Adding a focal extender to the imaging train?

[kbrown]

I'm thinking of getting one of these for planetary imaging purposes:

https://www.firstlightoptics.com/barlow-eyepieces/explore-scientific-2x-barlow-focal-extender-2.html

At the moment the imaging train on my 10" newt is a coma corrector in front of a filter wheel and zwo ASI 183mc pro on the other side. This puts the ccd at the required distance of 55mm of the coma corrector. What I'm not sure of is would the focal extender work between the ccd and filter wheel?

[vlaiv]

4 hours ago, kbrown said:

I'm thinking of getting one of these for planetary imaging purposes:

https://www.firstlightoptics.com/barlow-eyepieces/explore-scientific-2x-barlow-focal-extender-2.html

At the moment the imaging train on my 10" newt is a coma corrector in front of a filter wheel and zwo ASI 183mc pro on the other side. This puts the ccd at the required distance of 55mm of the coma corrector. What I'm not sure of is would the focal extender work between the ccd and filter wheel?

No need to use coma corrector when doing planetary type imaging.

Each newtonian scope has "coma free" zone at the center of FOV. There is formula to calculate this zone and in most cases when you use small sensor and a barlow, you'll be operating within this zone where coma is smaller than airy disk and therefore not really an issue.

Taken from this page: https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

formula for diffraction limited field (strehl>=0.8) r = F^3 / 90

This means that for F/4.7 without barlow this gives radius of 1.1536mm, or diameter of 2.3mm. With x2 barlow / tele extender, this is diameter of 4.6mm

ASI183 has diagonal of about 16mm - so only 1/4 of it will be usable, but that is still plenty - just use ROI and go for something like 1200x800 central ROI (that should be 1/4).

Only issue is if you are trying to do lunar with larger FOV, then coma correcting barlow might be better option:

https://www.apm-telescopes.de/en/optical-accessories/barlow-lenses/1.25-barlow-lenses/apm-comacorrecting-1-1-4-ed-barlow-2.7-x.html

or

https://www.apm-telescopes.de/en/optical-accessories/barlow-lenses/2-barlow-lenses/apm-2-inch-coma-correcting-telecentric-1.5x-barlow