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(Another) Newbie question - Helical Focuser, eyepieces, barlows and the SW 130


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Hi SGL,

My lovely wife bought me a SW Explorer 130 with the basic RA drive for my 40th birthday 8 years ago, and I've enjoyed using it on and off ever since.  I've started getting more into it - it's a great beginner scope, but I'm - like lots of others on this forum - looking at upgrading.  One of the things I found tricky with the scope was getting the focus just right at high magnification (I'm sure that's kind of obvious!).  It does wobble quite a bit on the EQ2 mount when focusing.  I get pretty good - if small - views of Jupiter, Saturn, (the Moon, of course) and have had hints of seeing some DSOs.
I bought a 8mm BST Starguider, and it's a great improvement on the standard 10mm that comes with the scope, but if I push the scope by adding the 2x Barlow and the 8mm it's a blur and the focus seems to keep missing the sweet spot.  I saw a post elsewhere on this forum by MakTheNight, (looks like they've left now - last post was 2 years ago) who added a Baader Helical Focuser that attached directly to the T-thread on the focusing apparatus on the SW. Link to that post is below. I got one and it is indeed a great improvement on fine tuning - the problem is I can't use it with the Barlow as the helical focuser is about an inch long and the focuser can't move far enough into the scope to focus with the Barlow and any of my eyepieces (the 8mm BST, or the 10 or 25mm standard eyepieces)
.
Clearly MakTheNight managed to get their setup to work with some specific eyepieces (a Luminos 10mm and 32mm Plossl) and a specific Barlow, so I tried looking up how I could work out what combination of eyepieces and Barlow would work, rather than randomly buying kit and having to return it because it won't work with my scope, but I haven't found anything useful yet.  So I thought I'd take the plunge and ask here :)

Can anyone help me figure out how I can calculate what combinations of eyepiece and Barlow would work with this setup?  I'm also looking at upgrading the OTA and mount to a 200P and EQ5 at some point, so ideally would have a combination that could work with that too - the focal length and mirror is quite different - 1000mm on the 200P parabolic vs 900mm spherical on the 130.

 

Many thanks!

Simon

 

 

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The resulting magnifications are calculated by taking the focal length of your telescope (900mm) and dividing it by the focal length of your eyepieces (e.g. 8mm) 900 / 8 = 112.5x magnification. When you add a 2x barlow into the optical path your 8mm will essentially function as a 4mm eyepiece (resulting in 225x magnification). On Newtonian designs, it’s best to keep the eyepiece in ‘mm’ more than the focal ratio of the telescope. Your telescope has a focal ratio of f5 so, in my option, the resulting 4mm of the 8mm + barlow is a bit too much magnification for your setup.  

My suggestions for four eyepiece focal lengths would be:

25mm (36x), 15mm to 13mm (60x to 69x), 10mm to 8mm (90x to 112x) and 6mm to 5mm (150x to 180x). These would also scale up well to a 200p in the future.

You wouldn’t always want to use a barlow as it would limit your ability to enjoy wide field views. However, if you wish to use it as much as possible you should double the focal lengths suggested above (excluding the 25mm which should be used without the barlow). Therefore: 25mm (not used in the barlow), a 20mm (resulting in 10mm in the barlow) and a 12mm (resulting in 6mm in the barlow).

Edited by Rob_UK_SE
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Use the Televue eyepoece calculator.

Imputnthe aperture and focal legth of the scope and select magnification and it will give the max magnification recommended and a chart of eyepieces  Yes they’re Televue eyepieces but will show you what mag different focal length eyepieces will give among other info.

http://www.televue.com/engine/TV3b_page.asp?id=212&plain=TRUE

Edited by johninderby
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Thank you John and Rob. Sorry - My explanation of what I’m trying to work out was very poor. 
I’m trying to work out why some eyepieces will work with the helical focuser and Barlow in place and others won’t. It must be something to do with where the focal plane is in the eyepiece, I think, but I haven’t been able to find anything that explains this. 
 

MackTheNight managed to get a combination of The helical focuser plus 2x Barlow and 10mm eyepiece to work in the 130 scope. I have the helical focuser, the Barlow that came with the scope and the Super MA 10mm & 25mm  eyepieces that also came with the scope and also the 8mm BST Starguider I bought and can’t achieve focus at  all with any of them. The additional length the helical focuser adds to the focus apparatus together with the Barlow mean the eyepiece can’t achieve focus. I’d need to wind the focus into the scope beyond the hard stop.  I’ve heard of short barlows - maybe that would help. 
 

