Wednesday, March 18, 2009

Getting more field of view from a Celestron C5

Last time I went stargazing, I really wanted more true field of view. I used to play with my C8 with a 40mm SMC XL before, which yields around 1.3 degrees true field. That session, I used my C5 with a 20mm Widescan Type III, that gives around 1.3 degrees true field as well, but the view provided by the C5 is just not as attractive.

To get wider field of view, we can either use a reducer or a bigger diagonal. People always said the small baffle of the C5 will prevent you from using a 2" diagonal, but who knows how worst could the vignetting get? So, why not give a try?

First up is a Hirsch f/6.3 reducer. I used the 1.25" visual back and a 1.25" Tele Vue diagonal for testing, my 20mm Widescan Type III is used... Yes, the true field of view is just much wider than the 20mm alone without the reducer. The focus difference with/without the reducer is quite a lot, and it's not so convenient in the field especially you will want to remove the reducer for higher power.

Second up is a 2" diagonal. I used a 2" visual back together with a generic 2" diagonal for testing, my 40mm Pentax SMC XL is used. The true field is indeed much wider than the 20mm alone without the reducer. And my impression is that, it yields even more true field than the above setup! The image is also brighter since the exit pupil is bigger for this one. To use higher power, we just need a higher power eyepiece! This is just more convenient in the field!

Having said that, the 2" visual back plus the diagonal is making the C5 even more back heavy... and it seems funny to use those large accessories and eyepiece with such a small telescope.

My verdict? Visually, I would prefer the 2" setup based on my Pentax SMC XL 40mm, but mechanically, the reducer based setup seems a little bit better.

But since I'm primarily a visual observer, I would say I prefer the 2" setup more!


Remarks: I'm now more inclined to keep my C5 after tuning that PST etalon, the result is just fantastic given its low price. Also, my desire to go for a Lunt's CaK diminished after knowing that they raised their price recently. Given the chance about going for a Borg 101ED is further reduced along with that CaK, I guess that I shall maximize my C5 instead of buying/selling anything at the time being.

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