53 replies to this topic

[upwinddan]

I just bought this little scope for mainly  portable photography.  However, I'd like to use it as a grab and go for visual use also. I put the 2' to 1.25" adapter on but I can not get focus with any of my 1.25" eyepieces.  I've tried both a  star diagonal and direct focus. If I hold the eyepiece all the way out of the start diagonal I can get focus. 

 

Do I need an extender or spacers or just a better brain? I have had several scopes but have never experienced this issue before.

 

 

[Astrojensen]

The scope is designed to use a 2" diagonal, so if you're using a 1.25" diagonal, you need a short extension tube. About 2" long should do it. 

 

 

Clear skies!
Thomas, Denmark

[Shorty Barlow]

The scope is designed to use a 2" diagonal, so if you're using a 1.25" diagonal, you need a short extension tube. About 2" long should do it. 

 

 

Clear skies!
Thomas, Denmark

I'm not so sure it's designed specifically for 2" diagonals. Admittedly it's sold without an adapter, but that doesn't mean anything. I can use my Baader T2 but the Takahashi needs a slight extension, less than 2" definitely.

 

 

My Baader Zeiss 1.25" Amici also needs no extensions. 

 

 

I don't have enough back-focus with a 36mm Baader Aspheric and this 2" Baader dielectric.

 

 

My 1.25" TV Enhanced Aluminium has no focusing problems either.

 

 

I was using this TV in the 72ED a couple of nights ago, everything from a 36mm Baader Aspheric (1.25" barrel), a 25mm TS Planetary HR to a 5x Powermate combined with a 9.7mm Meade Plossl achieved focus. I find with all of my 2" diagonals (mirror or Amici prism) there is a point where there just isn't enough in-focus for any eyepiece less than 4mm. My APM 2" Amici won't focus at all. So I'm pretty certain it wasn't designed primarily for 2" diagonals. 

Edited by Shorty Barlow, 31 August 2020 - 02:09 PM.

[sg6]

Equally if you put a 2" diagonal in you will have to wind it in as far as possible to get focus.

I had to literally ram mine against the scope to just manage a viewable image - could have been the diagonal I suppose.

I eventually had 2.5mm skimmed off the lower lip of the 2"-1.25" adaptor so it sat lower and I had a little "spare".

 

Skywatcher don't make it easy at times. I suspect everything is just about useable somehow.

Edited by sg6, 31 August 2020 - 02:09 PM.

[sojourneyer]

I'm not so sure it's designed specifically for 2" diagonals. Admittedly it's sold without an adapter, but that doesn't mean anything. I can use my Baader T2 but the Takahashi needs a slight extension, less than 2" definitely.

 

 

My Baader Zeiss 1.25" Amici also needs no extensions. 

 

 

I don't have enough back-focus with a 36mm Baader Aspheric and this 2" Baader dielectric.

 

 

My 1.25" TV Enhanced Aluminium has no focusing problems either.

 

 

I was using this TV in the 72ED a couple of nights ago, everything from a 36mm Baader Aspheric (1.25" barrel), a 25mm TS Planetary HR to a 5x Powermate combined with a 9.7mm Meade Plossl achieved focus. I find with all of my 2" diagonals (mirror or Amici prism) there is a point where there just isn't enough in-focus for any eyepiece less than 4mm. My APM 2" Amici won't focus at all. So I'm pretty certain it wasn't designed primarily for 2" diagonals. 

good advice Shorty!

[Shorty Barlow]

In my experience all short tube refractors around 70mm ~ 80mm have back-focus or in-focus problems.

 

 

It's what extension tubes were invented for.

[Shorty Barlow]

good advice Shorty!

I must be having a good day lol.

[Redbetter]

In my experience all short tube refractors around 70mm ~ 80mm have back-focus or in-focus problems.

 

 

It's what extension tubes were invented for.

 

Having the right focuser mated to the OTA is a big part of this.  Synta has some models with issues in this regard.  Focusers with ~75mm of travel tend to do pretty well with a wide range of set ups.  Ones with only 50mm are more limited.  

