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SW 127 Mak OTA with 2" focuser


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4 hours ago, Relpet said:

Having started this thread I've been interested to see how it's developed, especially since I bought the Omegon from John!  I'm still awaiting delivery of a 2" visual back but as soon as that arrives I'll hope to make a comparison of lunar views using a 25mm ES 2" and a 27mm 1.25" Orbinar flat field.  Not a direct comparison but close enough, maybe.  As John has said in other posts, the Omegon is a true 127mm so vignetting should, I imagine, be even less noticeable using the 2" but I certainly hope my ancient eyes will get added benefit from a 2" option.  I asked the question originally because the Skywatcher 127 is available with a 2" option in Europe,  (and I believe Canada) at minimal extra cost, but not in UK.  FLO, at my request, were asking the importers why it was not on offer to UK buyers but I've had no response.  It's irrelevant now anyway since I'm delighted with the Omegon.

The factory 2" version of the Skywatcher 127 is quite literally the original 1.25" 127 with the optional Mak to SCT thread adapter and 2" visual back standard.  Thus, the rear port and baffle is exactly the same size as the original 1.25" version.  How do I know, because I've got both an older Orion 127 1.25" version and a newer Celestron 127 2" version, and they're identical at the rear ports.  Celestron is owned by Synta/Skywatcher, so they're the same scope.

I suppose @FLO could offer an upgrade package for the 1.25" version by adding the thread adapter and swapping the visual back for a 2" version until their importer sees fit to offer it in factory form in the UK.

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Am I missing something?  It seems to me that unless the internal diameter of the baffle tube is a full 2", there's little point in using 2" eyepieces. All the 127mm Mak's I've used have had a much narrower baffle tube. And supposing the baffle on a 127mm Mak was a full 2" internal diameter, it would eat up a significant amount of useable surface area on the primary; bearing in mind that the thickness of the baffle tube, and its retaining ring also intrude into the surface area. A 127mm Mak is a terrific scope, but its not a wide field instrument, and it seems to me at least, that trying to use it as such would have detrimental effects. Using a 2" back on a scope with a baffle tube less than 2" would surely cause vignetting? :icon_scratch:

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2 hours ago, mikeDnight said:

Am I missing something?  It seems to me that unless the internal diameter of the baffle tube is a full 2", there's little point in using 2" eyepieces. All the 127mm Mak's I've used have had a much narrower baffle tube. And supposing the baffle on a 127mm Mak was a full 2" internal diameter, it would eat up a significant amount of useable surface area on the primary; bearing in mind that the thickness of the baffle tube, and its retaining ring also intrude into the surface area. A 127mm Mak is a terrific scope, but its not a wide field instrument, and it seems to me at least, that trying to use it as such would have detrimental effects. Using a 2" back on a scope with a baffle tube less than 2" would surely cause vignetting? :icon_scratch:

Did you miss my empirical photographic posting earlier in this thread demonstrating limited vignetting with just such a setup?  It works just fine except when bright stars pass the edge of the rear baffle tube/port and create an ever larger oval reflection in the field of view.  I also wouldn't recommend it for lunar or solar work, either.

 

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4 hours ago, johninderby said:

The 127 SW mak now comes with an SCT thread on it so no need for an adapter anymore.

Are you sure it's natively an SCT thread?  My 2" 127 Celestron Mak version's SCT thread adapter just screws off revealing the original 45.4mm Mak thread.  I can't imagine Synta makes two different versions for the Celestron and SkyWatcher labels.  It was a bit difficult to get it to start, but it eventually unscrewed completely off.

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10 hours ago, Louis D said:

Did you miss my empirical photographic posting earlier in this thread demonstrating limited vignetting with just such a setup?  It works just fine except when bright stars pass the edge of the rear baffle tube/port and create an ever larger oval reflection in the field of view.  I also wouldn't recommend it for lunar or solar work, either.

 

No, I didn't miss your post Louis!  My issue is that no matter what is attached to the back end, the 127mm Mak can not use a 2" format to the full. A 2" eyepiece with a near full size field lens has its field lens effectively stopped down by the much smaller baffling. It can't use the full aperture of its field lens. 

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2" eyepieces are just 2" fit.  It's the field stop of the eyepiece that you have to consider.  Obviously, if the aperture feeding the eyepiece is smaller than the field stop there will be a degree of vignetting.  Visually, vignetting is not as noticeable as you might think.    🙂

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For the cost and trouble of the changed visual back and a decent 2 inch diagonal one could almost have bought a pre-owned a 130mm F/5 newt or 120mm F/5 frac and have a proper wide field scope to compliment the 127 (ish ?) mak-cassegrain rather than going to great length to squeeze a few extra arc minutes of view from a scope who's strengths lie elsewhere.

