Does a Parracorr alleviate the need for expensive EPs in a fast Dob?

[Bart]

Just wondered?

I have a 10" and if I come into money in the next year or so, might convince my wife that I NEED a 16" Dob. I wondered if the EPs I have now for the 10", Explore Scientifics and BSTs, will fail terribly in the 16". Would a Parracorr help? or will the purchase of a 16" F4.5 mean the need to add another €1k for premium EPs?

God help my bank account............

Thanks in advance for comments, or sympathy!

Bart

[Damo636]

I may be wrong Bart (probably in fact) but i think the Paracorr will only help with the coma associated with fast scopes.

[Moonshane]

This is a very good question and although I have a Paracorr and indeed a 16" dob I am not sure I can answer it.

A coma corrector corrects the coma (a feature of the mirror) rather than any faults in the eyepieces so I suspect that defects would still show given the speed of the mirror. In truth I don't know how CC's work just what the effect is - that they make a massive difference in my f4.

Another 'problem' is that I only have good quality eyepieces. That said, I am sure that the view through the 16" and Paracorr would be as good across the field as the 10" but with the coma removed. If you are not bothered with any defects (there may be none that are that obvious) currently then I think you'll be OK. I'd suck it and see.

[cs1cjc]

In my experience a coma corrector (Altair Astro in my case, aka GSO/Astro-Tech), makes a terrific difference even with modest eyepieces.

[Bart]

This might be stupid question then, but is coma something to do with false colour/spikes and astigmatism a flaw to do with image distortion. Apologies for the probably basic question. I'm hanging my head with shame...

Bart

[andrew s]

Bart - Coma is a specific optical aberration as is astigmatism. False colour is also an aberration due to refractive optics and spikes are a diffraction effect of the spider vanes. The speed of the scope F4.5 is the challenge to eye pieces and a Parracor may help a little here as it increases the focal ratio of the scope a bit (not all coma correctors do this). However, its prime role is to remove the coma which is due to the Newtonian primary mirror and inherent in the design. The coma increases with the speed of the scope. The lower the focal ratio the more obvious the coma.

Regards Andrew

[Bart]

Thanks Andrew. The bigger Dob will present some challenges so.

Just a thought then following on from what you said. If increasing the focal length helps, would I be better off leaving a good quality barlow in there and use EPs twice the focal length of what I actually want, if that makes sense? Or am I just messing at this stage?

Bart

[andrew s]

Bart - You just move the problem to the barlow which has to deal with fast F#. So you would need a quality barlow designed for F4.5 so your into expensive ones or Powermates. In fact the short focal length Tele Vue eye pieces have barlows built in.

A 2" x2 Tele Vue barlow might be a good inverstment. You may also need a coma corrector unless you are happy to put up with the coma. Look a the Tele Vue web site as it has some good discussions about these issues if I remember correctly.

Andrew

[cs1cjc]

If increasing the focal length helps, would I be better off leaving a good quality barlow in there and use EPs twice the focal length of what I actually want, if that makes sense?Bart

Coma is caused by the fast parabolic mirror. A barlow will not reduce it.

Coma is zero on axis and increases linearly as you go off axis. Its impact is proportional to the apparent distance off axis so it is more visible with wide angle eyepieces. With higher magnification, the airy disc is is apparently bigger and the image is darker so the visibility of coma is reduced.