 

I don't recall any of mine in this size range having these sort of problems, even with 2" focuser upgrades.  ST80 & Meade AS 80, AT72EDII, Orion ED80, and Celestron LT80.  That is with a wide range of 2" and 1.25" eyepieces and filters.  But the focusers used have ~75mm of travel and are also well mated to those OTA's. 

 

There are other attachments that can prove more problematic with certain set ups, such as ADC's and binoviewers.

[MortonH]

Apparently Skywatcher have shortened the tube in the latest version...

 

[Baatar]

Another experience in favour of 1.25" format.

 

I have this scope and I use only 1.25" diagonal and EPs without any focusing issues.  Before getting this I read many posts that 2" diagonals would indeed be problematic in reaching focus as SG6 and Shorty Barlow mentioned above.  But this depends on what combinations of 2" diagonals and EPs are used, I guess.

 

Other reasons of going for 1.25" format for me was:  balance issue, less ergonomics, less heavy, and may be less costly etc.  And simply, using 2" EPs wasn't needed for me.  The ES68 24mm already gives the widest TFOV in 1.25 format, which on this scope is 17x magnification and 3.9 degrees field of view.  For me, this is enough for low power sweeping, and am not sure if one needs to go even lower or wider (requiring 2" format, except for other considerations such as exit pupil, even wider AFOV etc.).

Edited by Baatar, 01 September 2020 - 08:54 AM.

[PatrickVt]

As others have mentioned already, the older version of this scope has a slightly longer tube which does make it difficult to come to focus with some 1.25" eyepieces when using a 2" diagonal.  The newer version of the scope is slightly shorter to allow for focus.  Skywatcher really made a boneheaded move on that first scope.  I have one and it is a problem for me but only for solar viewing.  

 

For nighttime viewing, I simply use a 1.25" diagonal.  I have no problem with that.  It works fine.  I even use my Baader Aspherical 36mm in it with no problems although I suspect I am not noticing some vignetting.  I might actually have a view closer to using my GSO 32mm eyepiece because of vignetting in the 1.25" diagonal.  Again, I get a nice, bright, wide view with the 36mm anyway.

 

For solar viewing, I was planning to use my 2" solar wedge but, because the wedge is a bit longer than standard 2" diagonals, there is no way to focus any 1.25" eyepieces...  none.  Now, that is a problem.  

 

Cutting the tube isn't an option because the focuser screws onto the end of the tube.   If I cut the tube, then I need to somehow thread it again so I can screw on the focuser again.  Paying someone else to thread the tube isn't worth it...  cutting the tube and then purchasing a new/different focuser isn't worth the cost...  I might as well just purchase one of the new versions of the scope.  I don't want to reward Skywatcher with another sale though simply because of their boneheaded design flaw.  If I do replace the scope, I'll purchase a different brand.  

 

My temporary solution to my solar issue is to purchase a 1.25" solar wedge for use with the Skywatcher ED 72mm.  I'm not too happy about that either since I already have a 2" wedge but it is the least costly option as well as an easy solution.  It really annoys me that the tube is so long on that first version of this scope.

 

These scopes that are too long really need to be labeled in product descriptions as such.  They should also be marked down in price so they can unload these scopes.  The long version is definitely a 1.25" version when using a diagonal.  For imaging with no diagonal, 2" works just fine.

 

Patrick

[Shorty Barlow]

Being a centimetre shorter would certainly help with in-focus issues, not so sure about back-focus though. I only use 1.25" diagonals on my 72 now, but this is for reasons of convenience. I have a visual back with a compression ring which aided rotating a 2" diagonal. Unfortunately, I'm partially paralysed in my right arm and hand and using rotating focusers on other scopes has spoiled me. 

 

 

As I stated earlier, these sort of issues are well known with many short tube refractors. Which is why extension tubes exist in the first place.

 

 

After I removed the TS Optics (GSO) focuser from my ST102 I replaced it with this Sky-Watcher (probably Long Perng).

 

 

It was an improvement to the GSO which had rotating issues. It was also heavier and longer. There wasn't enough in-focus for my 14mm Morpheus and a 2" APM Amici, although most of my eyepieces were fine.

 

 

Eventually I replaced the Sky-Watcher with a MoonLite. And I can now use the Morpheus again! The Long Perng developed a tendency to shift focus. I fixed this with an Allen key but by then I decided to get a MoonLite anyway.

