Star elongation with 2600MC Pro and Z73

[smr]

I haven't been happy with the Stars in my images for some time and really want to get to correct it so that I can take images where the stars are round in all corners.

Take the FITS image attached. If you zoom in on it you can see the Stars are pulled in different directions in each of the corners...

 

Top Left

 

Top Right

 

Bottom Left

 

Bottom Right

 

Middle

 

 

Any ideas as to what this is ? Is it Tilt? 

 

L_2022-07-27_00-40-07_Bin1x1_180s__-10C.fit

 

[Elp]

If the stars at the edges look like they're radiating out from the centre of the image it's likely your camera is too close to the telescope focal point. Have you tried adding in another 1-2mm into the backspacing to see if it makes a difference?

What field flattener are you using? There's also a chance due to sensor size and the flattener being used it may cause elongation at the edges.

Edited July 27, 2022 by Elp

[symmetal]

It does look like a tilt issue. The right side corners look pretty reasonable while the left corners have elongated stars pointing generally inwards. This implies that the left side of the image, particularly the bottom left is closer than the optimum FF distance. Here's a CCD Inspector analysis

It's worth building a test jig as several others have done using pieces of wood and a cheap laser pen as shown here. Trying to adjust tilt by analysing star image tests is a never ending nightmare. If the camera doesn't have tilt adjustment then a tilt adjuster can be fitted in the image train before the camera, or the filter wheel if a mono camera, and the camera, filter wheel and tilt adjuster mounted together on the jig.

Alan

[smr]

31 minutes ago, Elp said:

If the stars at the edges look like they're radiating out from the centre of the image it's likely your camera is too close to the telescope focal point. Have you tried adding in another 1-2mm into the backspacing to see if it makes a difference?

What field flattener are you using? There's also a chance due to sensor size and the flattener being used it may cause elongation at the edges.

Thanks for the reply. Here's a pic of what I have... it's the WOZ73 1:1 Field Flattener with spacers...

[smr]

25 minutes ago, symmetal said:

It does look like a tilt issue. The right side corners look pretty reasonable while the left corners have elongated stars pointing generally inwards. This implies that the left side of the image, particularly the bottom left is closer than the optimum FF distance. Here's a CCD Inspector analysis

It's worth building a test jig as several others have done using pieces of wood and a cheap laser pen as shown here. Trying to adjust tilt by analysing star image tests is a never ending nightmare. If the camera doesn't have tilt adjustment then a tilt adjuster can be fitted in the image train before the camera, or the filter wheel if a mono camera, and the camera, filter wheel and tilt adjuster mounted together on the jig.

Alan

Thanks. I'm not that brilliant at DIY stuff but if this is the only way to do it I'll have to give it a go. Is there anything obvious in the image above which might be contributing / causing it?

[Elp]

Just to confirm the spacer rings you are using, do they measure 37.3mm total length (bottom of thread connected to flattener to camera face)?

That should eliminate the spacer issue.

[smr]

30 minutes ago, Elp said:

Just to confirm the spacer rings you are using, do they measure 37.3mm total length (bottom of thread connected to flattener to camera face)?

That should eliminate the spacer issue.

Do you mean this ... ?

[Elp]

At the top of the yellow line is that where your flattener ends (and the bottom of it's screw thread)? The bottom of the yellow line should be on the black face of the camera (the face pointing at the telescope).

[smr]

This is how the flattener came.

 

[Elp]

This distance (blue line):

[Elp]

Your flattener has a back focus distance from the bottom of it's thread of 54.8mm, the distance from the front of your camera to the front of the sensor is 17.5mm, so 54.8-17.5=37.3mm should be the blue line distance (ie your spacer distance bottom of thread to bottom of thread).

You can measure with a ruler near enough but better measured with vernier calipers.

Edited July 27, 2022 by Elp

[smr]

8 minutes ago, Elp said:

Your flattener has a back focus distance from the bottom of it's thread of 54.8mm, the distance from the front of your camera to the front of the sensor is 17.5mm, so 54.8-17.5=37.3mm should be the blue line distance.

Thanks, yes just measured it, it is 37.3mm.

[symmetal]

3 hours ago, smr said:

Thanks. I'm not that brilliant at DIY stuff but if this is the only way to do it I'll have to give it a go. Is there anything obvious in the image above which might be contributing / causing it?

Focuser slop may cause it, where the focus tube can move slightly side to side. The easiest way to check is to take an image after a meridian flip and see if the elongated stars change position in the image. if they do change then it's likely the focuser tube moving side to side.

If there's no change then take another image with the camera rotated 90 degrees.  If there is again no change  then the camera is causing the tilt. If there is a change it's the imaging train before the camera.

I've found that the camera is almost always the cause and putting them on the test jig, (once you've made it 😀) enables you to fix it in 5 minutes during the day. Your camera has tilt adjustment so it should be fairly straightforward.

The wood frame box doesn't have to be fancy, just rigid enough that it doesn't move while the camera is rotated on top. The top and bottom of the box don't have to be exactly parallel to each other either for it to work.😊

Alan

Edited July 27, 2022 by symmetal

[michael8554]

Some corner, spacing too close.

Some corners, spacing too far.

It's Tilt.

I believe the camera has Tilt adjustment screws.

Michael

 

 

[newbie alert]

Top and bottom left share the same angle of stretch

Same as the right

 

I'd say it's tilt rather than spacing issues