Coma correctors

[neptune1]

I'm not considering buying one until I look down my F4 reflector without but was wondering at what magnifications they are most helpful? Presume so at wide field rather than high mag???Thanks

[oldpink]

they are normally used for astro photography to deal with stars close to the edge of field getting bloated

it also acts as a reducer so drops the F ration down

[neptune1]

Thanks oldpink, I'm thinking for visual use rather than photography.

[wolfpaw]

they are normally used for astro photography to deal with stars close to the edge of field getting bloated

it also acts as a reducer so drops the F ration down

Hmm...I would disagree with that, tbh. If coma is visible across 50% to 80% of the field of view when using a fast Newtonian and wide angle, low power eyepieces for visual observation then a coma corrector becomes something of a necessity. 'Edge of field' has nothing to do with it in my experience.

As I get significant and noticeable coma in my f/4.7 I suspect neptune1 will experience even more aberration in an f/4. In fact it must be a mathematical certainty that f/4 will show a lot more coma than f/4.7.

[Moonshane]

I'm not considering buying one until I look down my F4 reflector without but was wondering at what magnifications they are most helpful? Presume so at wide field rather than high mag???Thanks

coma is most obvious in low power wide field eyepieces but to be honest if you buy a paracorr/altair astro or similar they act like a barlow but one that's always left in the focuser.

[oldpink]

personally for visual I wouldn't bother
but being I'm into AP its a purchase I will make soon, it will most likely be used with my DSLR mounted onto the T ring

[Moonshane]

Hmm...I would disagree with that, tbh. If coma is visible across 50% to 80% of the field of view when using a fast Newtonian and wide angle, low power eyepieces for visual observation then a coma corrector becomes something of a necessity. 'Edge of field' has nothing to do with it in my experience.

As I get significant and noticeable coma in my f/4.7 I suspect neptune1 will experience even more aberration in an f/4. In fact it must be a mathematical certainty that f/4 will show a lot more coma than f/4.7.

you are quite right in that coma starts small and expands across the field. this is why the 'coma comets' are smaller in the centre of the field than the edge. as you increase magnification though (or narrow the field) the level of apparent (i.e. visibly detectable) coma reduces. in all cases it reduces with a coma corrector. personally, I would recommend a coma corrector from f5 or faster but many don't. in fact I use no corrector in my f5 scopes but do in my f4 scopes.

[Moonshane]

this oft quoted article http://www.umich.edu/~lowbrows/reflections/2007/dscobel.27.html shows the effect of coma and other aberrations very well.

[wolfpaw]

you are quite right in that coma starts small and expands across the field. this is why the 'coma comets' are smaller in the centre of the field than the edge. as you increase magnification though (or narrow the field) the level of apparent (i.e. visibly detectable) coma reduces. in all cases it reduces with a coma corrector. personally, I would recommend a coma corrector from f5 or faster but many don't. in fact I use no corrector in my f5 scopes but do in my f4 scopes.

This does appear to be true, although I'm surprised so many people are happy with half of the field of view consisting of out-of-focus stars.

[Moonshane]

as always it's a matter of compromise. a 10" dob is easy to use and handle and gives great aperture for not a (relative) lot of money. a 10" refractor, SCT or Mak is far more costly, possibly heavier and in the case of the refractor, somewhat rarer. if you want a completely coma free and aberration free image for <£500 then buy a 80mm F7 APO. the views of faint objects will bear no comparison though with the 10" newt.

[mitchelln]

This does appear to be true, although I'm surprised so many people are happy with half of the field of view consisting of out-of-focus stars.

This was discussed at length on another thread with conflicting reports regarding the severity. Visually, it is down to whether the coma you see is acceptable to you or not. On my f4.9 dobo without a CC I only get coma toward the edge, not anywhere near 50% of the field.

It is worth noting that the Baader MPCC, unlike most CC's, does not affect the focal length or field size. It does a great job of ridding the optics of coma if they bother you.

[Pete Presland]

i bought the Skywatcher comma corrector for my 200PDS which also acts as a 0.9x reducer so F5 becomes F4.5. it was the first purchase i made for it, primarily for imaging but i felt it was worth while for visual as well.

[Mr Spock]

When I had a 10" Newt I had a Baader MPCC. I found with the 22mm Nagler it was essential.