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6 hours ago, LondonSi72 said:

Thank you John and Rob. Sorry - My explanation of what I’m trying to work out was very poor. 
I’m trying to work out why some eyepieces will work with the helical focuser and Barlow in place and others won’t. It must be something to do with where the focal plane is in the eyepiece, I think, but I haven’t been able to find anything that explains this. 
 

MackTheNight managed to get a combination of The helical focuser plus 2x Barlow and 10mm eyepiece to work in the 130 scope. I have the helical focuser, the Barlow that came with the scope and the Super MA 10mm & 25mm  eyepieces that also came with the scope and also the 8mm BST Starguider I bought and can’t achieve focus at  all with any of them. The additional length the helical focuser adds to the focus apparatus together with the Barlow mean the eyepiece can’t achieve focus. I’d need to wind the focus into the scope beyond the hard stop.  I’ve heard of short barlows - maybe that would help. 
 

Assuming that everything is setup correctly with the focuser, it sounds like you might need more inward travel in order to bring the eyepiece/s into focus. Barlows require more inward travel than simply using the same eyepiece alone.

It would be worthwhile sending MackTheNight a message about the ‘winning’ combination and how it was achieved. I suspect that it’s probably the result of a specific eyepiece, specific barlow and possibly a small modification to the focuser?

I have not used the 130 explorer, but some of the other Sky Watcher newtonian scopes include a short extension tube. If this the case for your one too, you would -at present- be missing sufficient outward focus to bring the eyepiece into focus. I am purely speculating with this suggestion though. 

Can you achieve focus without using the barlow?

Edited by Rob_UK_SE
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6 hours ago, Rob_UK_SE said:

Assuming that everything is setup correctly with the focuser, it sounds like you might need more inward travel in order to bring the eyepiece/s into focus. Barlows require more inward travel than simply using the same eyepiece alone.

It would be worthwhile sending MackTheNight a message about the ‘winning’ combination and how it was achieved. I suspect that it’s probably the result of a specific eyepiece, specific barlow and possibly a small modification to the focuser?

I have not used the 130 explorer, but some of the other Sky Watcher newtonian scopes include a short extension tube. If this the case for your one too, you would -at present- be missing sufficient outward focus to bring the eyepiece into focus. I am purely speculating with this suggestion though. 

Can you achieve focus without using the barlow?

Hi Rob.  I'll try Mak the Night - they last posted on the forum 2 years ago so suspect they aren't monitoring it these days.  Yes I can focus without the Barlow - though a large part of the point of getting the helical/fine focuser was to focus at higher magnifications.  I could buy a higher mag EP, and not use the Barlow, but that's not as flexible as having the Barlow / medium mag EP combination.  Mak didn't modift the scope - the helical focuser attaches directly to the T-thread that's on the focusing system and then accepts 1.25" EPs.  It just adds about 1" to the length of the focusing tube - and that's enough to prevent focusing for the Barlow/EPs I have.

What I was trying to work out is the how to calculate where an eyepiece would need to be in the focuser to achieve focus.  There must be some optical calculation that would explain this, but i haven't found anything - nor any specifications on EPs that would help.  I wondered if eye relief may be a factor, but in reverse - if you know what I mean.  Longer eye relief may mean the focuser has to be closer to the OTA than eyepieces with shorter eye relief - or vice versa.  I know some ranges of EPs are described as being parfocal - you don't have to refocus when you switch between magnifications in the same set, so there is some maths being done there in terms of the focal plane.
 

I guess there isn't some common knowledge telescope maths here that I'm missing.  I've just seen comments from other members about EP combinations that suggested they knew in advance if an EP and telescope combination was going to work - and not just in terms of maximum potential magnification, focal length and exit pupil stuff.

 

I'll keep researching the setup Mak used to see if there's anything special about the combination.

 

Thanks again for repsonding :)

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Look for an older, long Barlow.  They usually require a fair amount of out focus rather than in focus.  My favorite very long Barlow is the 1.25" Orion Fully Baffled 2x made in Japan in the 90s.  Side by side testing on the Trapezium showed it is slightly sharper than my Tele Vue 1.25" 2x and Meade 140 2x, both of which are phenomenally sharp and mid-length Barlows.  The Orion is about 6 inches long and focuses about 2 inches out, IIRC.  I just don't insert it all the way into the focuser rather than crank the focuser that far out.

