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Newbie looking for some purchase advise

Celestron refractor
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#1 thewellofascension

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Posted 15 September 2020 - 11:00 PM

Hello friends,

 

N00b here looking for a bit of advise. I had been saving up for a 8' Dob for X'mas as my first telescope, but as luck would have it, a mate from work was giving away his Celestron Astromaster 70AZ for free so I jumped at the chance and have been enjoying the night skies with it !!

 

The telescope is pretty old (my friend also got it second hand) so it has a few issues.

 

a) The finder scope also seems a bit useless as the battery cover is not there. I am not sure if it the cover can be replaced? Should I get a new finder scope? It doesn't look like the current one can be removed from the scope though? As I cannot see the red dot I am currently just moving the telescope roughly into position and moving it around to find my target. It's been fun smile.gif

 

b) I am not sure if the eye pieces I have are the best. I believe they are the ones that came with the scope. A 10mm and a 20mm. But they've served me well so far with good views of the Moon, Jupiter, Saturn, Mars etc.

 

I am looking at making a few purchases to enhance my views and am looking for some advice on what those items might be (budget friendly too hopefully !)

 

Seems like the best options are

 

1) A barlow lens. Not too sure what the best Barlow would be for me. The website says the highest useful magnification is 165x for this scope. But reading through the posts here it seems to be more like 50x-60x per inch of aperture so that puts it between 138-165. 

 

2) New eye pieces (am I better off just getting higher powered eye pieces over a barlow)?

 

Both 1, 2 appear to be future proof purchases as I could use them with the Dob too eventually ?..

 

3) Moon and Solar filters

 

4) Maybe the diagonal as it's a plastic one and reading through the forums, suggests a better one will improve things (then again am I better off saving that money for the Dob?)

 

5) New finder scope?

 

Anything else?

 

Thanks everyone!

 

Details and photos of the telescope below.

 

Celestron Astromaster 70AZ

Aperture 70mm (2.76")
Focal Length 900mm (35")
Focal Ratio f/13
Focal Length of Eyepiece 1 20mm (.8")
Focal Length of Eyepiece 2 10mm (.4")

 

 

IMG-6513.jpg

 

IMG-6606.jpg


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#2 Taosmath

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Posted 15 September 2020 - 11:20 PM

My overarching comment is don't put any money into the refractor if it takes that money away from the Dob.

 

For the finder, get an eraser and cut a chunk off it (circle/Square/octogon) that will fit in the battery cavity.  Then buy a battery, put it in the cavity and put the eraser on top.  Hold the eraser and battery in place with duct tape or maybe a big binder clip if you have one large enough.

 

If you have the money, I'd buy good eyepieces that you can transfer to the Dob when you get it.

 

One eyepiece to consider is something like a 6.7mm Explore Scientific.  That would be close to the highest power you would need for your Dob and would also be useful in your refractor.

Another eyepiece I like in my Dob is an 8mm-24mm Zoom.  They are in big demand now, and most stores have them on back order. The Celestron/Meade/Agena models are all rebadged versions of the same unit and are good value for money in my opinion. If you look for used ones you can get one for $60 or so.

 

The only other EP I would get for the Dob would be a 2" wide field unit such as an Agena 32mm 70 degree SWA or any of the 82 degree 30-ish mm EP's that you can find at a good price. (Meade, Celestron, Explore Scientific, Televue if you want to go premium).  However they won't fit in your refractor, so I wouldn't buy that until you have your dob.

 

Those three EP's are all I need for my Dob, so i wouldn't buy any others.

 

I would not worry about moon or Solar filters unless you really think you'll be doing lots of solar observing (in which case neither the dob nor the refractor would be a great choice).  You can make a moon filter with a sheet of cardboard and a hole cut in it.

 

In the meantime have fun with the refractor !


Edited by Taosmath, 15 September 2020 - 11:21 PM.

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#3 mrlovt

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Posted 15 September 2020 - 11:35 PM

Congratulations on the new scope!  As you have already experienced, there's a lot of fun you can have - and a lot you can see - with inexpensive gear. 

