Coma Correctors?

[Alfian]

Hi all, I'm ruminating, as ever, about possible additions to the viewing stock, weighing up the needs and wants, the pros and cons, and what in the end it will cost me and how long it will consequently take me to acquire  the readies! 

Perhaps irrationally, I have something of a resistance towards f5 reflectors although I admit I do have a couple of older cheaper smallish examples which may not help my case!

Anyway to the point - in looking at coma correctors.  I've noticed that for example  in the Skywatcher 0.9 x item it makes the point that it reduces an F5 to a  an f4.5. Does this mean that "reverse barlow  fashion" this effectively would reduce say an F5 150/750 to an f4.5  "675mm", and if so does this automatically reduce magnification/increase  fov with any given EP? Sounds like a daft question now. Someone put me out of my misery!  The other question that follows is that given the expense of a Coma Corrector in relation to a scope OTA are they worth the outlay for visual  (no AP planned) purposes only?

[Tinker1947]

I have a Baader and never noticed any difference when viewing, it certainly does a good job with imagine.....

[estwing]

shouldn't this be in eyepieces section mods?

[John]

shouldn't this be in eyepieces section mods?

Yep - I've popped it over there with a link in the original place

[YKSE]

A question might be in place first, do you see that you need a coma corrector?

Coma do exist in a f5 scope, most visible in low magnifcation eyepieces, and the wider FOV of an eyepiece, the more coma will be there.

Then, another important thing is how much an observer sees coma, some sees 5coma in 0% FOV of 24mm 68 deg with 10"f4.7 dob, while there're others don't see coma in the same type of scope and eyepiece, that's what I've read here. Also, how much are you bothered by the coma seen? 

[russ.will]

I second that.

Assuming visual use; Coma Correctors are a luxury that aren't really 'needed'. If you have an EP case lined with UWAs of long focal lengths, then yes, it's a nice thing to have. I'd also say that I didn't remotely feel compelled to own one until I had a 12" F5 scope. Even then, I didn't really consider it added much at sub 20mm EP focal lengths and when I had an 8" F5 scope, I wasn't bothered at all.

In other words, spend the money on EPs and worry about CCs when the outer aFOV starts to bother you.

Russell

[Alfian]

Thanks guys, and sorry about this being misplaced. As things stand at the moment I certainly do not need a CC, but looking forward I was pricing up various alternative packages. Thinking I might(?) need a CC to get acceptable views through a larger F5 scope I was wondering just how necessary it would be for visual work, and I think you've answered that, reassuringly. I think I can cross it off my list  and  yes I would prefer to spend that kind of money on an EP.  Thanks again.

[YKSE]

Here's Bill Paolini's comments about eyepieces after having eyepiece book published, worth to think over it:

http://stargazerslounge.com/topic/187546-incoming-new-book-on-eyepieces/?p=2045228

[Paul73]

I have a Televue coma corrector for my f4.7. It is a luxury that I sometimes use to gat an alternative view. With my 30mm 82°, it certainly improves the outer 15%. But that outer 15% is generally context arounIt the target(s) so the coma isn't noticed.

Conclusion. Spend the cash on good eyepieces.

Paul

Ps. My corrector was bought 2nd hand in anticipation of a faster scope in the future.

[estwing]

I have a Televue coma corrector for my f4.7. It is a luxury that I sometimes use to gat an alternative view. With my 30mm 82°, it certainly improves the outer 15%. But that outer 15% is generally context arounIt the target(s) so the coma isn't noticed.

Conclusion. Spend the cash on good eyepieces.

Paul

Ps. My corrector was bought 2nd hand in anticipation of a faster scope in the future.

...got the fever Paul?

[alan potts]

I bought a coma corrector purely because I don't see the point in paying out on top line eyepieces then to have less than sharp at the edges in a fast scope. I do find the Paracorr helps the view especially in the very wide field eyepieces, 82 degree plus. I tried a few targets with only the 41mm and 35mm Panoptic's and you could see the wider field even though it is only 15% difference. I could also see coma which i have to say was not as annoying as I thought it might be.