[scarp15]

I think that it also depends on what it is you are aiming to observe. For example a faint galaxy group framed in the centre of field with a low power wide field eyepiece or small planetary, can be studied without distraction regardless of any hint of outlining coma towards the edge of field. Many open and Globular clusters once located, usually look best at high power, in which the impact of coma is less detectable. I do not use a coma corrector at F4.6 as it does not detract or impact the subjects that I view enough to warrant and I only notice it if I am consciously looking for it. However I may consider a baader MPCC one day based on the reasons stated above for low power perhaps. F4 though could be a different consideration. Yet depending on the range and quality of eyepieces and how you intend to observe specific objects, still best to try without and evaluate before making a purchase.

[Mr Spock]

Depends on whether you want wide field or just to go deep. If it's the latter then an ortho is better than anything

[mitchelln]

Depends on whether you want wide field or just to go deep. If it's the latter then an ortho is better than anything

Orthos are lovely:)

Sent from my GT-N7105 using Tapatalk

[AlexB67]

they are normally used for astro photography to deal with stars close to the edge of field getting bloated

it also acts as a reducer so drops the F ration down

Not as a general rule. The paracorr/GSO/ES coma correctors increase the f ratio a bit, depending on which one you buy, for example the paracorr increases it by 15%. The Baader Mk3 does not alter f ratio at all.   Worth bearing in mind while the difference is not substantial, when buying eyepieces your mags and exit pupils will be shifted a little bit.

If you end up being one of those observers  that may leave the coma corrector in place in a really fast scope such as f/4 for visual use all the time, and want to use it while looking at planets, a 200x mag eyepiece will become 230x.  You may just push it out of the zone you'd ideally want, so in that case a 7mm may be more suitable where a 6mm would have been without such a coma corrector.

Worth planning the eyepiece selection in advance a bit with the coma corrector in mind you are going to buy. With a paracorr your f/4 scope will effectively become a f/4.6 in terms of the magnification change, a bit of a shift, and an f/4 will become like a sort of f/12 or even better in terms of coma reduction with a paracorr 2. 

[wolfpaw]

I think that it also depends on what it is you are aiming to observe. For example a faint galaxy group framed in the centre of field with a low power wide field eyepiece or small planetary, can be studied without distraction regardless of any hint of outlining coma towards the edge of field. Many open and Globular clusters once located, usually look best at high power, in which the impact of coma is less detectable. I do not use a coma corrector at F4.6 as it does not detract or impact the subjects that I view enough to warrant and I only notice it if I am consciously looking for it. However I may consider a baader MPCC one day based on the reasons stated above for low power perhaps. F4 though could be a different consideration. Yet depending on the range and quality of eyepieces and how you intend to observe specific objects, still best to try without and evaluate before making a purchase.

This might be true of large apparent objects but not with smaller ones. For example, the last time I used my f/4.7 telescope was to look for the Eskimo planetary nebula. I was expecting it to be very small, almost like an out-of-focus star and so I put in the 24mm 68 degree eyepiece and tried finding it. Unfortunately, with 50% of the field of view obscured by coma, it was impossible to work out what was a faint planetary and what was just a coma-affected star. The only option was to being every part of the field of view into the central 50% of the field of view. Scanning the entire field of view simply wasn't possible because of the aberration. I gave up after a while and haven't used the telescope since.

[wolfpaw]

*'bring' not 'being'...

[scarp15]

That does appear to be quite severe if you cannot distinguish the nebula apart from the star field. It would be interesting to learn if the additional glass elements via a coma corrector has any effect on image? I am sure that It probably does not and I often for example, use filters to enhance certain objects, adding glass to the optical path. I read somewhere that certain particularly faint and challenging objects, such as the Horse Head nebula may benefit (besides the list of well debated requirements and filter type) from a plossl type eyepiece, Not just to narrow the field and cut out brightness, but because it is only constructed in four glass elements and so helps to tease out any clarity, perhaps in a similar way as a planetary eyepiece. 

[andyman]

*'bring' not 'being'...

Dont be too hard on yourself...we all make mistakes  

[neptune1]

The coma correctors like the Paracorr 2 look quite large. Does that then limit the size of the EPs the reflector on a dob mount can support? If someone could post a picture of their reflector with a coma corrector and eyepiece in situ I would be very grateful.

[neptune1]

Depends on whether you want wide field or just to go deep. If it's the latter then an ortho is better than anything

Mr Spock -are the Orthos very different to the Televue Plossls? Thank you.

[Moonshane]

here's my OOUK 12" f4 on home made base with paracorr and 26mm Nagler

[Moonshane]

here's the thread with more pics

http://stargazerslounge.com/topic/181536-12-f4-orion-optics-ota-on-home-made-dob-base-my-latest-project/