 

It looks like the finder on that scope is usable - just not illuminated with the battery out.  If that's the case, I would use it as it is.  If you can get it to illuminate, great!  But if not, it doesn't appear to be made to easily swap out finders.

 

Those eyepieces are just fine. They might not be the fanciest, but they'll certainly do the job.  

 

I'd say spring for an inexpensive 2x barlow, as that's a great way to get more usefulness out of your eyepiece collection.  But since the eyepiece combination you have is 20mm and 10mm, and the 2x barlow will give you effectively another 10mm and 5mm, you'll only get one extra focal length as the 10mm is redundant.  Maybe find an odd power barlow, or just pick up a good eyepiece that will transfer with you over to the new scope when you get it.  

Skip the filters and the diagonal for now, as this isn't your long term scope.  A polarizing filter to tame a full moon is nice, though.  The only kind of sun filter you should ever buy will fit over the objective, not the eyepiece.

 

I say for the most part, don't spend any money on this scope that won't transfer directly to the new one you plan to get.  Enjoy what you have and learn with it!

 

 


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#4 Jethro7

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Posted 16 September 2020 - 12:22 AM

Hello

Welcome to the Addiction. And wellcome to CloudyNights Astronomy forum. I am going to third the suggestion dont spend your savings on this scope. ( FORGET THE DIAGONAL)  you wont need it for the Dob.That 70AZ scope cannot hold a candle to  what the Dob can do. If you have enjoyed 70AZ scope you are in for a mind melt when you get your Dob.. If you do spend anything please do it for items you will need for the Dob in mind. With that said I would tell you to get that Dob first, before you start buying any accessories for it. Then If I were to advise on a eyepiece I would say go for a Celestron 8 X 24 Zoom and 2X Barlow and a variable Polarizing filter for viewing the bright Moon.  The rest of the accessories  can be figured out as you go. That Dob should be your priority.

 

 

HAPPY SKIES TO YOU AND KEEP LOOKING UP Jethro


Edited by Jethro7, 16 September 2020 - 12:23 AM.

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#5 havasman

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Posted 16 September 2020 - 12:27 AM

My overarching comment is don't put any money into the refractor if it takes that money away from the Dob.

 

In the meantime have fun with the refractor !

That. And what Jethro said.


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#6 sg6

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Posted 16 September 2020 - 01:52 AM

If the eyepieces are 1.25" then it might be worthwhile getting say 2 budget plossl's. Thinking $25 each cost area. One obvious would be a 30/32mm one simply to find objects easier and that can transfer to the 8" later.

 

Not sure of the other as something around 8mm would be useful but I suggest not an 8mm plossl, simply too tight on eye relief. And I wouldn't spend more at this time, it delays the dobsonian purchase if you bought say a Paradigm at $60.

 

Don't think a barlow helps, as I think a 30/32 would be useful and barlowing the 20 or 10 is pointless. The 20 acts as a 10 which you have and the 10 would act as a 5 which would be too much. And a barlow will degrade the final view at least a little.

 

Maybe search out Vixen NPL plossl's they are good, just unsure of cost.

 

Your real maximum will be around 100x.

 

Filters, cannot think a moon filter is actually much use and a solar filter I suggest you leave until after the dobsonian. Then having said that a solar filter is actually useful. You can go outside in the day and look at the sun, not 100% sure how much use you would make of it however and again the money "should" be directed at the dobsonian. The catch to all this is that you have the 70mm and you don't (yet) have the dobsonian. The other negative of a solar filter is that you have to find and track the sun manually.

 

A solar filter is the sort of thing that they are nice to have and allows daytime observing. 

 

I think the scope at present has a correct image diagonal, they do not help the quality of the final image if a "proper" 45o diagonal could be fitted I suspect that would improve the final viewing. "Old" 1.25" diagonal seem to be often spare - I have 3 lying around. I keep replacing them with better 2" ones and the 1.25's get bagged and stored.

 

Finder - if you can fit a CR2032 can the finder be tried and if the RDF lights up then work out how to make some form of cover and use it.