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On 03/09/2020 at 19:47, Louis D said:

Look for an older, long Barlow.  They usually require a fair amount of out focus rather than in focus.  My favorite very long Barlow is the 1.25" Orion Fully Baffled 2x made in Japan in the 90s.  Side by side testing on the Trapezium showed it is slightly sharper than my Tele Vue 1.25" 2x and Meade 140 2x, both of which are phenomenally sharp and mid-length Barlows.  The Orion is about 6 inches long and focuses about 2 inches out, IIRC.  I just don't insert it all the way into the focuser rather than crank the focuser that far out.

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Thank you Louis.  I'll have a look for a long barlow and see if that does the trick.  They seem to be harder to find these days, as I guess the optical quality of short barlows has improved, and they're more convenient.  (I'm still intrigued as to how you can determine where a particular eyepiece / barlow combination would need to be in the focuser for it to focus)

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2 hours ago, LondonSi72 said:

Thank you Louis.  I'll have a look for a long barlow and see if that does the trick.  They seem to be harder to find these days, as I guess the optical quality of short barlows has improved, and they're more convenient.  (I'm still intrigued as to how you can determine where a particular eyepiece / barlow combination would need to be in the focuser for it to focus)

You can't even use long focus Barlows in a refractor's diagonal because they require more in-focus than most of them have to spare.  The difference is that a Newtonian focuser essentially has no bottom (at least until you hit the secondary) while a refractor's diagonal only has about room to accept the first 1.5" of a barlow before bottoming out on a hard stop or hitting the diagonal's mirror/prism in a 2" unit.  Thus, you can just keep inserting the Barlow in the Newtonian focuser until it comes to focus which is usually before you contact the shoulder.  This can be 2" to 3" or more.  This just isn't an option in a diagonal, thus the popularity of shorty Barlows which can be used in either.

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another option perhaps would be to raise the mirror using longer screws & springs if there's not enough travel left on the OEM ones. That'd move the focal point further up the focuser tube to allow you to reach focus. You would need to redo collimation but it's a relatively simple mod to do. Your eyepieces sans barlow would find focus a little further out from where they currently do, as in the amount you shifted the mirror. Trick would be to determine how much to adjust for to reach the barlow focal point. How much extra length does the helical add to the focuser compared to the SW OEM holder ring?

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  • 2 weeks later...
On 08/09/2020 at 09:06, DaveL59 said:

another option perhaps would be to raise the mirror using longer screws & springs if there's not enough travel left on the OEM ones. That'd move the focal point further up the focuser tube to allow you to reach focus. You would need to redo collimation but it's a relatively simple mod to do. Your eyepieces sans barlow would find focus a little further out from where they currently do, as in the amount you shifted the mirror. Trick would be to determine how much to adjust for to reach the barlow focal point. How much extra length does the helical add to the focuser compared to the SW OEM holder ring?

Thanks Dave.  I saw a post from a guy who moved his mirror like that on the same scope as me - but it was like open heart surgery!  The primary isn't attached to the tube in a very user-customisable way on some of the SW130s - including mine.  There are non-captive nuts inside the tube holding the side bolts in place that hold the mirror assembly.  Undo the bolts and the nuts fall free and would have to be super-glued in place to allow you to screw the bolts back in securely.  I think I like the long Barlow option better (I'm a whimp!)

I've started looking at the SW 200P as a nice upgrade to my current scope, so may just hold out for that before I change my EP and accessories set, as with the different focal length and aperture it will all be different.

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sounds sensible, mine's a black tube 130/900 SW130 and I've had the cell out with no issues, but if the nut does come away for sure it'll be a challenge to refix them, epoxy would likely be better than superglue but either way allow a lot of time to outgas before refitting the optical bits. I was thinking about lifting the mirror to use a modified CCTV camera with it but I've other scopes so no real need. I did lift the mirror in the TAL-1 so it could use regular 1.25-inch eyepieces which was simple to do, just used longer screws and springs and it's been very stable.

With the SW130 I've had no issue with eyepieces or barlows but I've not fitted a helical to it. I guess that's increased the length of the focuser tube which causes you to not have the in travel needed.

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18 minutes ago, DaveL59 said:

sounds sensible, mine's a black tube 130/900 SW130 and I've had the cell out with no issues, but if the nut does come away for sure it'll be a challenge to refix them, epoxy would likely be better than superglue but either way allow a lot of time to outgas before refitting the optical bits. I was thinking about lifting the mirror to use a modified CCTV camera with it but I've other scopes so no real need. I did lift the mirror in the TAL-1 so it could use regular 1.25-inch eyepieces which was simple to do, just used longer screws and springs and it's been very stable.

