Altair astro coma corrector ?

[AlexB67]

After reading this thread,

http://stargazerslounge.com/topic/205950-do-i-really-need-a-coma-corrector/

it got me thinking again, but for the sake of not hijacking, it sort of set me on a tangent reading about coma correctors ( again ). 

This solution along with some reviews, for the price it does seem rather attractive. 

http://www.altairastro.com/product.php?productid=16363

Am I correct that it works similarly along the same principle as a paracorr with the 4 element design ?, the very slight magnification factor would not bother me, and the spacers, once done is perhaps not as fiddly as I thought, after some extra reading how some achieve this and this great review by Chis.

http://stargazerslounge.com/topic/158529-astro-techaltair-astrogso-coma-corrector-and-user-guide/

So some questions, How heavy is it ? Would a standard Crayford focuser such as the ones found the SW Dobs begin to slip or hold up okay ? Balance of the Dob can always be cured cheaply. 

With all the extra elements in there, is there any perceptible change in contrast, seeing even 4 elements eyepiece would now become 8. Any extra noticeable effects, things like scatter and so on ? I do wonder if the extra costs of the TV paracorr is not only due to the tuneable top as well as superior glass.  For < 100 pounds, it does seem like great value and a bit of too good to be true perhaps ?. For those that have been privy to comparing this solution to a paracorr, Is it as good or even nearly as good in terms of  coma correction and that extra benefit of a flatter field. ?

The intention is to use it in my f4.7 DOb, so while fast, not that fast. It should turn my scope into twice that ratio according to the product page, so my scope would effectively be  > f/9 in which case all coma should be as good as gone with all my eyepieces, none of which exceed 68 degrees anyway.

I also had a look at the baader MPC but that works a bit differently,  seems to me for visual use I prefer the sound of this one. Also, I have no intention of using it for imaging so that needs no consideration.  

All in all, for this price it seems like an extra something I may get sooner than later perhaps, it does all sound rather good, or am I dreaming. 

In the end of the day it is not as if I have a coma crisis with the eyepieces all under 70 degrees in an f4.7 Dob, all the same, it would be nicer without, especially at that price tag. I think I could see this extra be an almost permanent fixture in the focuser, apart from when looking at some very faint targets like galaxies perhaps and when contrast/transmission is more critical.

Look forward to you thoughts and any reviews you are aware of, I only just started looking at it more seriously, more reading to be done 

[Moonshane]

I bought one on a whim for £55 used as I wanted to compare it with the used TV paracorr I already had which cost £165.

Whilst I have now sold it, I have to confess it did work very well indeed in my f4 12" dob. I used just a 28mm Baader fine tuning ring to get the spacing within the visual range.

After using it for a few minutes with my 26mm Nagler, I then switched back to the paracorr and there was more of a difference than I expected in the sharpness of the views. I cannot really put my finger on it but the view was crisper and cleaner with the Paracorr and I suppose this is to be expected at three times the price. Swapping back and forth a few times confirmed these initial thoughts. There is also the convenience factor of the PC with the tunable top and lack of an barrel undercut (this hung up badly in two of my focusers in the Altair). In fairness with further fine tuning of the spacers it might be possible to get a closer match but I could not be bothered with what I saw as messing about.

Don't get me wrong the Altair did a very good job of cleaning up the coma and is excellent value for money but the Paracorr was the clear winner for me. If I didn't have the paracorr I would certainly use the Altair quite happily but based on my experience I'd be saving for and snapping up the first used Paracorr I could afford.

[AlexB67]

Very helpful moonshine, thank you. Indeed after more reading as good as it may be, I get more of a feeling there is no beating around the bush here, some good discussion on cloudy nights too. A good as it may for the buck, it is not a substitute for the paracorr I get the impression. Should a second hand paracorr 1 turn up second hand for < 200, I'd have not hesitation at some point, there is no rush with it 

In your tests, at higher magnification did you find it to be less effective as well, in other words,  did the gap to a paracorr become even more apparent ?. I suppose there is the ES option too at lesser cost, but still a lot more compared to altair astro, but seems to me it is still a bit of a lesser known beast as to how good it really is.

Coma corrector do kind of fascinate me for some time. I always did have the feeling however they are well worth it. Coming from a very reputable source at CN too echoed my feelings I've had for some time. To put it in m terms then,  a BST explorer say with coma correction coupled with a altair astro coma corrector, still a lot less than a delos in total. If I am going to invest in that premium eyewear in future and allow it to shine I should consider one.  I would not actually be one bit surprised if I preferred a coma cleaned up view in medium range eyepiece such as the ones I own compared to that of a really costly premium eyepiece in certain cases, depending on what it is I am looking at, I hope I am not going to start a flame war with that statement

[Moonshane]

to be honest I didn't try it at high power as coma becomes less pronounced

if I understand your last point you are suggesting that a BST and coma corrector would be comparable to a Delos with no coma corrector? I doubt it to be honest but give it a go sometime

nowadays I don't even think about the paracorr, the money's gone and it's always in the focuser, quietly doing its job. I feel it's one of the best investments I have made.

