Revalation Coma Corrector

[bomberbaz]

Up intil recently I haven't been bothered by comas but the other night I found it rather annoying.

I am not about to rush out and buy a paracorr as to me the expense is far too much but the revelation offering seems rather well priced and so was wondering if anyone uses one can they tell me the following?

Does it alter focal ratio?

Does it dim at higher magnifications?

Is it worth it for F5?

Anything else you can tell me!

http://www.telescopehouse.com/acatalog/Revelation-2--Coma-Corrector-for-Newtonian-Telescopes.html

Like I say, toying with the idea. One of the things buying one would force me to do s have to alter my eyepiece case again gff 

So I need to feel this will be worth it before I shell out my cash

cheers in advance all

Steve

[estwing]

eh yup steve...my paracorr put 15% on my focal length..doesnt dim the view as far as i can tell anyway!!..if its bothering you i'd get one

[Dude_with_the_tube]

looking at your scope and your eye pieces Steve,it would be a shame not to support them with a good quality coma corrector.Think of it as a long term investment.Also it is not really necessary to go for the type 2,myself and Shane aka Moonshane using Type 1 paracorr and it works like a champion. Got mine from US for 150quid all in all.Type 1 come up quite often on Cloudy Nights classifieds,if not,you can always put a wanted add,same like I did and found one in a matter of couple of days.

[cs1cjc]

Up intil recently I haven't been bothered by comas but the other night I found it rather annoying.

I am not about to rush out and buy a paracorr as to me the expense is far too much but the revelation offering seems rather well priced and so was wondering if anyone uses one can they tell me the following?

Does it alter focal ratio?

Does it dim at higher magnifications?

Is it worth it for F5?

Anything else you can tell me!

http://www.telescopehouse.com/acatalog/Revelation-2--Coma-Corrector-for-Newtonian-Telescopes.html

Like I say, toying with the idea. One of the things buying one would force me to do s have to alter my eyepiece case again gff 

So I need to feel this will be worth it before I shell out my cash

cheers in advance all

Steve

I understand that the GSO is the same coma corrector as the Astro-Tech and Altair Astro with optics designed by Roger Ceragioli.

It increases focal length by about 10%

It does not significantly dim the view at high magnifications, but planetary viewing is the only time I remove mine from the focuser.  I feel that it reduces the central sharpness/contrast of the image is very very slightly (I would not want to try an objective trial).

I feel it is worthwhile for f/4.6, but have no experience of using it a f/5. Sensitivity to coma seems to be a very personal thing.

Some have found getting the correct back focus an issue.  I use eyepieces that are (nearly) parfocal so a single setting suits for all my usual eyepieces.

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

[Bart]

I bought a GSO cc. I have F4.5 and F5 scopes. Yes it alters ratio by 10%. Makes a noticeable difference in outlying stars. However, I found coma bothers me less now then it did, don't know why! Glad I didn't spend big bucks on one. Sometimes coma annoys me but mostly it stays in the case. I do prefer galaxy hunting, esp with the F4.5 16". I find that when I'm viewing galaxies, the surrounding stars are just a frame for the main attraction. If would be different if I am viewing the double cluster, I find in that case, more than any other, a cc is invaluable because the stars fill the FOV.

Barry

[bomberbaz]

I bought a GSO cc. I have F4.5 and F5 scopes. Yes it alters ratio by 10%. Makes a noticeable difference in outlying stars. However, I found coma bothers me less now then it did, don't know why! Glad I didn't spend big bucks on one. Sometimes coma annoys me but mostly it stays in the case. I do prefer galaxy hunting, esp with the F4.5 16". I find that when I'm viewing galaxies, the surrounding stars are just a frame for the main attraction. If would be different if I am viewing the double cluster, I find in that case, more than any other, a cc is invaluable because the stars fill the FOV.

Barry

you see this is where i differ barry in that i really enjoy viewing open clusters and doing my best to fill the FOV and so this is where I am coming from. However, and this is a BIG HOWEVER, I dont want the Fratio increasing. I wouldnt mind so much a decrease say to 4.5 from 5 but not the other way. 

I know the skywatcher version does thhis http://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html

have you or anyone eklse used one (new thread might be coming up  )

[Moonshane]

Hi mate

you would be welcome to come over to me with your scope and try my paracorr 1 in it. Bear in mind that some focusers have a lip preventing full insertion of these barlow style CCs.