 

I think the key to the 70 is do small inexpensive changes, but then keep the 70. Bet that 6 months after you get the 8" dobsonian you find you will use the 70 as much. Just quick and easy to take outside, also no real learning curve for people not familiar with the functioning and operation of a dobsonian.

 

You have a bit of a balancing act between the 70 and the dobsonian. The advantage the 70 has is that you have it at this time. You could take it out put in the 10mm and look at Jupiter tonight, you cannot do that with the dobsonian. Just if you divert any funds make sure there is a good and valid reason.

 

Add a location, it seems irrelevant to many but a meaningful one really helps. Plain useful to know where you are.


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#7 Redbetter

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Posted 16 September 2020 - 02:19 AM

Welcome to Cloudy Nights!

 

About two years ago I picked up the LT80 version of the same scope at a garage sale for $10 without eyepieces, messed up diagonal that probably didn't go to it, and a focuser that was having issues.  Anyway, when I tested the 80mm it turned out to have very good optics, so I replaced the focuser and attached a new dovetail so that I could better balance it and use 2" eyepieces.  The tripod is going to be pretty inadequate for 900mm focal length (and mine had some damage as well...so it and the OTA have not really been paired together.)

 

If the optics are good in that scope and the diagonal is decent, the 10mm should work well for planets.  The plastic rack and pinion focuser with integrated RDF is the weak point of the optical tube.  Hopefully, it will prove functional.  I wouldn't advise trying to replace it as it won't make economic sense.

 

The battery for the integrated RDF is a CR1620 for the focuser I have which looks like yours.  It is an odd little unit; I had it working, but wasn't really fond of it...hard to say how much of that was due to problems with the focuser itself and how much of that was being unfamiliar with the particular finder.    A simple Celestron Starpointer RDF (or generic equivalent) is inexpensive and has various mounting feet.  These RDF's are handy to have around, so it is something you will probably find a use for later.  One mounting foot could be screwed directly to the plastic focuser body on the opposite side--mark for proper hole spacing of the foot, then drill two evenly spaced holes of a size that the screws can self-tap into.

 

That appears to be an RACI diagonal, not a good match for high magnification/planetary viewing.  A decent 1.25" mirror diagonal should serve the scope well (and an be used on other refractors later) if it is reasonably well aligned/collimated.  It is the housing/nosepiece/barrel holder of the cheaper 1.25" diagonals that is usually the problem in my experience.

 

The mount is going to be a real impediment to higher power observing.  It isn't a good match for a long optical tube like this.  Satisfactory mounts for this long of a tube are not cheap, so I am not going to suggest you go the route of buying another mounts.

 

If the optics are good, this scope could have some potential for planets because of the relatively long focal ratio.  This results in CA # of nearly 4.7, which is close to the Conrady criteria of 5 where chromatic aberration should be minimal for an achro.  I would expect it to perform well with an 8mm eyepiece on planets, and possibly even a 7mm.  (I use a 7 for planets with the LT80 which has the same focal length.)  An eyepiece you buy for the refractor will work in the Dob.

 

You might want to consider a maximum-true-field eyepiece for the scope, it will make star hopping somewhat less difficult.  This would be a 1.25" eyepiece with a field stop of about 27mm.  Typically this would be a 32mm Plossl or something similar for this scope.  That would provide ~1.7 degrees of field, while that 20mm probably only provides about 1.1 degree of sky, or less.

 

I wouldn't try adding a higher power eyepiece without getting a better diagonal.


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#8 JohnnyBGood

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Posted 16 September 2020 - 07:36 AM

In my opinion, you've got a perfectly nice scope that probably deserves a lot more credit than it gets. It's perfectly capable of seeing nearly every object in Turn Left at Orion or Nightwatch, especially if you have access to darker skies now and then. I think 70mm is a perfect size for learning the basics of using a telescope and finding things to see. The only risk to buying one is that if you really get into the hobby you may outgrow it and want a larger scope within a few months. In your case, that's not an issue because you got it for free and already plan to get a larger scope.

