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View Full Version here: : Newtonian f/5.6 and coma corrector with APS sensor, needed or not?


bojan
14-05-2018, 10:30 AM
I intend to use my old 10" Newt (permanently mounted on EQ6, with enlarged secondary) for AP with DSLR (APS format).
For this setup I an not sure I really need the coma corrector... I definitelly don't want to spend too much, I am already in trouble with SWMBO..

Any thoughts about low-cost solution?

Thanks in advance!

Atmos
14-05-2018, 12:29 PM
For visual it would be fine but you’ll notice it on an APS-C sensor. As for how much, your best bet would be to try it and see if you mind the amount of coma.

Imme
14-05-2018, 12:41 PM
....Startools has a lovely 'Lens' module that can remove minor coma without degrading the rest of the issue too badly.

It's an option!

raymo
14-05-2018, 12:48 PM
You could also produce slightly wider fields than you actually want, and crop
off most of the coma.
raymo

bojan
14-05-2018, 12:57 PM
Thanks guys, that's what I thought it would be..
Long time ago I took one frame of M42 (to see how the whole setup behaves), but it was when Newt was on AltAz mount (motorized of course) and I think I remember the coma was not a big issue then (maybe because of field rotation).
Anyway, this is only planning exercise... I may be tempted to use some sort of focal reducer to widen the field a bit, but then again, mosaic (with Registar) may be a better option regardless of more time needed.

billdan
14-05-2018, 03:33 PM
The coma free zone is pretty small and outside this zone the coma gradually gets larger in a linear fashion.

The formula is --- Coma Free Zone = 0.022mm x FR³
So for your scope its
F5.6 = 0.022 x 5.6³ = 3.86 mm

You could make an off-axis mask, say a 4 inch hole fitted between the spider vanes. At your focal length of 56 inches with a 4 inch aperture would give you a F14 scope, pretty much coma free. With no diffraction spikes will give refractor type performance.

xelasnave
14-05-2018, 03:58 PM
Hi Bojan,
I purchased my f5 and never looked to see what the coma presented.
In reflection I wish I had worked with non coma corrected so I had something to look at the improvement.
I would try without which may tell you certain things.
In any event within software there is probably a tool that would enable a reasonable correction...You could see it as a chalenge to fix coma via softeware maybe.
And if you know what you are working with and know what will work for you. ..you woll know what to look for as a good buy in the classifieds.
I have not even colrrrminated this ...well was new...8 inch f5...but my thing has been to take something..the cube is working a treat..its warmer for a start and pack up a zip...and this is allowing me to get auto guiding sortted...so much to do...
Stick the scope on the mount and see what you are working wiuth ..crop and twiddle and post a resultalex

xelasnave
14-05-2018, 04:03 PM
My approach is now ..if its clear I take anything..ie polar is off go 15 seconds..if polar is better go 30 seconds...if 30 is good look at fixing auto guide and camera control...there is so little time ...you gotto attack with the wepons that are working today...coma is just all those galaxies in the background☺
Alex

bojan
15-05-2018, 07:10 AM
Alex,
My approach is similar.. use what's at hand, as long as it is practical and good enough.

I am not sure if software can correct coma well enough (as PtLens removes CA, foor example).

Anyway, I wil live with what I have for a while. Maybe a bargain pops up sometime in the future.

bojan
15-05-2018, 06:30 PM
I found that image of M42, taken with 10" Newt in prime focus, Canon 400D, in March 2012.
Focus could have been better, but still coma is almost negligible..
This is stack of 8 frames, 5 sec each, at ISO1600.

I think I will not need coma corrector... ;)

bojan
29-06-2018, 12:45 PM
Working on focuser... preparing my 10" Newt for AP.

I did some reinforcement (added ring on focuser body to keep it from stretching), so now I have to move the mirror for ~3mm up, to compensate for added height. This is easy, I just have to adjust the nuts & screws a bit and re-colimate.

xelasnave
30-06-2018, 10:00 AM
That looks rather nice.
Alex

raymo
30-06-2018, 11:49 AM
The lack of coma looks too good to be true. Are you sure that
you didn't crop the image at the time?
raymo

bojan
30-06-2018, 01:32 PM
Nope.

Attached here is single frame, full size.
BTW,the FOW is consistent with 1422mm FL and APS sensor.
Besides, I have record file (also attached)

raymo
30-06-2018, 02:08 PM
It appears to be so minimal that you will be able to ignore it.
Raymo

bojan
30-06-2018, 03:03 PM
Yes...

It may be more prominent with brighter stars...