f/5 reflectors

Post by **VinSnr** » Tue Sep 27, 2005 11:05 pm

Hi guys,

Anyone here has a f/5 reflector? How bad is the coma?

Thanks

Post by **weixing** » Wed Sep 28, 2005 12:13 am

Hi,
I got the 6" F5 Newtonian... I didn't actually look for the coma, so can't tell you how bad is it... I'll try to estimate how bad it is next time.

Anyway, I only notices the coma when at 30x (1.9 degree TFoV) using the Super Erfle 25mm eyepiece... I roughly estimate that coma start to appear at 10% - 20% of the edge of the Field of View... may be better if you use a better eyepiece.

By the way, if you use a 2" focuser, you can always get a coma corrector for it... Too bad my 6" Newtonian only got a 1.25" focuser...

Have a nice day.

Post by **VinSnr** » Wed Sep 28, 2005 1:36 am

weixing wrote:Hi,
I got the 6" F5 Newtonian... I didn't actually look for the coma, so can't tell you how bad is it... I'll try to estimate how bad it is next time.

Anyway, I only notices the coma when at 30x (1.9 degree TFoV) using the Super Erfle 25mm eyepiece... I roughly estimate that coma start to appear at 10% - 20% of the edge of the Field of View... may be better if you use a better eyepiece.

By the way, if you use a 2" focuser, you can always get a coma corrector for it... Too bad my 6" Newtonian only got a 1.25" focuser...

Have a nice day.

Weixing, let me know if you go observation next time. I want to try my Pentaxs on a f/5 and see if the coma is acceptable. I also want to see the quality of these Chinese mirrors. People raved about it, but I have not seen one.

Guan Sheng is claiming 1/12 wave for their mirrors and if that's true, it is quite incredible.

Anyway, if you didn't notice it, then I believe it shouldn't be that bad.

Post by **weixing** » Wed Sep 28, 2005 1:47 am

Hi,

Guan Sheng is claiming 1/12 wave for their mirrors and if that's true, it is quite incredible.

So you are looking at the 10" F5 or the 12" F5 dob... hee hee

Have a nice day.

Post by **acc** » Wed Sep 28, 2005 6:30 am

Coma is intrusive on my f4.7 with low-power eps between 20-30mm (even Naglers). Not a problem at very high (<15mm eps) or very low pwer (35 pan) eps. Not sure how that translates at f5 though.

MooEy · Post by **MooEy** » Wed Sep 28, 2005 6:37 am

i don't think coma is much of a problem for newtonian, it's often an overstated problem. i think it's the people who owns cheaper eyepieces with fast scopes that complain about coma.

on a 8" f/6, coma is almost non existence with any eyepieces i have used. on the r200ss, a 8" f/4 newt, coma creeps in only in the very last bit of the field in the tak 30mm LE. the cheaper 20mm vixen lathanum does show some coma in the last 10% or so. any cheaper eyepieces, esp in the 30mm range shows lots of coma. putting in the 15mm pano removes every single bit of coma. i think with ur collection of pentax, u can safely ignore any coma problems with a f/5 system.

~MooEy~

denebman · Post by **denebman** » Wed Sep 28, 2005 9:23 am

It is not surprising that the longer the focal length is the eyepiece, the easier you see the coma. Longer focal length eyepiece always has bigger clear aperature, which means that more light from the periphery of the light cone reflected from the secondary mirror is thrown into the eyepiece. The periphery light (more off-axis) from newtonian always has more coma.

Eyepieces with modern design eliminate astigmatism well, but they still lacks good coma and field curvature correction. Some plossl type ep is designed with negative coma as compared to newtonian mirror, it performs excellent on newtonian.

Canopus Lim · Post by **Canopus Lim** » Wed Sep 28, 2005 10:10 am

Yes agree with you guys. The smaller the F-ratio, the harder it is for the eyepiece to correct the coma as the ray angle is steeper. For on-axis rays (paraxial case).. the eyepiece performs the best as the angle is very small or zero; that's why star collimation requires to put the defocus star in the centre of the field. For bigger clear aperture, more off axis rays will enter the eyepiece and the aberration will be more pronounced. In any optical designs, the system is optimised for certain range of f-ratio or focal lengths. It can't possibly perform well in all cases. But generally the higher the f-ratio, the smaller the off-axis angle, the better the optical performance as the aberrations like spherical, coma, astig can be corrected more easily by the lens system. I guess.. more expensive eyepiece will be better optimised as they can afford to select more types of glasses (and hence a wider range of refractive indexes and possibly higher cost) and therefore better correction.