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GSO Coma corrector with 1.25'' EPs


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Hey guys,

I am (still) considering buying a GSO CC to use in my SW 250px. 

What I understood so far is that I would need 75mm from the top lens of the CC to the focal plane of the EP. This would be achieved by adding a 25mm spacer... Am I right so far? 

So am I supposed to put the CC straight in the focuser ? (without the 2'' adaptater)?

My second question is how do I use this with a 1.25'' EP? Am I supposed to add the 2'' to 1.25'' adaptater after the CC? Will that maintain the proper lightpass length?

Many thanks for your answers

Raph

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Yep, pretty much correct.  This all assumes that your eyepieces focus close to their shoulders (+-5mm is fine).  A counter example for me would be my 12mm Nagler T4 which focuses 19mm below its shoulder, so I added five 4mm thick 50mm ID O-rings and a 20mm 2" extension to the bottom of the eyepiece to make it closer to parfocal and still be able to be securely held in the focuser.  The difference in coma correction is noticeable.

For 1.25" eyepieces, just put an adapter in the CC and you're pretty much good to go.  Many 1.25" TeleVue eyepieces focus about 6mm below their shoulder, so the adapter would probably put the focal plane at just about the right distance.  The GSO CC is designed to have good correction from 70mm to 80mm of separation.  I don't notice much difference up to about +-10mm.  That 12mm Nagler is really the only eyepiece I have that really needs to be parfocalized.  Check your eyepieces to see how far they focus from their shoulder to get some idea about your own need for parfocalization.

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53 minutes ago, Louis D said:

Yep, pretty much correct.  This all assumes that your eyepieces focus close to their shoulders (+-5mm is fine).  A counter example for me would be my 12mm Nagler T4 which focuses 19mm below its shoulder, so I added five 4mm thick 50mm ID O-rings and a 20mm 2" extension to the bottom of the eyepiece to make it closer to parfocal and still be able to be securely held in the focuser.  The difference in coma correction is noticeable.

For 1.25" eyepieces, just put an adapter in the CC and you're pretty much good to go.  Many 1.25" TeleVue eyepieces focus about 6mm below their shoulder, so the adapter would probably put the focal plane at just about the right distance.  The GSO CC is designed to have good correction from 70mm to 80mm of separation.  I don't notice much difference up to about +-10mm.  That 12mm Nagler is really the only eyepiece I have that really needs to be parfocalized.  Check your eyepieces to see how far they focus from their shoulder to get some idea about your own need for parfocalization.

Thanks for your answer!

 

How can I know where the focus point of an EP is? Btw you can see what EPs I have in my signature.

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Ideally, put a piece of vellum/tracing paper/thin copier paper across the focuser opening and focus on the moon.  This can be difficult if the magnification is low due to a short focal length.  Note the focuser tube position.  Next, insert each eyepiece and refocus to see if they require in-focus or out-focus relative to the native focus position that you just determined.  Note the direction and how much.

Alternatively, start with your 11mm Nagler T6 and focus on anything.  All of the T6s focus 1/4" (~6mm) below the shoulder.  Note where each eyepiece focuses relative to the T6 and add or subtract the 6mm offset appropriately to determine each offset from their own shoulder.  If one focuses 10mm further in, that would be 4mm above the shoulder (10-6=4), for instance.  On the other hand, the 12mm NT4 would be 19mm-6mm=13mm further out (what you would measure), thus 13+6=19mm below the shoulder.

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6 minutes ago, Louis D said:

Ideally, put a piece of vellum/tracing paper/thin copier paper across the focuser opening and focus on the moon.  This can be difficult if the magnification is low due to a short focal length.  Note the focuser tube position.  Next, insert each eyepiece and refocus to see if they require in-focus or out-focus relative to the native focus position that you just determined.  Note the direction and how much.

Alternatively, start with your 11mm Nagler T6 and focus on anything.  All of the T6s focus 1/4" (~6mm) below the shoulder.  Note where each eyepiece focuses relative to the T6 and add or subtract the 6mm offset appropriately to determine each offset from their own shoulder.  If one focuses 10mm further in, that would be 4mm above the shoulder (10-6=4), for instance.  On the other hand, the 12mm NT4 would be 19mm-6mm=13mm further out (what you would measure), thus 13+6=19mm below the shoulder.