Alan

[Paul73]

...got the fever Paul?

With bells on!

Sadly, I am not exactly awash with cash at the moment.....

Paul

[Alfian]

Here's Bill Paolini's comments about eyepieces after having eyepiece book published, worth to think over it:

http://stargazerslounge.com/topic/187546-incoming-new-book-on-eyepieces/?p=2045228

Definitely food for thought and does nothing for my wariness of short focal length scopes.

[John]

Definitely food for thought and does nothing for my wariness of short focal length scopes.

I've often wished that the 10" Skywatcher dob was an F/5.3 rather than F/4.7.  The tube would be a little longer but coma would be much less and the scope would be kinder to low cost eyepieces. Collimation would be a little easier to get spot on and maintain too.

[cs1cjc]

A question might be in place first, do you see that you need a coma corrector?

Coma do exist in a f5 scope, most visible in low magnifcation eyepieces, and the wider FOV of an eyepiece, the more coma will be there.

Then, another important thing is how much an observer sees coma, some sees 5coma in 0% FOV of 24mm 68 deg with 10"f4.7 dob, while there're others don't see coma in the same type of scope and eyepiece, that's what I've read here. Also, how much are you bothered by the coma seen? 

I am not wholly convinced that it is "visibility" of coma that is the issue.  Off axis on fast newtonians, coma sprays sprays light so widely that it is generally only visible on bright objects at low magnification.  However it is just as damaging the image at higher magnifications and on fainter objects.  I have found that the Astro-Tech/GSO/Altair Astro model improves the view signficantly at wide apparent angles at F/4.5.

[Alfian]

I used the word "wariness", which doesn't quite hit the spot. I think it just rubs a little that you have to spend more on equipment, which admittedly is undoubtedly good and nice to have in its own right, to compensate for the abberations created by the fast nature of the scope. Its swings and roundabouts and its the way optics work, but as you say, John,  slightly slower scopes would be nice. Altair do the 150/F6 which I imagine is a nice scope an of course there are the F8 and F6 Dobs. Perhaps I'm just being grumpy!

[John]

Another reason I'd prefer a slightly slower newtonian is that it's easier to make, say, a good F/6 10" mirror than it is to make the same aperture at F/4.5. Maybe modern mirror manufacturing methods have reduced inconsistency but when I was in my early years in the hobby I felt I was more likely to get a good mirror if it was a slightly slower focal ratio, especially at the price I was able to afford

[alan potts]

It is very true if my Dob was F6 instead of F 4.3 I would not need a Paracorr and could have saved the money, however the cost of the corrector would have been spent on a set of strong 8 foot folding  ladders, so there is no escape for the wallet.

Alan

[John]

It is very true if my Dob was F6 instead of F 4.3 I would not need a Paracorr and could have saved the money, however the cost of the corrector would have been spent on a set of strong 8 foot folding  ladders, so there is no escape for the wallet.

Alan

Yep - when the aperture gets big I can understand the "need for speed". I just don't see why Synta went for F/4.7 at the 10" aperture when their 8" was F/5.9 and their 12" was F/4.9

I heard a credible theory once that it was to do with shipping / packing economies rather than being driven by what the customer wanted.

[alan potts]

To be honest John I think you have a very good point, what you put forward would be better all round for the beginner. I think I am about the same size as you 5, 11, was more once, over 6 foot. I think I would find the slower scope much more back friendly unless the other size is easy to use sitting. The only was I can use mine sitting is view below 25 degrees, then I am on a house brick as well.

Alan.

[YKSE]

I am not wholly convinced that it is "visibility" of coma that is the issue.  Off axis on fast newtonians, coma sprays sprays light so widely that it is generally only visible on bright objects at low magnification.  However it is just as damaging the image at higher magnifications and on fainter objects. 

Thanks Chris, good to know that