 

The weakest link in this case is the mount, which is very hard to use at higher powers. While your scope is (in my opinion, based on an optically identical Meade Polaris 70 I tried out) perfectly capable of 140x magnifications on the moon (w/ 6.4mm Ploessl) it's extremely hard to make the small movements necessary to track an object as it moves across the sky with that mount. My son uses the same mount on his short tube 70mm scope and while it's fine at 17x it's very hard to use at 100x. It's really hard to balance the scope as you move the vertical angle above 45 degrees or so unless you get really creative with springs or makeshift counterweight rods. The tripod itself is perfectly good, though. It it were my scope, I'd probably look around on Goodwill, eBay, or the classifieds here for an EQ-2 mount head (even if the used scope is broken junk the mount is probably still fine). Most of the modern ones will accept a dovetail connection like your scope has. You can probably get one for around $35 or so. An EQ-1 could be made to work but an EQ-2 would feel more solid and less wobbly.

 

I personally don't like finders like the telescope came with and replace all mine with traditional optical finder scopes, usually 6x30 ($25-$50) though sometimes you can find an okay 5x24 (you want at least four adjustment screws, not three) for much cheaper ($10-$15) though they are noticeably less effective at picking out dimmer stars. I find an optical finder to be more useful in my light pollution because it enables me to see fainter stars that I couldn't see naked eye, which in turn is useful for following along with a basic star chart or the directions in Turn Left At Orion. To do so you'd need to drill a couple mounting holes in the telescope tube. It's not that hard to do, but may not be worth the cost/trouble.

 

I wouldn't bother with a Barlow since it wouldn't give you anything but too much power without buying additional eyepieces. I think the ones you've got are Ploessl designs, which are plenty sufficient. If it were my scope, I'd pick up a generic 32mm Ploessl ($25-$30) for better low power viewing and to make it easier to find things and maybe a 6.4-6.5mm ($10-$15 depending on brand) if you aren't bothered by tracking objects with the 10mm. That gives you four choices along the full range of magnifications you can get from the scope and they should still be useful with the next scope.

 

Even after you upgrade to a bigger scope it's handy having a smaller, more portable scope for nights you don't feel like dragging out the big, heavy scope. I spend as much time using my 50-90mm scopes as I do my 130mm and 203mm scopes and have just as much fun with them.


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#9 rhetfield

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Posted 16 September 2020 - 10:06 AM

My suggestion would be to just get a 2x barlow.  Preferably one that the lens element can come off and be used on the end o an eyepiece to act as a 1.5x barlow.  This will give you a very nice range of magnifications with that scope.

 

This fry's kit is not too bad.  It has was appears to be a celestron omni barlow that works decently.  The filters will not be too useful, but they come in nice holders.  The little box it all comes in can be turned into an eyepiece holder box.  With shipping, it cost me $20.

 

https://www.frys.com...FromSearch=true

 

Red dot finders are pretty cheap.  If you can't get the built in one to work, look at using double sided tape to attach something like this:

 

https://www.astronom...dot-finder.html

 

After that, save up for the dob.  I would wait until you have the dob to think about additional eyepieces.  


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#10 thewellofascension

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Posted 17 September 2020 - 04:22 AM

Thank you everyone for the responses and suggestions. 

 

Add a location, it seems irrelevant to many but a meaningful one really helps. Plain useful to know where you are.

I am based in Sydney Australia, but about 40km's out of the city in the burbs. If anyone here is based in Aus, I would love some suggestions on what online stores to try buying some of these accessories and where I might find the best prices.

 

Yes, I am quite weary of spending too much on the current scope that might not transfer over to the Dob eventually. My plan was to spend roughly a max of about AUD 100 and see what I can get for that. Looks like the best suggestions are to get a 30-32mm / 6-6.5mm Plossl and/or possibly a 2x Barlow that could be used as a 1.5x Barlow.

 

I will also try the suggested tips on getting the RDF to work. Thanks!

 

I do plan to keep the 70mm as a grab and go in the long run too, which will make these investments worth while I think.