 

 

In my experience with short tube refractors, finding what combinations of diagonals and eyepieces attain focus is a matter of trial and error. 

 

 

It's been almost exactly two years since I first bought my 72ED DS Pro. 

 

 

Despite minor issues with focusing, it was still 245 quid well spent!

[PatrickVt]

Being a centimetre shorter would certainly help with in-focus issues, not so sure about back-focus though. I only use 1.25" diagonals on my 72 now, but this is for reasons of convenience. I have a visual back with a compression ring which aided rotating a 2" diagonal. Unfortunately, I'm partially paralysed in my right arm and hand and using rotating focusers on other scopes has spoiled me. 

 

 

 

As I stated earlier, these sort of issues are well known with many short tube refractors. Which is why extension tubes exist in the first place.

 

 

 

After I removed the TS Optics (GSO) focuser from my ST102 I replaced it with this Sky-Watcher (probably Long Perng).

 

 

It was an improvement to the GSO which had rotating issues. It was also heavier and longer. There wasn't enough in-focus for my 14mm Morpheus and a 2" APM Amici, although most of my eyepieces were fine.

 

 

Eventually I replaced the Sky-Watcher with a MoonLite. And I can now use the Morpheus again! The Long Perng developed a tendency to shift focus. I fixed this with an Allen key but by then I decided to get a MoonLite anyway.

 

 

 

In my experience with short tube refractors, finding what combinations of diagonals and eyepieces attain focus is a matter of trial and error. 

 

 

 

It's been almost exactly two years since I first bought my 72ED DS Pro. 

 

 

 

Despite minor issues with focusing, it was still 245 quid well spent!

 

As far as a rotating focuser...  yes it is convenient but isn't it just as easy to just rotate this very short tube in it's rings when necessary?

 

I've checked out the Moonlite website in the past to see about focusers and did not see a flange listed for the Skywatcher ED72.  I know there is a flange for the AT72ED...  does that one fit the same for the Skywatcher ED72?  I might consider that option as an upgrade.  I could check on it myself but you probably know off the top of your head, is the Moonlite shorter than the stock Skywatcher 2 speed focuser?  That would buy me some much needed focus range.  Perhaps I should look into this option. 

 

Patrick

[Redbetter]

That seems a lot of trouble just to reach focus with standard eyepieces.  I don't really understand some of Synta/Sky-Watcher's OTA/focuser design choices--and that goes for their Dobs as well when I have fiddled with them.  Too much fiddling with extension tubes for visual.  I have been observing for 25 years and have a variety of scopes and haven't needed extension tubes for observing with any of them with one exception, and that exception needs an asterisk because it is the Sky-Watcher 50ED that is intended as a photographic guide scope not visual.  Other than the extension tubes I bought just for the 50ED, I have zero, and have needed zero from eyepieces in the range of 31 Nagler (max inward travel) to 40mm Plossl (max outward in 1.25") and 55 Plossl (max outward in 2".)

 

I don't really care for the Sky-Watcher 72ED's design.  The OTA is too long, then paired to a focuser with a short draw tube, something like 38 or 39mm that needs extensions (an additional 12mm extension is included from what I gathered.)  Fixed (but removable) dew shield that makes the set up even longer than it needs to be.  An included dovetail with hardware that sticks out--Synta dovetails in general are not well thought out in this regard.  

 

Compared to the AT72EDII the Sky-Watcher focuser doesn't rotate, is too short, etc.  AT has 86mm of travel.  And the included AT dovetail allows somewhat better dovetail positioning for balance--as well as featuring recessed hardware.  The AT's sliding dew shield results in an effective packed OTA length of about 320mm vs. 420mm for the SW, making for a much more compact package with the AT despite having so much more focuser.  The SW's overall length is slightly longer than the Apache 3800 case that I use for the AT.  (The two areas where the SW has some advantage are in supplying a case and a finder dovetail.)  

[Shorty Barlow]

As far as a rotating focuser...  yes it is convenient but isn't it just as easy to just rotate this very short tube in it's rings when necessary?

