visual and AP performance between f/5 and f/10 scope

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cloud_cover
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Re: visual and AP performance between f/5 and f/10 scope

Post by cloud_cover »

I'd like to correct a few issues here
1. Eyepieces and off axis aberrations:
Most cheaper eyepieces assume the light coming in is in parallel rays. When the f ratio is small, then this doesn't hold true as the angle of the cone of light is quite steep. This produces coma-like aberrations away from the center of the eyepiece and is worse with widefields. Televue eyepieces are able to correct for this defect down to f/4
On the other hand, the primary optical disadvantage of a newtonian telescope, Coma, is not corrected for except in a very few, special and out of production eyepieces. This too causes seagull shaped stars away from the center of view and worsens with decreasing f ratio. This optical aberration requires a coma corrector, such as the Baader MPCC, Astrotech Coma Corrector or the much vaunted Televue Paracorr. Merely sticking a TV eyepiece in would not correct for Coma.

On a personal level, I have a 14" f/4.7 newtonian in which I don't see the need for a Paracorr visually even when using my widest EP, a 31mm 82deg. When using a CCD though, the co,a will be very apparent.
Also, poor collimation will produce optical defects far worse than coma so the mechanical construction of the telescope in how well it holds its collimation and how easily adjustable it is to collimate is important.
Again, having said that, its not difficult and I get very good views after less than 10mins collimating my 14"

2. Focuser: Crayfords are not necessarily superior to R&P designs. What matters is the quality of the machining. A well made R&P will outperform a poorly or even average-ly made crayford as R&P, when well constructed, tend to bear loads better without slipping. I'm not saying they are a superior design therefore, but just to point out one design is not necessarily inferior to the other.
Hope it helps :)

Finally, if you intend to mate a camera, do ensure your choice of scope has sufficient infocus to allow you to do so and come to focus!

Hope it helps :)
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orly_andico
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Re: visual and AP performance between f/5 and f/10 scope

Post by orly_andico »

good one kev.

i would like to point out though: while mis-collimation will produce terrible optical defects, collimation is much less critical above f/5. and even more so at f/6.

so a slower scope would be more forgiving of collimation, be probably of better optical quality (it's easier to make an f/5 or f/6 mirror than an f/4 mirror) and not require a Paracorr and some spendy eyepieces.

also, kev - the Paracorr is a mild barlow. So even though as you say it is primarily a coma corrector, it also helps poorer quality eyepieces correct for edge of field defects, because it slows down the entire optical train (an f/4 would become an f/4.6)
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antares2063
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Re: visual and AP performance between f/5 and f/10 scope

Post by antares2063 »

Having owned the Vixen R200S newtonian (f4) a few years back , i realised that the coma corrector is quite important for my visual use when paired with WA eyepieces..:) bought a corrector sometime later. I guess it really depends on personal basis.

cloud_cover: 14inch newtonian? (jaw drops)...how do u store such a tube in dry cabinet!

Regards,
Junwei
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cloud_cover
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Re: visual and AP performance between f/5 and f/10 scope

Post by cloud_cover »

Came with a raincoat :) previous owner must have kept it in a shed or outdoors. Now I just cover it with the tubular plastic sheet, ruffle the bottom a bit to make a sort of skirting and then use a $65 electronic dehumidifier inside the plastic cover. Keeps the RH at about 37-40% Just nice :)
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