Telescope Reviews: Mushy Coma

Hi,

After a few posts to a flat size thread on the ATM pages it set me off thinking about differences of opinion on the need for a Coma Corrector.

If you are running an undersized flat in a Newtonian then once the field illumination drops off the aperture is no longer circilar but a circle masked by another one and you end up with a sort of bi-convex lens shape.

As this is smaller radially ot would tend to expand the Airy 'disk' radially and presumably increase the coma as well.

I have never seen a spot diagram where this is taken into account. Perhaps someone with a suitable program or the maths to amend one could produce spot diagrams off axis for various under sized flats.

It would also be interesting to have reports of various sized flats and off-axis performance for a scope.

Cheers. Andrew.

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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x