 

I've checked out the Moonlite website in the past to see about focusers and did not see a flange listed for the Skywatcher ED72.  I know there is a flange for the AT72ED...  does that one fit the same for the Skywatcher ED72?  I might consider that option as an upgrade.  I could check on it myself but you probably know off the top of your head, is the Moonlite shorter than the stock Skywatcher 2 speed focuser?  That would buy me some much needed focus range.  Perhaps I should look into this option. 

 

Patrick

A rotating focuser would add weight as well though. Personally I don't think rotating the entire scope would be more convenient for me. All of my refractors (and my 150mm Newtonian) are fitted with Baader 2" ~ 1.25" adapters. These have two screws, a compression ring and a T - adapter thread. IMHO these are the best adapters on the market and make it easier and safer to rotate 1.25" diagonals for me. 

 

I don't know if MoonLite make a flange for the 72 ED DS Pro. I doubt it.

 

 

I have some MoonLite literature but I can't find anything for the 72mm Evostar. AFAIK the AT72ED isn't marketed in my country and is more expensive than the Evostar. It might be worth asking MoonLite to make a bespoke flange for the 72ED DS Pro. 

 

The 72ED DS Pro is a bit sui generis as a Synta scope. I'm not really sure what the intended market was or is. It was very competitively priced here, especially at launch, and sold very well. Probably intended as a quality entry level short tube ED doublet for visual and AP. The early focusers had a lot of problems and were replaced. Mine's one of the new focusers and is actually very good. Synta have never released details about the ED element, although mine has a Schott crown. I'm guessing CDGM Glass Co., Ltd (Chengdu, Sichuan) as they are the biggest manufacturer of optical glass in the world.

[Shorty Barlow]

Back to the OP:

 

 

This can also solve back-focus issues, Takahashi collet issues, and often makes it easier to view when the diagonal is rotated. lol

[jeffmac]

I just bought this little scope for mainly  portable photography.  However, I'd like to use it as a grab and go for visual use also. I put the 2' to 1.25" adapter on but I can not get focus with any of my 1.25" eyepieces.  I've tried both a  star diagonal and direct focus. If I hold the eyepiece all the way out of the start diagonal I can get focus. 
 
Do I need an extender or spacers or just a better brain? I have had several scopes but have never experienced this issue before.

I have that scope and had the same problem. I couldn't get most of my 1.25" eyepieces to come to focus. I was using a 2" Tele Vue Enhanced diagonal with a Tele Vue Hi-Hat 2" to 1.25" adapter. I bought a Tele Vue low profile 2" to 1.25" adapter and now all of my 1.25" eyepieces will focus. (at least I haven't used any in that scope that don't focus now).

[Shorty Barlow]

I have that scope and had the same problem. I couldn't get most of my 1.25" eyepieces to come to focus. I was using a 2" Tele Vue Enhanced diagonal with a Tele Vue Hi-Hat 2" to 1.25" adapter. I bought a Tele Vue low profile 2" to 1.25" adapter and now all of my 1.25" eyepieces will focus. (at least I haven't used any in that scope that don't focus now).

Was this an in-focus problem?

[Shorty Barlow]

OK, scrap what I said earlier regarding the Baader 36mm Hyperion Aspheric.

 

 

I've been doing some daylight tests on distant targets and the Baader dielectric will achieve focus, although there's not much wiggle room. The Altair is similar but the Sky-Watcher gave me the most back-focus. I think when I originally couldn't get a focus with the Baader it wasn't pushed all the way into the visual back. This is probably due to my physical disability. It's a distinct possibility the physical dimensions of the diagonal housings are the root cause of the focusing discrepancies. 

 

The Baader has a straight nose with safety smurfs (or something). When I loosen the compression ring to rotate it can drop somewhat. The Altair and SW have flared nosepieces which enable them to hang slightly on the compression ring. This actually makes it easier for me to safely rotate the diagonal and push it back without it falling out. As the Altair is over half a kilo the SW wins. TBH I can't tell the difference between them visually anyway. 

 

This can enable me to use the 36mm Aspheric in 2" configuration, giving 11.6x. 

Edited by Shorty Barlow, 02 September 2020 - 08:11 AM.

