Will this work??

[emadmoussa]

I was thinking in order to avoid my eyepieces coming in contact with the thumbscrews in the Skywatcher Flextube 350P focuser, it might be worth it installing a 2" adapter with compression rings permanently in the focuser.

The other thing is to use the 2" coma corrector as an 2" adapter permanently living in the focuser, and then attach a 2" to 1.25" self-centering adapter to the coma corrector. This way I'll have corrected field of view, avoid thumbscrews and more back travel to achieve focus.

Any thoughts??

[rwilkey]

Hi Emad, yes, that will work very well in my view, I already use the Orion Self Centering Adaptor and it is great.  Never used a CC as have never noticed any coma in my relatively small 200P.  Looks good to me, go for it!

[knobby]

Looks fine Emad

[emadmoussa]

One question though, the coma corrector won't change the focal length or affect the eyepiece FOV? Just making sure...

[Tinker1947]

You could go for a Baader Click-Lock.....

[AlexB67]

One question though, the coma corrector won't change the focal length or affect the eyepiece FOV? Just making sure...

It will slightly for the one shown in your picture, which is the altair astro, but it may not bother you, it has a multiplication factor of 1.1.  One way to look at it is that a 28mm will become a 25mm roughly, or alternatively the net TFOV, assuming linearly for simplicity will roughly be reduced buy the same factor, a 1.5 degrees become 1.26 degrees.

Third way, probably simplest, think of your eyeoices as they are but working in a slightly slower scope, a f4.5 scope will effectively behave as a f4.5 * 1.1 = f4.95 with the altair astro CC. 

If you don't want that sort of alteration the Baader MPCC mk3 will maintain the f ratio as is. Jut to point out, in both scenarios you will also need to some tuning/extension/spacer rings to dial in each eyepiece to work with the CC so allow for that in budget.  The baader does come with a set, but best I can tell  it will not suffice for all scenarios, though it may be close for a lot of eyepieces, you may need a couple more depending on your eyepieces.

See for example here:

http://www.alpineastro.com/optical_accessories/MPCC/mpcc_visual.htm

[John]

As above, I think coma correctors do change the effective focal ratio / focal length slightly. The exact amount varies depending on the manufacturer.

I think they eat up a little of the inward focuser movement too and again that varies between the manufacturers. you just need to make sure that between the 3rd party eyepiece adapter and the coma corrector you don't use up enough inward focus travel that you can't get eyepieces to focus !

I'm probably being pessimistic here but I've had some great plans in the past thwarted by such little but frustrating details 

[emadmoussa]

Oops...that is the Revelation CC and according to Telescope House it doesn't alter focal length...just found out: )

[Daniel-K]

just buy a parracor

[AlexB67]

As above, I think coma correctors do change the effective focal ratio / focal length slightly. The exact amount varies depending on the manufacturer.

I think they eat up a little of the inward focuser movement too and again that varies between the manufacturers. you just need to make sure that between the 3rd party eyepiece adapter and the coma corrector you don't use up enough inward focus travel that you can't get eyepieces to focus !

I'm probably being pessimistic here but I've had some great plans in the past thwarted by such little but frustrating details 

The Baader is almost guaranteed to work with all focuser like the SWs focusers, it is more an issue with the paracorr like correctors or the ES which require that much extra travel I believe.  In a way the Baader is a neat package, also lighter.

Personally I would still go for the altair astro though if on a budget because of how the optical design works, but since I don't like the hassle of fumbling with rings and all that, I have just admitted to the fact I'l just have to bust my wallet when the day comes when I do buy a CC it will have to be  paracorr, or may be ES, the tuneable top is just so handy

[swamp thing]

Alex

If you get the SIPS feather touch. You'll get not only a Paracorr that you don't have to adjust but a rather reasonable focuser to boot.

[AlexB67]

Alex

If you get the SIPS feather touch. You'll get not only a Paracorr that you don't have to adjust but a rather reasonable focuser to boot.

