Coma comparison, C8 vs C8 with Hirsch reducer/corrector vs C80-ED

[rikyuu]

I've had the chance to photograph M57 with my C80-ED refractor, the C8 SCT and the C8 SCT with a Hirsch 0.63x focal reducer/corrector.

I've attached the images for comparison described below with estimated FOV's, which are close to what CCDCalc calculates.

Image 1 - C8 SCT F/10 60sec ISO 800 FOV=40arcmin

Image 2 - C8 SCT F/6.3 (FR attached) 20s ISO 800 FOV=1deg

Image 3 - C80-ED F/7.5 18sec ISO 1600 FOV=2deg 5arcmin

Let me know your opinions whether you think the Hirsch has helped with coma.

It's clearly increased the FOV by the rated amount of 0.63 and reduced exposure times. I think coma has improved near the edges. There's a small amount of vigentting too.

Of course, the refractor appears to be relatively free of coma.

Thanks

[arad85]

And the images?????

[rikyuu]

And the images?????

Thanks, spotted that as soon as I clicked submit, but it took then 5 minutes to upload them due to being on mobile broadband. You were 2 minutes too early! :)

[arad85]

Let me know your opinions whether you think the Hirsch has helped with coma.

Difficult to see as the images are so small relative to the pixels. By the looks of things, I'd say the coma was MORE in the reduced one

[rikyuu]

I see what you mean, it's tricky to call, but the 3 stars on the lower right seem coma(tosed?!) near the edge of the FOV on the focal non-reduced imaged, but still have coma on the reduced image, but closer to the centre, thus indicating it may be worse.

I must also add that I did a very good star test collimation on the night of the non-reduced image, but it was a bit rushed on the night of the reduced image. Unfortunately, there weren't take at the same time. I think a more direct comparison is called for.

I also wonder if these reducer/correctors are meant to make the field completely flat across a wider FOV or just make it flatter than it would be with only a reducer with no correction?

The Hirsch is meant to be a copy of the Celestron FR/Corrector and I've read good reviews about it, but I wonder if anyone has done a direct back to back comparison. It would be interesting to know as the Hirsch is £46 vs £150 for the Celestron part.

If nothing else, at least it give a good 1deg FOV with the C8 and a 2.5x brighter image. That's got to be better than imaging at F10!