 

For the generic Plossl's would these be ok?

https://www.bintel.c...?v=322b26af01d5

 

For the 2x Barlow's that can also be used as a 1.5x is this the type that will work?

https://www.bintel.c...?v=322b26af01d5

 

Thanks in advance again friends!



#11 thewellofascension

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Posted 17 September 2020 - 04:26 AM

This results in CA # of nearly 4.7, which is close to the Conrady criteria of 5 where chromatic aberration should be minimal for an achro.

 

Sorry  but this was a little over my head. I know CA means chromatic aberration but I don't fully understand what that is yet and the references to 4.7 etc..Can you dumb it down for me smile.gif



#12 Redbetter

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Posted 17 September 2020 - 05:56 AM

Sorry  but this was a little over my head. I know CA means chromatic aberration but I don't fully understand what that is yet and the references to 4.7 etc..Can you dumb it down for me smile.gif

"CA Ratio" is the term that was escaping me when I posted. For basic achromatic doublet refractors there are some very old standards for evaluating how much color blur they will have.  The CA Ratio is a standard way of evaluating it.  Very low numbers mean lots of chromatic aberration and badly harm planetary contrast even if the figure is perfect.  High numbers mean very little chromatic aberration, so if the figure is good the image will be sharper at high power, particularly on bright stars or bright extended objects (planets and the Moon.). 

 

The formula is simple:  CA Ratio = focal ratio / aperture in inches

 

So a 70mm scope with a 900mm focal length has a focal ratio of 12.9, while the aperture is 70/25.4 = 2.76 inches.  CA Ratio = 12.9/2.76 = 4.67. 

 

CA Ratio is a continuous scale and there are some points along it that are used to bin expected performance. 

  • Below about 1.2 there is unacceptable damage to the image that is noticeable across the range of magnification. 
  • Above 1.2 up to about 3 there is still quite a bit of color blur that will limit high power effectiveness, but yellow filters can be used to get rid of some of the more egregious violet/blue halo.   The common 80 f/5 achro is on the lower end of this range and it does well at low and mid power.  
  • The Sidgwick standard is 3.  3 and above is where the color blur is becoming less damaging, even though it is still readily visible on bright objects.  My LT80 works out to ~3.6.  I can still readily see the violet around bright white stars and Jupiter, but the planetary contrast is still sufficient for about 40x/inch to my eye before the scope has "topped out" in terms of detail.  
  • By 5 and above, a scope will show very little color aberration and can essentially operate to the aperture's potential.  Color can still be found at high enough power and/or on bright enough objects.


#13 thewellofascension

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Posted 18 September 2020 - 02:11 AM

 

"CA Ratio" is the term that was escaping me when I posted. For basic achromatic doublet refractors there are some very old standards for evaluating how much color blur they will have.  The CA Ratio is a standard way of evaluating it.  Very low numbers mean lots of chromatic aberration and badly harm planetary contrast even if the figure is perfect.  High numbers mean very little chromatic aberration, so if the figure is good the image will be sharper at high power, particularly on bright stars or bright extended objects (planets and the Moon.). 

 

The formula is simple:  CA Ratio = focal ratio / aperture in inches

 

So a 70mm scope with a 900mm focal length has a focal ratio of 12.9, while the aperture is 70/25.4 = 2.76 inches.  CA Ratio = 12.9/2.76 = 4.67. 

 

CA Ratio is a continuous scale and there are some points along it that are used to bin expected performance. 

  • Below about 1.2 there is unacceptable damage to the image that is noticeable across the range of magnification. 
  • Above 1.2 up to about 3 there is still quite a bit of color blur that will limit high power effectiveness, but yellow filters can be used to get rid of some of the more egregious violet/blue halo.   The common 80 f/5 achro is on the lower end of this range and it does well at low and mid power.  
  • The Sidgwick standard is 3.  3 and above is where the color blur is becoming less damaging, even though it is still readily visible on bright objects.  My LT80 works out to ~3.6.  I can still readily see the violet around bright white stars and Jupiter, but the planetary contrast is still sufficient for about 40x/inch to my eye before the scope has "topped out" in terms of detail.  
  • By 5 and above, a scope will show very little color aberration and can essentially operate to the aperture's potential.  Color can still be found at high enough power and/or on bright enough objects.