Well that's my take on it anyway :dontknow:

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9 minutes ago, Peter Drew said:

2" eyepieces are just 2" fit.  It's the field stop of the eyepiece that you have to consider.  Obviously, if the aperture feeding the eyepiece is smaller than the field stop there will be a degree of vignetting.  Visually, vignetting is not as noticeable as you might think.    🙂

Exactly what I found in testing 2” eyepieces in the 127.

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8 minutes ago, John said:

For the cost and trouble of the changed visual back and a decent 2 inch diagonal one could almost have bought a pre-owned a 130mm F/5 newt or 120mm F/5 frac and have a proper wide field scope to compliment the 127 (ish ?) mak-cassegrain rather than going to great length to squeeze a few extra arc minutes of view from a scope who's strengths lie elsewhere.

Well that's my take on it anyway :dontknow:

If you already have the bits needed why not give it a try but I found it wasn’t really worth the bother for a small increase in fov. It’s never going to be a wide field scope. 

Edited by johninderby
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4 hours ago, John said:

For the cost and trouble of the changed visual back and a decent 2 inch diagonal one could almost have bought a pre-owned a 130mm F/5 newt or 120mm F/5 frac and have a proper wide field scope to compliment the 127 (ish ?) mak-cassegrain rather than going to great length to squeeze a few extra arc minutes of view from a scope who's strengths lie elsewhere.

Well that's my take on it anyway :dontknow:

Yes, but if you already have a 2" diagonal, the cost of a 2" visual back and Mak-SCT thread adapter is well under $50.  I spend $300 on used sets of step-up eyepieces just to see what the fuss is about, so $50 is chump change.

With the 2" visual back, I can use my 2" eyepieces like the 22mm NT4, and 17mm and 12mm ES-92s in the little Mak.  Since the visual back is solidly attached to the rear plate, I'm not concerned about mechanical deformation with these heavy eyepieces, either.

Also, I don't always have the AT72ED or 90mm APO attached to the other side of my DSV-2B, so it's nice to be able to get to a 1.7 degree instead of a 1 degree TFOV to better take in the surroundings of larger starfields without having to swap scopes or mount an additional scope.

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I wish I could advance this discussion with the same level of knowledge and experience as you chaps but my 2" visual back arrived today.  Focusing on a neighbour's chimney stack about 100 yards away I compared the performance of a 1.25"  25mm Celestron plossl with a 25mm ES 2".  With the plossl I counted 7.5 courses of brickwork  but, with the ES, 11 courses, a difference  in view of 47%.  There was no vignetting as far as I could discern.  However, the plossl is reckoned to have an AFOV of 52° while the ES is marked as 70°, a difference of 35%.  To my (probably over-simplified) calculation it would suggest therefore, that by using EPs with identical fields of view there would be a net difference of 12% in favour of the 2" format.  In some circumstances that  12% might make a difference sufficient to justify to some observers the use of a 2" diagonal with 2" EPs.  If my view of this suffers from being too naive treat me kindly and blame it on my age.  As it was, I was equally thrilled with the image I got through both EPs as my new set-up is now complete.

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Does anyone make a 1.25" only Crayford or R&P focuser? I'm guessing not but it could be an option for people who wanted to move away from the stock focuser without wanting the extra weight of a 2" focuser on the back of the scope?

Edited by GazOC
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9 minutes ago, GazOC said:

Does anyone make a 1.25" only Crayford or R&P focuser? I'm guessing not but it could be an option for people who wanted to move away from the stock focuser without wanting the extra weight of a 2" focuser on the back of the scope?

There is the Feathertouch for the Lunt HA scopes. Could that be adapted ?:

https://www.firstlightoptics.com/specialist-focusers-borg-tele-vue-lunt-gso-rc/feathertouch-125-dual-speed-focuser-for-lunt-50mm-solar-telescopes.html

I think Moonlite might have something similar ?

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49 minutes ago, johninderby said:

The focuser isn’t too expensive but the custom adapter to fit an  SCT thread isn’t cheap. However still way cheaper than an FT. Good focuser too. Sort of a mini Steeltrack.

Do you have the TS link for the SCT thread adapter?  Thanks! 😁

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