I does look like a dream solution, but I can see myself with two scopes at some stage and having it fixed in the focuser would mean I'd have to buy two. If money grew in my wallet the way nettles appear in my garden I'd happily do that 

[tingting44]

you got me thinking of this now lol just more on a budget for me tho lol...

http://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html

http://www.altairastro.com/product.php?productid=16455

[neptune1]

Hi can you explain to a clueless newbie why you don't want eyepieces coming into contact with the thumbscrews? What should we be aware of? Thanks all!

[emadmoussa]

Hi can you explain to a clueless newbie why you don't want eyepieces coming into contact with the thumbscrews? What should we be aware of? Thanks all!

Two things: screws, or at least the majority of them - will mar and scratch the eye piece barrel. The last thing you want if you have expensive EPs.

And, screws are.less effective than compression rings in keeping the eyepiece or camera centered and tight in the focuser tube. This particularly important if you are collimating the scope or using a heavy eyepiece.

And, yes, they are ugly and old fashioned

[neptune1]

Ah, thanks. I thought it might be the obvious but you never know with this hobby! Do only cheap focusers come with screws or do you need an adapter for all? 

[emadmoussa]

Ah, thanks. I thought it might be the obvious but you never know with this hobby! Do only cheap focusers come with screws or do you need an adapter for all?

Generally cheap scopes and sometimes expensive ones, Skywatcher for instance, come with single speed/ thumbscrew focusers. The GSO / Revelation scopes, albeit relatively cheap, often come with good quality dual speed / compression ring focusers. There are always exceptions of course.

[emadmoussa]

There is also the element of what you need, your accessories and how fussy you are..to like/dislike stock focusers.

I personally cannot go back to thumbscrew/ single speed focusers

[neptune1]

ah thanks 

[emadmoussa]

again, the the CC above is Revelation Astro - not Altair Astro

[AlexB67]

Oops...that is the Revelation CC and according to Telescope House it doesn't alter focal length...just found out: )

I would really double check that, telescope house may have the advertising wrong, or of course I may have it wrong, but it looks like it could be the same coma corrector to me under a different name

for example see here for GSO

http://agenaastro.com/gso-2-coma-corrector.html

and here

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

They all look so similar.

Vendors can get that sort of info wrong at times, as FLO will tell you for example about the skywatcher CC acting as a reducer, which many places will not tell you.  The fact the add at telescope house says helps with reducing coma and reduce field curvature. It's afaik usually the optical 4/2 design with the small magnification factor ( and thus in increase in effective focal ratio ) that achieves improved flattening of the field.

[emadmoussa]

They look awfully similar - I was reading the ''good'' reviews on the GSO/Revelation CC and one guy says clearly that it effectively turned his ES 20mm into 18mm...hummm, while it might not be a big deal considering this CC is reasonably priced and works very well, but change of focal length is the last thing I want. I'll commit myself to 3-4  eyepieces only and that will make some difference that I don't want.

[AlexB67]

If you are going to just use a couple of wide angle lower mag eyepieces and not high mag I would still consider the Baader Mk3. The way I see the two compared in strengths and weaknesses are a bit like this. I've never looked through either   but based on reviews, and theory behind the various designs at telescope optics.net I draw the following

Baader

Pros

1. Light in weight.
2. No magnification change
3. transmission very high, I think it is possibly a rowe like or 3 element design,  ( I am guessing though ) , I believe Baader claim close to 98% transmission. The other 4 /2 design  or I believe the paracorr 2 even has 5, it would be tough to compete on transmission grounds I suspect, that is one nice thing about this CC I like the sound of. Nothing much is known about the GSO/altair or similar makes about transmission, coatings I am aware of. No doubt it is very good as well, though not paracorr quality me thinks.
4. Focus travel likely not an issue with pretty much all eyepiece and standard focusers.
5. Comes as a whole ready set with spacers and all, the altair does not, you may need a couple more spacers though.  I expect when you take into accout the extra spacers both would work out similar in price. 
6. Quite a bit of info is beginning to appear on the net how to configure spacers for known eyepieces with the Baader, the altair not so much info. Once you read about how to go about it is it is only an initial pain to go through to dial them in. Not hard it seems if just a tedious exercise.