With the SW130 I've had no issue with eyepieces or barlows but I've not fitted a helical to it. I guess that's increased the length of the focuser tube which causes you to not have the in travel needed.

Dave - yes - in-focus is exactly the problem.  That's what I was trying to work out - if there's some way you could calculate where the focal point would be for a given telescope, barlow, eyepiece combination based on the information the manufacturer's provide with these items.  It seems not.
The helical focuser is probably about an inch in length, so quite compact.

If you have the SW130 then you may be able to answer another question I have.  I noticed that the EP holder in the SW130 screws into the focusing tube, and the holder is just about the length of the helical focuser.  I was wondering if I could take the EP holder off the tube and screw the helical focuser to this other thread by a short adapter.  Perhaps getting someone to 3d print one for me, if there isn't one commercially available.  Is the thread on the focus tube that the normal holder screws into a standard size for telescopes?  I looked on eBay for adapters and there are so many different combinations I couldn't figure out what I might need.

 

(And you're right - epoxy resin would be a much better choice than superglue!)

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I believe on the SW130 the inner silver drawtube has a T-thread (or T2?) but I can't confirm for certain, having neither manuals or a T-thread to try on it. That would likely make a big difference tho as you'd gain at least half an inch of in travel which may be enough. Is your helical threaded, in which case its worth a try?

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2 minutes ago, DaveL59 said:

I believe on the SW130 the inner silver drawtube has a T-thread (or T2?) but I can't confirm for certain, having neither manuals or a T-thread to try on it. That would likely make a big difference tho as you'd gain at least half an inch of in travel which may be enough. Is your helical threaded, in which case its worth a try?

The end of the EP holder has a T-thread - that's actually what the helical focuser screws on to - but the silver tube is different - much wider pitch, and slightly smaller diameter (I think).

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doing a search, turns out someone here already crossed the bridge of finding an adaptor:

Looks like what you need to fit direct and then hopefully have enough in travel.

Edited by DaveL59
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As you say the 130 is a spherical 900mm mirror I would really not expect great results from it at the high magnifications.

It comes out as F/6.9 and many would suggest that a 7mm was a good idea and maybe a maximum. Giving the scope the benefit of the doubt you could try a 6mm but I doubt that a 5mm would be worthwhile.

The barlow is another vague area, they are not all equal and some may work with a scope and others may not. Throw in that the barlow works on the scope focal length, actually image size, not on the eyepiece. So the barlow is affecting the output of what is a spherical mirror. As that likely is not starting out good the barlow is likely just going to highlight the abberations from the mirror. Previous person may have had a selection of barlows to try and naturally selected the one that worked best.

However for the nature of the mirror the use of a barlow and an 8mm eyepiece is I expect too much for the scope.

A finer focus adjustment is likely to make no difference. What you have not indicated is that you get momentarily to a good focus but cannot stop at that place. Meaning to me that there is no good or sharp focus. Exactly what I would expect.

I would say that a 6mm eyepiece is going to be in effect as good as it will get, that is 150x. Seriously doubt there is any more to squeeze out of the scope.

Concerning 200P use if the 1000mm focal length version f/5 then select a 6mm carefully. The William Optics ones were said to be good but not best in an f/5 scope, worked well in an f/6 and slower. Altair sell a 6mm however I suspect, from the appearance, it is a rebrand of the WO item so may suffer in an f/5. Would suggest the f/6 200P if possible. The slower scopes seem a little easier to use. Other then that a 6mm should, focal ratio considered, be a good option in a 200P. The question being "Which 6mm?"

Edited by PEMS
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2 hours ago, DaveL59 said:

doing a search, turns out someone here already crossed the bridge of finding an adaptor:

Looks like what you need to fit direct and then hopefully have enough in travel.

You absolute STAR (excuse the pun)!  I looked on this forum - and many others - to try and find anyone else who had this issue, and this post didn't show up (my search criteria, I guess).  That's exactly the same setup.  Found and ordered on eBay.  Bit of a wait (20 October), but I'll let you know if it works.

 

Thank you :)

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1 hour ago, PEMS said:

As you say the 130 is a spherical 900mm mirror I would really not expect great results from it at the high magnifications.

It comes out as F/6.9 and many would suggest that a 7mm was a good idea and maybe a maximum. Giving the scope the benefit of the doubt you could try a 6mm but I doubt that a 5mm would be worthwhile.

