Coma correctors

[Nick P]

I would like to call on the collective experience of SGL for help as I am sitting here pondering the best way to eliminate coma (for visual use) from the views in my 4.5 newt and can only really find two options, being a TV Paracorr or a Baader/skywatcher coma corrector.

As the Paracorr comes in at over #300, is it really that much better than a Baader/Skywatcher coma corrector at a third of the price?

[Moonshane]

I see nobody has answered this.

I am debating the purchase of a Paracorr and have never used a coma corrector at all so cannot really advise. I would say though that people sometimes confuse astigmatism etc in their eyepieces as coma. Coma is usually masked by this unless you buy top quality eyepieces.

If you have such eyepieces then OK.

I feel the benefits of a Paracorr over a MPCC etc is that you can use the paracorr like a barlow and not have to keep changing things round. also, they have a tunable top allowing you to fine tune the correction to the eyepiece currently used. this is much more tricky with a BMPCC I suspect.

hope this helps a bit.

[Ben Ritchie]

AIUI the Paracorr is good if you regularly switch between longer focal length eyepieces (particularly with large AFOV) in a fast Newtonian, as it can live in the focuser and you just swap eyepieces and adjust the tuning if necessary. That's much easier than threading/unthreading a MPCC (or Skywatcher equivalent) in the dark everytime you swap over.

However, it depends on how much you care about coma and/or how fast your Newtonian is. I find I only really want a corrector for ultrawides greater than about 20mm focal length, and generally only own/use one of those - so there's no real benefit in the flexibility of the Paracorr for me. Plus the MPCC is much cheaper. YMMV, of course.

[PeterW]

The MPCC needs to have precise positioning away from the focal plane of the eyepiece (55mm), which for imaging is "easy" as you know where the CCD plane is. Eyepieces do not give you much help in where their focal planes are, so you'd have to fiddle about with spacers for each eyepiece you'd want to use. A pain, but a cheaper one than than a parracorr. Having just bought an F4 newtonian I was mildly shocked by the excessive coma round the edges of the field of view..... I intend to image, so I am not too worried.

Cheers

PEterW

[Ben Ritchie]

This helps a lot for positioning

MPCC

I guess my other thought is that the Paracorr is apparently the 'better' design optically (although I can't remember the details of why this is) so I suppose the usual weakest-link argument applies to some extent if you're using high-end eyepieces and high-end optics.

[cs1cjc]

The Altair Astro coma corrector (same as the Astro-Tech, optical design by Roger Ceragioli, manufactured by GSO I understand) works well for me.

Altair Astro Photo Visual Coma Corrector and Field Flattener for Newtonian reflectors

It is *much* cheaper than the Televue products and with a design spacing of 75mm, it is more tolerant of eyepiece focus position so does not need a turntable for a reasonable collection of eyepiece, but you should measure focus positions. It is supplied, with a 50mm 2" adaptor, so it needs additional spacing with at least a 14mm Baader Hyperion fine tuning ring and perhaps and empty 2" filter or two, dependent on eyepiece set and the height on your 1.25" adaptor. It adds nearly 50mm of out focus so I have a (cardboard) washer between the spacers and coma corrector to add height.

[Ben Ritchie]

Interesting, I wasn't aware of that one

[Bender]

There are quite a few discussions on CN and it seems that the MPCC corrects better for coma but on the other hand introduces , some say and objectable amount, of sperical aberations.

I just got my f4.5 newt back and wait for a MPCC to arrive so I am currious as well what works better.

The come certainly is there at the monet without a corrector when using a Pan 24. Stars out side the axis clearly develope into seagulls making it impossible to tell doubles from single stars.

If you have only TV 1.25" eps, as I do, than its nice and easy to build a setup with the MPCC that you can leave in the focuser when swapping eps.

[Moonshane]

AIUI the Paracorr is good if you regularly switch between longer focal length eyepieces (particularly with large AFOV) in a fast Newtonian, as it can live in the focuser and you just swap eyepieces and adjust the tuning if necessary. That's much easier than threading/unthreading a MPCC (or Skywatcher equivalent) in the dark everytime you swap over.

However, it depends on how much you care about coma and/or how fast your Newtonian is. I find I only really want a corrector for ultrawides greater than about 20mm focal length, and generally only own/use one of those - so there's no real benefit in the flexibility of the Paracorr for me. Plus the MPCC is much cheaper. YMMV, of course.

hi Ben

Another member on another thread suggested that the difference with Ethos is quite staggering so (although I presume you already have?) might be worth trying your 13E?

I am going to eventually get a Paracorr as I have plans for a f4.5 and I suspect it will even improve the views in my 5.3.

Best wishes

Shane

[cs1cjc]

For a given focal ratio, coma is proportional to the distance off axis in all newtonian telescopes. This is best illustrated by Al Nagler's Paracorr spot diagrams (apologies for the reference to another site):

http://www.cloudynights.com/ubbthreads/attachments/4265205-Paracorr%20Graphs.jpg

This might give the impression that coma will look larger for longer focal length eyepieces, but it is not the case, because shorter focal length eyepieces magnify the image (and coma) more, exactly balancing the real decrease in coma. However at higher magnification the image is dimmer so the coma may be less visible and at some point coma disappears into the airy disc, for example at about an image radius of 1mm for an f4.5 mirror or an image diameter of 2mm. For comparison the stop size of a 40* 9mm ortho is 6.1mm diameter and an 80* 4.5mm would have the same stop size. At the edge of field in such eyepieces, uncorrected coma in an f4.5 newtonian extends to 3x the airy disc. The paracorr 1 (and one hopes other coma correctors...) reduces the coma into the airy disc.

There is a good non-mathematical description of newtonian coma at:

http://www.opticalmechanics.com/technical_articles/about_coma.html