I love mine (about 160 used) and although in my f4 scopes there was an apparent difference between it and a gso one the latter was very very good for about 60 used.

[bomberbaz]

I might take you up on that Shane old son, lets see where things go with this first. 

I am also looking into this one http://www.altairastro.com/product.php?productid=16363  from altair astro, seems like a decent offering but hard to find info on it

[Bart]

I might take you up on that Shane old son, lets see where things go with this first. 

I am also looking into this one http://www.altairastro.com/product.php?productid=16363  from altair astro, seems like a decent offering but hard to find info on it

I understand that the altair astro and the GSO are the same thing

[Moonshane]

It looks like the one i had. You would probably need a baader 28mm tuning ring for visual.

[Moonshane]

In fact it is the one i had

http://stargazerslounge.com/topic/206099-altair-astro-coma-corrector/

[Rustysplit]

Steve,

I am using the SW CC in my f5 dob, and it works a treat. There is not not much coma anyway, but tbh, coma really bothers me, so cleaning it up is important to me. The .9 reducing effect is quite handy, giving you that bit wider fov. I have never used a parracor of either type, so can't comment on whether or not they are noticeably better. At f4.5 and higher I suspect it would make little real difference. At the faster f ratios is where the parracor may start to show it's worth. The ES tunable CC may be worth considering too??

[russ.will]

I had the GSO/Altair Astro CC and was very pleased with it. It cleaned up the outer stars of my longer focal length EPs very nicely, which is what it is supposed to do. It was also un-fussy about exacting tuning the setting distance and a 24mm Baader FTR was fine with my then ES82 EPs.

I'm planetary puritan, but I found it had no discernable impact at higher magnifications. The corollary being that there is also no need for it a high magnifications and if you've gone to the length of reducing air glass surfaces by using Orthos, it would seem pointless to leave it in the optical chain as it can't have a positive effect on contrast.

Now, the warnings;

Using any CC with a big heavy EP creates a lot of leverage on the OTA wall, which if it's of the thin rolled steel variety, bends, thus undoing the finer tuning of your collimation. In my old SW 300p Flextube it was enough to shift a laser from dead center of the donut to a good couple of mm off.

Once I was using EPs below about 24mm, I couldn't be bothered to insert it if it wasn't already in the focuser. You could see a difference but for me, with all but the longest FL EPs, coma isn't that much of an issue, unlike astigmatism, which good EPs eradicate anyway. Of course, personal sensitivities mean YMMV!

Russell

[bomberbaz]

Thanks Russell you make a very good point there about the wweight. My Axiom 31mm is a heavy beast and so without doubt must cause some flexing of the wall. I guess it depends on the weight of the coma corrector. 

Alan, how heavy and long is the SW coma corrector please, it is the focal reduction that interests me also as it will improve my FOV and enable me to use the longer focal length EP's more often.

Steve

[Rustysplit]

The weight according to some very dodgy kitchen scales is around 130gms The overall length is 85mm, but the eyepiece holder only sticks out about 48mm from the focuser if that makes sense? The altair/gso/rev cc that I saw was very well made, but seemed very chunky too.

[russ.will]

From memory, the AA/GSO CC (not the SW - No idea about that) acts as a 1.1x extender. Thus, your focal length will now be 1650mm (approx) and your focal ratio increased to F5.4.

The CC is very light - no more than a 2" Barlow - but like a Barlow, it pushes the weight of the EP 2-3" further out from the focuser giving it's weight greater leverage on the OTA wall. Stick a Plossl or Ortho in it and the combination would tip the scales at less than an Axiom, ES82 30mm, 31mm Nagler, etc. It's less about the weight and more about the length.

Russell

[bomberbaz]

thank you guys, some really good and usefull info there and much apprectiated.  I think going off what you are saying a SW CC would be perfect. I do not want to extend the F ratio as this will render my little used 5mm XW useless and force a sale. A reducer on the other hand would bring it into use more often as well as the Delos 6. 

I also currently use a 50mm extension tube so sounds to me like the SW CC will replace that too, it all seems to be a perfect match! 

 big thanks again 

Steve

[cs1cjc]

Just to note that coma is independent of eyepiece focal length.  Essentially coma is linear so at double the magnification, there is at half the coma but you have magnified by exactly 2. Coma may appear to be less at high power but this is likely to because of the nature of objects viewed, or because the image is less bright.  This holds until at very high power, beyond the magnification of normal use, the airy disc will grow to subsume coma.