[AlexB67]

I meant a Delos or any any premium eyepieces without coma correction, versus say a medium one in a fast scope with coma correction, my MV would be abetter example with coma correction, I wonder what I would prefer in some cases, since no eyepiece no matter how good is going to solve coma problems.  I did not mean so much in general, but say wide star cluster, in that case perhaps coma  may well be the thing that annoys the most.  other aberrations  may well override that anyway In a poor eyepiece, but it would be an interesting test for me to try. Perhaps not at the FOV ranges and f ratios  I am using.

In any case, it seems to me it would not make much sense to buy a nagler or an ethos and use it in a scope with f ratio say around f 4.5 and not coma correct it, perhaps even slower scopes at f5 this could be true. There was great post by John Isaacs at CN that pretty much stated that, he summarised the whole thing quite nicely with fast newt and when buying premium eyepieces, it made a lot of sense to me. This comes form someone with the hands on knowledge and long term experience I do not have, however I can't find it now. if I do I'll link it here. 

[Moonshane]

in all honesty my 26mm Nagler is not that bad with no coma corrector at f4 but it's definitely better with. in fact in the 12" f4, when looking at M45 I take the PC out as it increases the field enough to fit it all in (1.6 degree field with and 1.8 degree field without the PC).

[AlexB67]

okey dokey. In any case I've come around to the paracorr I think as the only option in the long run anyway.  Should one of those altair Astros  turn up second hand for 50 or so I may well bite to try one out for a bit  and see how I find it at some point. After all, it is not much to pay out for an interesting little experiment and could well do for a while if I like it enough.   

[AlexB67]

Moonshane, On second thoughts I don't think I would buy the altair astro second hand. I've been reading about it a lot the potential for extras needed, potential focuser mods etc. possible issue with heavier 2 inch eyepieces and  crayford focuser with its own adapter approach for 1.25 and 2 inch eyepieces in my Dob.  I can see myself buying this that and the other, before I know it  I'll have pay out for extras for something I may not ultimately want, and could set aside for other things I am more definite about at this time.

If it was a bit more plug and play ready perhaps without extra cost may be I would consider, or I'd have to see one in action on an SW Dob such as mine and be convinced about the whole setup.  Nice thought and diversion, but the more I read the more I see it as a mistake, for me anyway at this time.

[Moonshane]

I had no issues with weight/focusers/2" eyepieces. I just put in a baader 28mm FTR and put it in the focuser. all of my focusers cope with a 26mm Nagler and Paracorr so would cope with the AACC.

[AlexB67]

I don't know what focuser you have, but the SW one I read some threads about it with regard to this setup. Handy thing is, luckily lots have been there buying this CC, so there is lots of info available looking around with the various focusers and setups.  None of these things can't be circumvented, but it means chopping and changing some things potentially. I  guess I rather do some more learning and reading to know if this whole thing is good way to go, test/measure some thing out with my eyepieces and setup and so on, then ask around some more where needed. 

Some experiments in the pipeline so I am clearer as to what extras and mods I may or may not need to do or replacing the focuser setup. That is not unique to this CC mind you and the SW Dobs, some things in general  that would apply with the setup if I got a paracorr also.   

[Moonshane]

I have two revelation dual speed crayfords and a baader steeltrack.

[cs1cjc]

to be honest I didn't try it at high power as coma becomes less pronounced...

This is not strictly true.  For a given telescope, coma depends only on the the apparent field of view of the eyepiece.   Coma increases linearly with distance off axis so with a higher magnification the coma is smaller, but it is magnified more.  These effects exactly balance each other, so coma is independent of eyepiece focal length.

HOWEVER for very high magnification coma will be lost into the airy disc and other aberrations of the view such as seeing.  Also, coma is a diffuse aberration which is is more apparent to the human eye when the image is bright, so at a lower magnification.  Cameras pick up coma very clearly though many human observers are not bothered by it.

[Moonshane]

I am sure that this is correct as far as theory goes but to my eyes at least (and they're the ones that matter to me) looking through e.g. a 15mm plossl and a 32mm plossl there is a lack of visible coma in the 15mm cf the 32mm. it's what I can see that bothers me not what I should see.