[PatrickVt]

This discussion seems to have moved into a confusing area and I fear that some people who don't own this scope might get the wrong idea about these focusing issues.  All I am going to write, below, applies only to the older version of this telescope which has a very slightly longer tube than the newer version.  From what I understand, there are no focus/backfocus issues with the newer version of this scope.

 

When using any 2" eyepieces in a 2" diagonal, I have never encountered a problem.  That doesn't mean that there isn't a 2" eyepiece out there that requires so much backfocus that it won't come to focus in this scope but it would require a lot of backfocus before there is a problem focusing and I have not come across that problem yet. 

 

Using my Baader 1.25"/2" eyepieces configured as 2" eyepieces, you save upwards of 27mm in optical path length because more of the body then slips down inside the diagonal and there is no 2" to 1.25" adapter to add more optical path length.  In short, none of my Baader eyepieces have any problems coming to focus when used in 2" mode with a 2" diagonal.  It is not even close.

 

On the other hand, when using a 2" diagonal with a 2" to 1.25" adapter inserted, I did find I had problems with a few of my 1.25" eyepieces coming to focus.  The problem eyepieces were the GSO Super Plossl 20mm, my Baader 36mm (in 1.25" configuration only) and my Baader 24mm (in 1.25" configuration only) was too close for comfort.  Since we're using a 2" diagonal, it makes little sense to use the Baader eyepieces in 1.25" configuration when we're able to easily use them as 2" eyepieces and resolve the problem completely.    

 

In fairness, I should also list the eyepieces that did work.  The entire Orion Expanse lineup worked fine.  My GSO Super Plossl 40mm and 32mm worked just fine although the 40mm vignettes but that is a different issue and completely expected.  My Baader Morpheus 6.5mm, 9mm and 12.5mm focused just fine as 1.25" eyepieces.  My Baader Hyperion 8mm and 17mm focused just fine as 1.25" eyepieces.  I have a Celestron 25mm that came with a Celestron scope which, I think, is probably a Starguider rebrand and that worked just fine.  Again, the only Baaders that I had a problem with were the 24mm and 36mm in 1.25" configuration only.

 

There are three things to point out about optical path length.

 

1.  Different diagonals have different length optical paths.  This can vary by a few mm's and upwards of 10mm or so when compared to prisms.  This is a relatively fast scope, though, so I would not recommend a prism diagonal even though they have a shorter optical path.  I have found that I see quite a bit of CA in this scope when using a prism diagonal.  Some prisms probably work better than others but the general advice found online is to avoid prisms in fast scopes.  (Good 2" prisms cost more than the cost of this entire scope anyway.)  For my focusing tests today, I used a 2" High Point 99% Dielectric diagonal.

 

2.  Purchasing a recessed 2" to 1.25" adapter will save you a few mm's for most 1.25" eyepieces.  (See my photo, below.)  I use one of these but I should point out that large eyepieces like the Baaders are not narrow enough to sit down inside this adapter so it still sits on top of this whole adapter.  For most 1.25" eyepiece, though, this recessed adapter will save you a few mm's in optical path length.

 

3.  Adding a compression ring to this ED72 focuser will eat up some of your valuable optical path length.  My guess is that the one compression ring that I know is available for the SW 72ED eats up about 6mm of optical path.  The rotator version will eat up even far more optical path than the compression ring.  My advice is don't use these "upgrade" accessories with the older, longer version of this optical tube because they will cause bigger focus issues especially when using a 2" diagonal.

 

So, to sum up, you should not have any problems with any 2" eyepieces in a 2" diagonal on this scope if you don't add accessories that cut down on your optical path length.  Also, focusing problems are minimal even when using 1.25" eyepieces with a 2" diagonal as long as you don't add accessories such as compression rings or rotators.  Swapping out a standard 2" to 1.25" adapter for a recessed one will save you a few mm's too and will definitely help in most cases.  Purchasing one of these is a very small price to pay to gain a few valuable mm's (some of my diagonals and Barlows came with these recessed adapters so I didn't need to purchase one).  Understand that if you do add accessories to the focuser tube, such as compression rings or rotators, you will have even more problems when using a 2" diagonal.  One last point is if you only plan to use 1.25" eyepieces, then it would make sense to use a 1.25" diagonal to avoid all potential problems.