The barlow is another vague area, they are not all equal and some may work with a scope and others may not. Throw in that the barlow works on the scope focal length, actually image size, not on the eyepiece. So the barlow is affecting the output of what is a spherical mirror. As that likely is not starting out good the barlow is likely just going to highlight the abberations from the mirror. Previous person may have had a selection of barlows to try and naturally selected the one that worked best.

However for the nature of the mirror the use of a barlow and an 8mm eyepiece is I expect too much for the scope.

A finer focus adjustment is likely to make no difference. What you have not indicated is that you get momentarily to a good focus but cannot stop at that place. Meaning to me that there is no good or sharp focus. Exactly what I would expect.

I would say that a 6mm eyepiece is going to be in effect as good as it will get, that is 150x. Seriously doubt there is any more to squeeze out of the scope.

Concerning 200P use if the 1000mm focal length version f/5 then select a 6mm carefully. The William Optics ones were said to be good but not best in an f/5 scope, worked well in an f/6 and slower. Altair sell a 6mm however I suspect, from the appearance, it is a rebrand of the WO item so may suffer in an f/5. Would suggest the f/6 200P if possible. The slower scopes seem a little easier to use. Other then that a 6mm should, focal ratio considered, be a good option in a 200P. The question being "Which 6mm?"

Yes - I suspect I'm pushing my scope beyond its limit - which I'm sure every newbie has a tendency to do :).  Theoretical max magnification of the SW130 / 900mm scope is x260.  The 8mm with a 2x barlow is x225, which I thought would be OK as it's almost 15% less than the max magnification - if you follow me.  Maybe that was too optimistic.  (I have a Baader 2.25x Barlow as well - now that would be optimistic at x253 !)
The Helical focuser works really well without the barlow - a very smooth, fine adjustment, so if I can't get this setup to work with the Barlow as well, I'll just go with the helical at lower magnification.

 

One of the many challenges for a newbie working through the options in astronomy, is being able to truly understand what a view will be like in THEIR scope for various EPs and Barlows etc.  As I said - theoretically my scope could provide 260x, but if you're saying I shouldn't expect anything better than 150x, that's hard to workout until I see the result and determine if it's good enough for me.  Like most things, that comes with experience, I guess.

At least there's an active market in EPs so it's almost like loaning them, if you don't buy them from new :)

Thanks.

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  • 2 weeks later...
On 03/09/2020 at 13:47, Louis D said:

Look for an older, long Barlow.  They usually require a fair amount of out focus rather than in focus.  My favorite very long Barlow is the 1.25" Orion Fully Baffled 2x made in Japan in the 90s.  Side by side testing on the Trapezium showed it is slightly sharper than my Tele Vue 1.25" 2x and Meade 140 2x, both of which are phenomenally sharp and mid-length Barlows.  The Orion is about 6 inches long and focuses about 2 inches out, IIRC.  I just don't insert it all the way into the focuser rather than crank the focuser that far out.

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I was out last night with my Dob and binoviewer.  I was able to verify that using a Meade 140 Barlow nosepiece as the reference distance that a Celestron Ultima type shorty barlow required about another 1/2" of in-focus while the above Orion long barlow required about 1" of additional out-focus.  This was also true with just an eyepiece.  The shorty Barlow required in-focus while the long Barlow required about double that out-focus.

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  • 2 weeks later...
On 28/09/2020 at 02:59, Louis D said:

I was out last night with my Dob and binoviewer.  I was able to verify that using a Meade 140 Barlow nosepiece as the reference distance that a Celestron Ultima type shorty barlow required about another 1/2" of in-focus while the above Orion long barlow required about 1" of additional out-focus.  This was also true with just an eyepiece.  The shorty Barlow required in-focus while the long Barlow required about double that out-focus.

Thank you. That’s really useful to know for future. 
 

However... The converter / adapter I ordered from eBay - as Dave spotted from another post in SGL - arrived last week (early) and I had a clear night so managed to try it out. It works a treat! 
The eyepieces I have all worked with my short 2x Barlow (and even with the 2.25x - though it wasn’t an improvement in clarity)

 

This setup is really nice. I’ve attached a photo of the converter and helical focuser. The converter is the first narrow black ring in the picture, attached to the silver thread on the focusing tube, and the helical focuser attaches snug up to this. 
 

The helical focuser has three grub screws to tighten the compression ring and a fourth to lock focus in position. It’s a very fine adjustment and works well in conjunction with the normal rack and pinion focuser of the SW130. 
 

Thanks for everyone’s help on this. 
The icing on the cake would be if this works on a SW200 too...

BB2FC141-7CEC-4EFE-B135-F5782D2AC65A.jpeg

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