It might sound like a stupid question but I ve got to ask at one point or anouther... What do you call the shoulder exactly? 🙂 

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1 minute ago, Raph-in-the-sky said:

It might sound like a stupid question but I ve got to ask at one point or anouther... What do you call the shoulder exactly? 🙂 

It's where the insertion barrel (1.25" or 2") meets the wider part of the eyepiece that stops further insertion into the focuser.  In TeleVue's eyepiece specifications page, the following diagram shows this clearly as where the white part meets the black part.  Dimension F is the focus position in their table.  Positive means below (the little x in the diagram) and negative means above the shoulder.  Often, the focus position does not correspond to the physical field stop if there is a Smyth lens in the insertion barrel, so I think their diagram should have labelled F as focus position rather than field stop (perhaps effective field stop would also be acceptable), because that is what it indicates in my experience with their eyepieces.

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Hi Ralph. I recently bought the Revelation/GSO CC to use with my SW 10” dob. I mainly used it with 2” eyepieces. Whilst it did improve the edge correction, I found that the transmission let it down a little. I did a lot testing on Comet C/2017 T2 and found using the same eyepiece, it was harder to pick out the comet with the CC compared to not using it. It’s not a big loss of transmission by any means and if you’re not looking at faint objects then it will make no difference to you. One other lesson learned for me was to collimate with the CC in place. I initially had some issues with a loss of sharpness on axis. Collimating with three CC resolved that. Ultimately, I decided to return the CC because of the transmission loss but that’s more down to my preference than a serious defect with the CC. Hope that helps!

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The only issue I've noted with the GSO CC is at small exit pupils where the image is not as sharp as without it.  I'm guessing it has some residual spherical aberration at high powers.  I've never noticed a decrease in transmission, but I've also never gone looking for it.  I've noted a similar issue with the TSFLAT2 field flattener in my 90mm APO.  The image has some chromatic aberration at small exit pupils with it in place, so I remove it under those conditions.

For the $75 I picked it up for used, I have been very impressed with the GSO CC.  I imagine the Paracorr T2 would certainly be better, but I can't justify the nearly $500 price tag for a slight improvement.  Perhaps if I was an imager I could justify it.

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1 hour ago, Louis D said:

The only issue I've noted with the GSO CC is at small exit pupils where the image is not as sharp as without it.  I'm guessing it has some residual spherical aberration at high powers.  I've never noticed a decrease in transmission, but I've also never gone looking for it.  I've noted a similar issue with the TSFLAT2 field flattener in my 90mm APO.  The image has some chromatic aberration at small exit pupils with it in place, so I remove it under those conditions.

For the $75 I picked it up for used, I have been very impressed with the GSO CC.  I imagine the Paracorr T2 would certainly be better, but I can't justify the nearly $500 price tag for a slight improvement.  Perhaps if I was an imager I could justify it.

I agree, Lewis. Very good value. I suspect use of a CC implies more interest in stars rather than faint objects. It certainly wasn't a big difference but enough to be noticeable. I was using the Revelation branded version which I think is the same as the GSO. The next step up would be the Explore Scientific CC but even that's about 4 times the price of the Revelation. All comes down to what your preference is and how big your wallet is ;) 

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12 minutes ago, Littleguy80 said:

I agree, Lewis. Very good value. I suspect use of a CC implies more interest in stars rather than faint objects. It certainly wasn't a big difference but enough to be noticeable. I was using the Revelation branded version which I think is the same as the GSO. The next step up would be the Explore Scientific CC but even that's about 4 times the price of the Revelation. All comes down to what your preference is and how big your wallet is ;) 

Yep, mostly starfields big and small because of the level of light pollution in my backyard.  It wasn't bad 25 years ago when the nearest small town was 4 miles away.  Now I'm surrounded by a tollway, tens of thousands of new homes, several giant shopping centers, multiple schools and their lighted fields, car dealerships, etc.  I'm seriously considering moving to darker skies when I retire.

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1 hour ago, Raph-in-the-sky said:

$75 sounds pretty good. 

If I cannot find it used, I will get the Omegon copy which seems identical but cheaper (at least for me when shipping costs are taken into account).

https://www.astroshop.eu/flatteners-correctors-reducers/omegon-pro-koma-korrektor-fuer-astrograph/p,53821

Can you buy from Telescope House?  Their Revelation version is only £65.10.

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