 

Thanks for that detailed explanation. Makes things much clearer.

 

Based on this calculation it would seem 4.67 will be pretty decent at higher magnifications? 



#14 thewellofascension

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Posted 18 September 2020 - 02:13 AM

Thank you everyone for the responses and suggestions. 

 

I am based in Sydney Australia, but about 40km's out of the city in the burbs. If anyone here is based in Aus, I would love some suggestions on what online stores to try buying some of these accessories and where I might find the best prices.

 

Yes, I am quite weary of spending too much on the current scope that might not transfer over to the Dob eventually. My plan was to spend roughly a max of about AUD 100 and see what I can get for that. Looks like the best suggestions are to get a 30-32mm / 6-6.5mm Plossl and/or possibly a 2x Barlow that could be used as a 1.5x Barlow.

 

I will also try the suggested tips on getting the RDF to work. Thanks!

 

I do plan to keep the 70mm as a grab and go in the long run too, which will make these investments worth while I think.

 

For the generic Plossl's would these be ok?

https://www.bintel.c...?v=322b26af01d5

 

For the 2x Barlow's that can also be used as a 1.5x is this the type that will work?

https://www.bintel.c...?v=322b26af01d5

 

Thanks in advance again friends!

Hopefully someone can give me a bit of feedback on these when they get the chance.


Edited by thewellofascension, 18 September 2020 - 02:14 AM.


#15 JohnnyBGood

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Posted 18 September 2020 - 09:05 AM

Looks like the eyepieces are made by GSO so they should be good quality. I've never had an optical quality complaints with even the cheapest Ploessl eyepieces I've bought. The only ones I wasn't totally happy with are some "Series 500" ones I bought. They were sharp as tacks but they had a slightly narrower field of view than other Ploessls. To be honest, I personally can't tell any meaningful difference between the no-name-brand eBay Ploessls and the couple Televue Ploessls one of my used scopes came with. I'm sure the Televues are better in every way but it's not noticeable to me, at least in my skies. Other people may have more discerning eyes.

 

The Barlow is probably fine optically. However, it has a pretty long "nose" on it that may not allow itself to be fully seated in a stock Celestron (or Meade) Amici prism diagonal. The Barlows many entry level Meade and Celestron scopes come with have that problem, which annoys me a little. It still works fine optically, but having hard objects come in contact with the surface of prism glass (or mirrors for that matter) makes me nervous. Granted, you're planning on using it screwed into the bottom of your eyepieces but you might want to check to see how much clearance you've got between the end of the eyepiece barrel and your diagonal. This isn't an issue for reflector scopes, since there's usually nothing for them to run into in the focuser drawtube.



#16 thewellofascension

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Posted 22 September 2020 - 05:13 AM

Looks like the eyepieces are made by GSO so they should be good quality. I've never had an optical quality complaints with even the cheapest Ploessl eyepieces I've bought. The only ones I wasn't totally happy with are some "Series 500" ones I bought. They were sharp as tacks but they had a slightly narrower field of view than other Ploessls. To be honest, I personally can't tell any meaningful difference between the no-name-brand eBay Ploessls and the couple Televue Ploessls one of my used scopes came with. I'm sure the Televues are better in every way but it's not noticeable to me, at least in my skies. Other people may have more discerning eyes.

 

The Barlow is probably fine optically. However, it has a pretty long "nose" on it that may not allow itself to be fully seated in a stock Celestron (or Meade) Amici prism diagonal. The Barlows many entry level Meade and Celestron scopes come with have that problem, which annoys me a little. It still works fine optically, but having hard objects come in contact with the surface of prism glass (or mirrors for that matter) makes me nervous. Granted, you're planning on using it screwed into the bottom of your eyepieces but you might want to check to see how much clearance you've got between the end of the eyepiece barrel and your diagonal. This isn't an issue for reflector scopes, since there's usually nothing for them to run into in the focuser drawtube.

Thanks for this info. I will go buy these and report back!




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