[bomberbaz]

Yeah I realise the coma will be virtually non existent at higher mag, but I if it replaces the extension tube it also saves me having to keep swapping it over 

[cs1cjc]

Nope.   I apologise if my post above was unclear, but coma is independent of magnification...

[bomberbaz]

Ok so received the SW CC today and afraid to say that it wasnt up to the job. Problem is that when fitted it reduces my already tight backfocus travel by a further 15mm, I was left with a little under 10mm by 3 of my Delos EP's with my current ext tube.

I did read up on this quite a bit and took a lot of advice but I mis-understood this bit that Rusty pointed out.

The overall length is 85mm, but the eyepiece holder only sticks out about 48mm from the focuser 

If this make sense. Current ext from focuser 35mm, with SW 48mm, dif 13mm. 

SO short circa 4 mm or more of back trravel to achieve focus.

Anyway, the guys at FLO are very accomodating and and Baader MPCC is on order. This has a removable collar and operates in a very differant way. 

Watch this space!

steve

[Don Pensack]

This appears to be the GSO-made unit.

Note that there is no easy way to adjust the eyepiece focal place-to-coma corrector lens distance in this unit other than by the use of barrel extenders and parfocalizing rings.

The process of adjusting each eyepiece for optimum coma correction will be done only once:

1) Take the eyepiece you currently have that requires the most in-travel of the focuser to get it to focus.  Insert it in the coma corrector, the ensemble in the scope, and then focus the center of the field.  Look at the edge.  If the star images are not tiny little pinpoints like the center, pull the eyepiece 3mm out of the coma corrector, tighten it down, refocus the scope, and look at the edge again.  If the star images got worse, the correct setting for this eyepiece might be a trace farther in than the coma corrector body allows, and the setting you'll use for this eyepiece is all the way in.  But, you won't use this eyepiece to adjust all your others.  If it gets better when you pull the eyepiece out 3mm, try another 3mm, refocus, and look at the edge.  When you have found the setting for this eyepiece that yields the best coma correction, freeze the focuser.  For the rest of the eyepiece adjustments, you will not use the focuser--you will use parfocalizing rings and/or barrel extenders on the eyepieces.

2) Since you know the ideal position for this eyepiece, go ahead and install a parfocalizing ring on it so it is in perfect focus in that "frozen" focuser position.

3) For every other eyepiece in your collection, insert the eyepiece in the coma corrector (using a 1.25" adapter if necessary) and slide the eyepiece in and out of the coma corrector until it is in focus.  For each eyepiece, the setting that allows the eyepiece to be in focus IS the correct setting for that eyepiece relative to the coma correcting lens.  When you're done, all your eyepieces may have parfocalizing rings on them, and all will be in focus at the same time in the coma corrector.

Now, simply insert each eyepiece when using the coma corrector and it will automatically be in the correct setting for coma correction.  And they will all be par-focal.  You might have to make tiny focuser adjustments, but that's OK.

If the original eyepiece needed to move in farther to correct the stars at the edge try an eyepiece that doesn't focus quite so far in to start your adjustment.

By doing this, you are fixing the coma corrector lens at the correct distance from the primary, and moving the eyepieces out and in until their focal planes are identical distances from the coma correcting lens.

It is this procedure the Explore Scientific and TeleVue coma correctors avoid, since they have a telescoping top that does the same thing without parfocalizing rings or barrel extenders.

[bomberbaz]

Thanks for that Don, sounds like a lot of fiddling but I guess you can do that even without a CC. However I have to say that the focal point difference between my Delos and Pentax is large that I doubt there would be much left on the pentax to put into the focuser. I think next time I use them I will get a picture to show this but good idea non the less. 

I had read about the paracorr adjustment thing but wasnt sure exactly what it was, now I know. Maybe one may come up at the right price and I would grab it but for now, I can wait, heheh 

[Rustysplit]

Sorry to hear that it did not work for you Steve. Good on FLO for sorting you out though.

[estwing]

And this is why the paracorr is a winner for me slip in and go!.. Hope your get sorted Steve