How much coma in a 10" F4 Newtonian

[CalvinKlein]

I am considering buying an inexpensive 10" F4 Newtonian mainly for capturing the ISS and for tracked planetary imaging. Until now I've quite successfully used a 7" triplet refractor for each but need more aperture for the ISS.

I use a Televue Powermate 4x and ASI183MC for the imaging.

Can anyone give me an indication of how much of the Field of View is totally useless due to coma -ideally with sample images.

For manual ISS tracking ideally its good to utilise the entire sensor but if, say, the centre 30% remains sharp-ish and the rest coma affected - that might still be acceptable - any less would be too difficult to get centred.

Thanks

[multiweb]

A 10" F/4 newt would be quite a challenge to collimate and correct. I'd say ~70% of the fov would be coma free.

[Merlin66]

According to Rutten and Van Venrooij "Telescope optics" p 47 it would suggest for an f4 system that there will be noticeable coma beyond about 5mm off axis.

[CalvinKlein]

Hmmmmm - 5mm off centre vs 70% coma free is a big difference.

I read some theory on one of the optics sites talking about aberrations close to the centre axis but i assumed that it would still be tolerable much further out than that. That would kind of imply that a 10" F4 is pretty well useless without a corrector which makes you wonder if the product is really fit for sale - rather than advertise "coma corrector recommended" advertising should read "coma corrector mandatory" - in which case why isn't it built in ?

[multiweb]

Not really. 5mm off center means you'll cover a 10mm diagonal chip. Depends on your sensor size . If it's a planetary camera your sensor will be tiny and you might not have any coma at all. Just got to figure out what your imaging circle is on an uncorrected 10" F/4 newt (I'd expect 15mm at least) then work your way back to your sensor size and work out the surface you can work with.

[Merlin66]

If you really want to read up on Coma:
https://www.telescope-optics.net/coma.htm

To quote from "Telescopes Eyepieces Astrographs" p172
For a 200mm f4.5 Newtonian ....only 7.5 arcmin off axis the coma has grown to over three times the Airy disk.

p175 they discuss wide field observing with an eyepiece (27mm) giving a 1.5 degree FOV (0.75 deg off axis) - the coma, towards the edge, would be 28 arcsec - "which is larger than the disk of Mars at it's closest"

[CalvinKlein]

Ahhh gotcha Lol i'm looking the other way at this - as in out the OTA into space :-) 5mm out of 254mm didn't seem right.

I'm using an ASI183 which is 13.2 x 8.8mm and about 20 megapixels so I might be okay then.

[multiweb]

You might get away with it.