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Originally Posted by Rodstar
Has anyone had experience with using a Paracorr? 
I am particularly interested in whether it has a significant negative impact on light transmission and image contrast. |
Rod
I have used a Paracor for a night comparing a 20" F4 with PCOR to a 20" F5 with and without.
What needs to be understood is that coma is a linear aberation that starts almost at the centre of the field . Whether you own an F5 or F4 etc you are losing faint stars from the view as when the aberation blurs out a faint star it disappears from your view. Check out the star spot radius graghs I've attached for the Paracor. Think of the inverse square of light, expanding a given spot of light to twice the size and you dim it by a factor of 4.
What you can see from the graph is that the radial comatic blur at the edge of an F4.5 field is 50 micron compared to 40 micron for an F5 and that the degradation is linear from centre to edge. Look at the effect th eParacor has in tightening star images across the field down to near the diffraction limit. I was startled by this effect visually and made me realise that an F4 Scope with a Paracor is a better visual scope than an F5 without.
The Net result is that in good seeing conditions, there are many more faint stars to be seen scattered across the field with the Paracaor, as the star images are tighter and brighter. Sure there is the usual 5% light loss , but this is below the human detection limit ( about 10%) and this is more than made up for by the tightening of star images.
The differnce between mirror Coma and eypiece astigmatism needs to be mentioned . The umbrella shaped aberation seen at the edge particularly badly in Chinese low cost low power eyepieces is Astgmatism. This come from the eyepieces in ability to handle the fast light cone. Naglers correct completely for astigmatiam and leave you with pure Coma. With addition of Paracor to Nagler you have pinpoint across the field. It needs to be mentioned too that the Paracor has a 1.15X Barlow effect. An F4 becomes F4.6 and and F4.5 becomes F 5.2.
Edge astgmatism usually about 4 X larger than Coma , so average eyepiece plus coma removing Paracor will only help marginally, as far as the very edge of the field is concerned. Panoptics only correct edge astigmatism partially so still look great with Paracor but not perfect. I did make the discovery recently that the TV Radian 18mm and 14mm have excellent astig and coma correction down to F4. At F4 you can let Jupiter drift into the field and see detail as it first drifts into the field. Amazing. With a rich 4.5mm pupil what a golden combination.
So I guess , you get what you pay for...Nagler plus Paracor is perfection, and Radian is narrower but a very cheap fix. I think at $500 or so the Paracor is well worth the money.
I found these two reviews on the Net which I think confirm my findings: an interesting read anyway.
http://www.excelsis.com/1.0/entry.ph...2866708e6f8668
Hope this helps.
Mark