Why are my stars odd shapes in the corners?

[Jasp05]

Hi guys,

Just trying to figure out what is causing my stars to be odd shapes in the corners of my images.

It seems like its coma but it is not uniform at each corner or edge of the frame. see example image attached (60sec ISO 400) . Astrobin Link for higher res. - https://www.astrobin.com/vwk25w/

the bottom corners seem to the be worst of the 4, the top right seems a bit out of focus and the top left is probably ok.. But the effect of the stretching stars seems to start quite early from the centre of the frame if you look closely. by the time your at 50% of the frame you can see the coma effect creeping in.


So my imaging train is as follows.

Skywatcher ED72 Evostar
Orion 0.8x Field Flattener reducer (Slip in fit, not screwed)
Canon T2 Adapter
Canon 550D astromodified.

I have tried using 2 different dslr's and the corner's of each frame end up looking the same which makes me think its not sensor tilt due to my modifying the camera. (the other dslr I tried was an unmodded 1200D).

So I "think" it could be one of the following..:question:

1. Incorrect spacing on the flattener. Although everything online says the flattener should work perfectly if used with a T2 adapter to DSLR. No spacing required. ( I believe 55mm Back focus is required. )
2. Pinched optics from the lens cell.
3. Focuser Sag
4. Me expecting too much from my field flattener. This is my first time using a flattener on a scope so maybe my expectation of the corrections these things make is inflated.

Would appreciate any input on what may be causing this issue and whether it is correctable or how to go about trouble shooting it.

[Imme]

I'll bet it's the slip in flattener....had the very same issue myself.

My solve - Joshua (member here) is currently making me a lovely custom screw in adapter at what I think is a pretty reasonable price

[tim.anderson]

If the spacing between your flattener and the camera sensor is too long, the stars will be tangentially elongated. If the spacing is too close, the stars will be radially elongated.

Your flattener-sensor distance is evidently too close, as your stars are radially elongated. Check the flattener manufacturer's spec to see what the correct spacing is and add a spacer. Or do what Jon and I both did - get Joshua to make a custom adapter that is exactly the right length.

[Jasp05]

Do you know what the correct length is for the spacing?

I assumed I would need an adapter of some sort but didn't want to go buying spacers without knowing exactly which one I would need.

And even though the coma is "Uneven" between the corners this would still be fixed with the correct spacing?


You guys mention Josh is making adapters. I'm not familiar the name on the forums sorry. Do you guys have contact details for him?.

[Nikolas]

55mm backfocus from the camera sensor to the field flattener glass, my bet is that your backfocus is not correct

[tim.anderson]

It isn't a back-focus problem (the stars in the image are in focus), it is a problem with the spacing between the flattener and the sensor. Too long and you get tangential elongation at the edge of the field of view, too short and you get radial elongation. Only the flattener manufacturer can tell you what the correct spacing is, unless you want to work it out by trial and error.

[Nikolas]

I meant the spacing between the sensor and the flattener apologies for the misunderstanding

[tim.anderson]

I've been wrestling with this star elongation problem for some time and I think I may now understand it. So here is a Star Elongation Summary 101, feel free to tell me that I am wrong:

1. Let us assume you are using a refracting telescope

2. The faster the F number of the telescope (the smaller the F number), then the more curved will be the focal position across the field of view of the camera sensor (the camera sensor is, in principle, perfectly flat)

3. If we don't do any corrections, then if the stars are in focus at the centre of the camera's field of view, the corresponding point of focus for stars at the edge of the field of view will be in front of the camera's sensor, and hence the stars at the edge of the camera's field of view will be out of focus

4. To correct this curvature problem, we put a field flattener in front of the camera sensor. A field flattener is essentially a "reverse lens" - it is designed to reverse the forward curvature of the focal position across the field of view to ensure that everything is in focus from centre to edge when the photons hit the sensor.

5. For a field flattener to work correctly, it has to have two properties:

a: its degree of reverse curvature has to match the degree of forward curvature of your telescope (not necessarily exactly, but at worst within an error smaller than the resolution of your camera sensor). In other words, the faster your telescope is, the more reverse curvature your flattener needsto have.

b. the distance from the back of the field flattener to the camera sensor has to be an exact match, which should be specified by the manufacturer of the flattener. If the manufacturer of your flattener doesn't specify a spacing distance, then dump your flattener and buy a good quality one. If you can't match the flattener-to-sensor distance correctly, then you will either have radially elongated stars (too short distance) or tangentially elongated stars (too long distance at the edges of your camera's field of view)

If you have invested in a good quality fast refracting telescope, and a good quality camera, then you deserve it to yourself to buy a good quality flattener and a spacer adapter to get yourself the right flattener-to-sensor distance.

Joshua Bunn makes such adapters to order and he can be contacted on this website. I thoroughly recommend his workpersonship.

[Jasp05]

Thanks Tim.

My understanding was the Orion Field Flattener I have needs a back spacing of 55mm. And from everything I've read this is achieved just by using a DSLR and a T-adapter.

I'm using the above combination but am still having an issue. I'm 3d printing some spacers in the range 0.5mm to 3mm to see if that helps with anything.

If it does I will invest in a proper spacer adapter.