Cons

1. potentially less effective at higher mag in particular with developing less sharpness on axis, whether this would be easily visible I cannot say, testing on doubles would be good one. Our often not so great UK skies my not even be able to push for the difference to be worth worrying about to see it.
2. No reduction in field curvature, I think this is an important point. If for example you use a coma corrector you want the field to be sharp. i.e. go all the way as much as possible, If your current combo of eyepieces and scope show field curvature you are still going to have fuzzy stars where there is field curvature visible. For me anyway, the coma correction is a bit of a waste if that still happens ( perhaps harsh, but that is how I see it ) even the 68 degree MV I know for example after correction from one review still has some residual field curvature in a paracorr 1 and does not tidy the field entirely, though it is a lot better. The Baader would struggle more.
3. Given 2, On the whole I don't think the coma correction and tidying up of the field in the Baader will be quite as good as the GSO, and further behind the paracorr and such like.  In much faster scopes this should become even more apparent.

probably other stuff I forgot, take you pick  . it is probably worth remembering that I Hope your eyepieces are well corrected for astigmatism or you may be disappointed with the results. some of these points may be knitpick.  Personally I hate to think eyepieces are being held back by a coma corrector  as much as possible in any other way besides what it is supposed to do, another reason I feel the paracorr is worth it in the end of the day if it is going to be a tool for life.

Personally I think in my scope at 4.7 I would probably use it for eyepieces above 15 mm or so most likely,  with that in mind I had a delos 17.3 in mind that would reduce down to about 15mm, similarly the two maxvisons I have will come down a bit, I do not mind, in fact it will bring the exit pupils down a bit to a more optimal range.  I can live with the FOV loss.  Not sure I would use the CC in planets. Eventually in the long run my low power will probaby be upgraded  anyway so I can plan accordingly with the CC in mind.

[emadmoussa]

If you are going to just use a couple of wide angle lower mag eyepieces and not high mag I would still consider the Baader Mk3. The way I see the two compared in strengths and weaknesses are a bit like this. I've never looked through either   but based on reviews, and theory behind the various designs at telescope optics.net I draw the following

Baader

Pros

1. Light in weight.
2. No magnification change
3. transmission very high, I think it is possibly a rowe like or 3 element design,  ( I am guessing though ) , I believe Baader claim close to 98% transmission. The other 4 /2 design  or I believe the paracorr 2 even has 5, it would be tough to compete on transmission grounds I suspect, that is one nice thing about this CC I like the sound of. Nothing much is known about the GSO/altair or similar makes about transmission, coatings I am aware of. No doubt it is very good as well, though not paracorr quality me thinks.
4. Focus travel likely not an issue with pretty much all eyepiece and standard focusers.
5. Comes as a whole ready set with spacers and all, the altair does not, you may need a couple more spacers though.  I expect when you take into accout the extra spacers both would work out similar in price. 
6. Quite a bit of info is beginning to appear on the net how to configure spacers for known eyepieces with the Baader, the altair not so much info. Once you read about how to go about it is it is only an initial pain to go through to dial them in. Not hard it seems if just a tedious exercise.

Very informative, Alex. Thank you!! With my previous f/4.9 I was tempted to get a Baader CC, but the scope disappeared before a I made up my mind. So yes, Baader CC is a possibility for my next scope - potentially f/4.5.

May I ask, what's the function of spacers?? Don't I just shove the CC in the focuser and go...?

Also, I heard some reviews on very long structure forming when using a CC and a big eyepiece... More like another scope growing on the side of the reflector...

[scarp15]

I tend to use 2" filters such as an Oxygen three in most of my observing sessions. I screw the filter directly to the extension tube. I can then interact amongst 2" and 1.25" eyepieces. It might be another consideration if you use filters and are considering a coma corrector.  would I be correct to assume that with a paracorr you would have to attach the filter direct to the eyepiece barrel? Fine if you are just using the one eyepiece, but in cold dark circumstances fiddling with the filter to replace onto more eyepieces will be no fun. Then if you have 2" and 1.25" eyepieces to use, you would require two of the same filter type.

Would I also be correct if I said that coma correctors that attach direct to the eyepiece barrel, with their extension rings might (such as MKIII)  penetrate too deep into the extension tube, so not permitting a filter to be used at all? However I guess if you wanted to, for example take a  look at the Veil, then the CC could be removed occasionally.