the focus is best across the whole image.  I've had pretty tight and even stars across the field with this method.

As Geoff mentions, could also be tilt.  Just thought I'd throw something else to consider into the mix.

[jerryyyyy]

To get some quantification you might want to use the FWHMEcentrity Script under image analysis.  I have been using this with CCDInspector to collimate a Takahashi 180ED.  Another person with the same scope noted that my STT 8300M will have different effects on the rotator/focuser depending on the orientation of the scope... this is not an issue with a light camera, but this one is 5.2lbs....

Ultimately I have gotten it down to a reasonable error, but it will change when I change the camera orientation... slightly. 

I cannot collimate for each shot so have found a bit of a compromise.  I have been running CCDInspector on a number of "test" images each night... there are 10s image unguided.  Then getting the mean collimation errors from CCDInspector, then every few days try to tweak the collimation to optimize. 

BTW, on the subscript selector script you can also get rid of particularly eccentric flats. 

BTW2, if the gradient moves with the rotation of the camera, this is a probably collimation error...

Good luck