 

Solar imaging is a bigger problem but really was not the topic in this thread but maybe I should quickly mention the issue.  A 2" solar wedge has an optical path of 114mm so it is considerably longer than the standard nighttime 2" diagonal.  I have found that I cannot get any of my 1.25" eyepieces to come to focus when using a 2" solar wedge.  So, for solar imaging with this scope, I'd recommend the 1.25" solar wedge. 

 

I hope this helps to clear up some confusion about the older version of this telescope.

 

Patrick  

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Edited by PatrickVt, 02 September 2020 - 09:56 AM.

[Shorty Barlow]

 

 

3.  Adding a compression ring to this ED72 focuser will eat up some of your valuable optical path length.  My guess is that the one compression ring that I know is available for the SW 72ED eats up about 6mm of optical path.  The rotator version will eat up even far more optical path than the compression ring.  My advice is don't use these "upgrade" accessories with the older, longer version of this optical tube because they will cause bigger focus issues especially when using a 2" diagonal.

 

 

Good point. I forgot about the possible extra optical path length an aftermarket visual back could create.

Edited by Shorty Barlow, 02 September 2020 - 10:04 AM.

[jeffmac]

Was this an in-focus problem?

Yes. Not sure why the OP's scope is requiring more out-focus when using a 2" diagonal.

[PatrickVt]

I forgot to add in my previous post (#20 above) that I don't even bother with a 2" diagonal anymore with this scope.  I pack an Apertura 1.25" diagonal in the case with the scope.  With this diagonal, I no longer have the occasional backfocus issue and I do not miss the 2" diagonal.  My Baader Aspherical 36mm and my GSO Super Plossls (32mm and 40mm) look great!  

 

The 2" focuser comes in handy for imaging with APS-C and Full Frame sized sensors.  When imaging, there is no diagonal eating up optical path space.  

 

Patrick

 

 

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[PatrickVt]

Yes. Not sure why the OP's scope is requiring more out-focus when using a 2" diagonal.

 

I assume the new version of this scope with the shorter optical tube is short enough to need a small extension tube for some eyepieces...  maybe? 

 

If so, this does make me wonder just how much shorter the new one is than the older model.  If an extension tube is required for a 1.25" eyepiece in a 2" diagonal (although, the OP never mentions a 2" diagonal...  only a 2" to 1.25" adapter), then an even longer extension tube will be required for a lot of other situations.  Unfortunately, the original poster didn't give us enough information and we haven't heard from him since he posed his problem. 

 

The known problem with this scope, particularly the older version of this scope, is that there is not enough inward focuser travel because of the longer optical tube which is probably why people went that direction in this thread.  Once people started discussing the lack of inward focuser travel, that potential issue needed to be cleared up.    

 

Maybe (hopefully) the original poster will chime in with more clarification.

 

Patrick

[Redbetter]

Having an absurdly short ~38mm of focuser travel is the root problem (even with the 12 mm extension).  It guarantees that the scope will have problems in either 2" diagonal mode, or 1.25" diagonal mode, regardless of the OTA's tube length.  They can supply extensions to compensate for one of the cases although that is not handy if one has to pull it in and out to swap eyepieces during a session, but the obvious solution would be for Sky-Watcher/Synta to select a proper focuser for it from the start with ~3" of draw tube travel, then cut the tube to a length that properly handles the range.

 

The narrower tube of the SW 72ED could be problematic in this regard.  (For comparison the AT72EDII has an 84mm tube O.D. and more room for internal baffling against the wall--it has a ton of baffles.)  It might be posing some sort of internal problem with respect to changing to longer focusers.  Or it might have something to do with Synta's stock ring diameters for the OTA and their resultant spacing or dovetail hole spacing etc.  Hard to say exactly what their corporate perceived limitation is that led to the design, but it seems that Synta needs to look at their full bin of parts more closely and arrive at a robust solution. 

 

Synta probably needs some better focusers for starters...none of theirs that I have tried have been very good whether on doublet refractors or Dobs.  Perhaps their higher end OTA's are better in that regard, but I have not used them so I don't know.