[cs1cjc]

I am sure that this is correct as far as theory goes but to my eyes at least (and they're the ones that matter to me) looking through e.g. a 15mm plossl and a 32mm plossl there is a lack of visible coma in the 15mm cf the 32mm. it's what I can see that bothers me not what I should see.

Accepted!

Do note though, that for faster mirrors and wider angles of apparent view, there is significant energy in the coma and this will surely take some brightness from stars, irrespective of whether the coma is above or below the threshold for vision.  I have found it interesting to note the amount of aberration (coma, tube currents, seeing, etc) which is visible when viewing very bright objects and in particular Jupiter and Venus.  Such aberrations are presumably detracting from the view of dimmer objects even though the aberrations themselves are no longer observable.

[AlexB67]

Hi Chris. After owning this bit kit, how do find it after using it for some time now, including the hassle of spacers and changing eyepieces and so on ?. Are you using it most of the time,  or just now and again, or does it I hardly ever leaves the focuser ?.  I doubt I'd get one at this point in the near future anyway with other things on the list. In the sort of 65 70 degree eyepieces at f4.7 I have I find to be very marginal,  it is more a curiosity thing as much as anything to see how much difference it would make. In the end of the day, the only way to know is to do the actual seeing with it, the best test of all . 

Still, if I did think about getting one 82 degree or wider FOV whopper to add to my collection,  I suspect it will become a different story, but a low power wide FOV is not a priority for me at this point to add to my collection.   In fact the 28mm MV 68 degrees does a sterling job already for that, but my FOV disease is not as bad as it is for some others I guess   If I really wanted a much wider FOV another scope may be a better option anyway.

[cs1cjc]

Apologies for my slow reply. 

I have had little time spare for participating in this forum recently, added to which my internet is, to put it mildly intermittent (I am hoping that talk Talk and BT Openreach will between them soon fix the problem and that I will no longer have to spend hours on the phone while tests are run), which has also been true of my viewing of late.  However if and when the clouds part and it is not misty or blowing a gale or freezing then I will certainly be using the Astro-Tech/Altair Astro/GSO coma corrector on deep sky objects.  I find it improves the view significantly.  The view feels "flatter" and does not "fade" to the edge. 

I will not be using the coma corrector on planets because I feel that the image is very slightly damaged at high magnifications, but then I am unlikely to be using a newtonian much on planets in any case.  I find the view through an apochromat to be crisper.  Now if you think a Newtonian should beat a smaller refractor on planets then I suggest that you get on very very early to allow your newtonian to cool and look at Venus and I would not be surpised if you see smearing or even multiple images due to tube currents.  A fan will  certainly help but to get a good look at planets I would normally use a refractor. 

Coping with different eyepieces is not an issue.  The  coma corrector spacing is not critical, so far as my eyes are able to detect (but note that I am over 60 and very short sighted).  To make life simple for myself I have bought eyepieces that are close to parfocal (within a few mm). I have a compromise spacing (one 14mm fine tuning ring plus one empty 2" filter) which with a 9mm or 10mm 1.25" to 2" convertor covers all my wideangle eyepieces, my ES82s, ES68s and (without the 1.25" adaptor) my 36mm Baader Hyperion aspheric.  These all lie well within the designer's quoted tolerance of +/-10mm of a nominal 75mm.  My Vixen plossls are also fine.  My orthos are the furthest away but I would normally only use these for planetary viewing for which I rarely use a Newtonian, see above...

I can well imagine that for those with a discerning eye and a deep enough pocket to spend £400 that the Telvue Paracorr is superior.  However I believe that the A-T/GSO/A A coma corrector at £85 will provide those of us on a smaller budget (and perhaps less acute eyesight) with a significantly improved view in a fast newtonian!

[Moonshane]

I agree with much of what you say Chris. I'd never spend £400 though, got my PC used for £165 which brings them a bit closer. the Altair/GSO unit really is very good.

[AlexB67]

Thank you Chris,  very informative post.  CC is an ltem that has been on my list to get at some stage. It is more a question of when on my to purchase list, what type/make of corrector I'll wait and see. I doubt I'd want to pay full bucks a parcorr new, but something like the price range that Shane mentioned I may stretch to when the time comes, a second hand one. Alternatively, it sounds to me from what you say and read elsewhere the altair astro is not at all a bad solution an performs very well indeed.

If  had an f/4 scope I probably feel a whole lot stronger about it wanting one as soon as possible, but luckily with my scope at f4.7, it not a crisis area for me as I said earlier when I considered buying one.

Definitely I'll not be buying this month though after my TV radian purchase, that's enough astro spending for me.  :0)

I imagine after another eyepiece or two upgrade I have planned for it may well come after that.