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Orion 120ST Testing

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#26 Jeff B

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Posted 29 September 2020 - 06:17 PM

Thanks.  So......

 

More DPAC!   And the crowd explodes!  Well, not really.

 

To investigate that center zone a little more, I made two quick and dirty (mostly dirty) aperture stops of 60mm and 80mm and had another look.  At 60mm, the lines in green were just dead straight, like at 52mm.  However, at 80 mm, I could just start to see some local effects of the zone at the edge, nothing radical, but there it is, as you can see in the attached color montages and high contrast green null shot.  I've also attached inside and outside of focus shots with a single green LED as I've found individual discrete color LEDs give a more high contrast image in their colors, especially since I can really skinny down the bandwidth of the sample color with my software.

 

At 80mm, the figure is still excellent.  And as you can see, at the reduced aperture, the red to blue spherochromatism is greatly reduced and the B,G, and R color focus points are much closer to each other as well, especially in blue.

 

Paul's estimate of  1/4 wave peak to valley for the zone looks to be pretty good and 80mm is the largest stop I'm going to use.  Anything larger than that allows that zone edge to dominate, degrading the overall wave front.

 

DPAC is very useful. grin.gif

 

Jeff

Attached Thumbnails

  • 120ST, 80mm, W, B, G, R, Inside.jpg
  • 120ST, 80mm, W, B, G, R, Outside.jpg
  • 120ST, 80mm,  Green, DPAC, Null.jpg
  • 120ST, 80mm, Green LED, Inside.jpg
  • 120ST, 80mm, Green LED, Outside.jpg

Edited by Jeff B, 29 September 2020 - 06:20 PM.

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#27 RLK1

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Posted 29 September 2020 - 07:01 PM

As to Orion, what happened when you  CALLED them?  You did try to call them, right?

I don' want to take away from the OP's excellent work but I followed Orion's procedure in contacting their tech department via email and getting an instant automated response with a ticket number telling me a tech would get back to me ASAP. I don't want to call them; I want a printable response via email, as per their procedure...


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#28 peleuba

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Posted 30 September 2020 - 08:32 AM

 

DPAC is very useful. grin.gif

 

 

Agree.

 

At 80mm its nearly an APO!  (well not really).

 

But the lens is actually not bad, at all.  Growing up, I would have been thrilled with a telescope like this.


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#29 peleuba

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Posted 30 September 2020 - 09:11 AM

First, I already have a pretty good refractor, an Antares 6" F6.5 with the objective spaced, centered and collimated by a refractor expert, John Pons. It throws up a very good image but it taxes my CG5 go-to mount and the diagonal can hit a tripod leg in certain orientations.

 

Wow,  Pons is now 79.  I would definitely hold on that Antares, especially if the views are good.  Is this you:

 

https://astromart.co...actorlong-37469

 

Dunno about Orion...   Though do seem to get squirrely when answering pre sales/post sales technical support questions about the equipment they sell asked by folks who've not actually purchased the equipment originally from them.  This is a somewhat recent phenomenon.  Back in the day, and I am sure you recall this, they were always willing to answer tech questions. 

 

I do know that Orion will not support any equipment that is not owned by the original purchaser.   



#30 Mitrovarr

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Posted 30 September 2020 - 11:06 AM

Have you tried the unit under the stars with the 2.5" focuser? I am very curious if any change in deep sky performance was obvious at the eyepiece.

#31 Jeff B

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Posted 30 September 2020 - 12:40 PM

Have you tried the unit under the stars with the 2.5" focuser? I am very curious if any change in deep sky performance was obvious at the eyepiece.

No, in fact, the scope has not been outside yet.  I suspect the differences would be a bit subtle though.  Just looking through the system with the stock focuser and the 2.5" ML and panning across the top of the 2" diagonal, I'd say, at the edge of the diagonal opening there is about a 25% clip of the aperture and maybe 10 % with the ML focuser.   With the stock focuser, the baffles on the inside of the draw tube were the offenders, while with the ML, the offenders were the baffles inside the main tube.

 

Those are just guesses. 

 

Jeff 


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#32 RLK1

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Posted 30 September 2020 - 01:17 PM

No, in fact, the scope has not been outside yet.  I suspect the differences would be a bit subtle though.  Just looking through the system with the stock focuser and the 2.5" ML and panning across the top of the 2" diagonal, I'd say, at the edge of the diagonal opening there is about a 25% clip of the aperture and maybe 10 % with the ML focuser.   With the stock focuser, the baffles on the inside of the draw tube were the offenders, while with the ML, the offenders were the baffles inside the main tube.

 

Those are just guesses. 

 

Jeff 

Perhaps you can frame that under practical terms for us, meaning Orion states that with a 25 plossl eyepiece, "this telescope serves up a sprawling 2.1° swath of sky!" So, is the baffling vignetting/reducing the field illumination and visibly compromising the operating parameter of the scope? 



#33 Jeff B

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Posted 30 September 2020 - 01:34 PM

So I set up for the indoor star test.  I used my TEC 160ED as a beam collimator for my artificial star.  The TEC has exceptional correction in green.  I also used my "leaning tower of barlows", which were a couple of high quality Celestron Silver Top 2x barlows stacked with a high quality Televue 2.5X barlow.  I chose these barlows as they are "parafocal" with eyepieces.  The barlows are used to create a point source for the star.  The artificial star is then inserted into the whole stack.  Both scopes are lined up.   Kinda scary looking in the pictures.

 

A trick I've used is to leave the test scope's focus position found in DPAC alone and swap out the screen holder for a 1.25" adapter.  The screen holder and adapter are of the same thickness.  The plossls I use position their field stops right at the top of the 1.25" adapter opening so I figure I'm at the infinity focal point for the scope.  Once the two scopes are aligned (rather easy with my laser) and the "tower" is installed in the TEC, I focus the star in the test scope by using the focuser in the TEC 160ED.   Seems to work fine for what I'm doing the test for which is basically evaluating astigmatism and coma.  Spherical evaluation may be a bit iffy with all of that extra glass, but so far it has correlated rather well with what I see in DPAC, at least in green light.

 

My only issue is my lack of experience in imaging the results.  So far, I've used my Cannon 6D with a barlow but images are rather grainy due to the 3200 ISO setting I need to even see the star in live view.  Sometime in the future, I'll excavated my two planetary cameras and have a go at it with them.

 

So, for now, just word.

 

The short story is that at ~150X at full aperture in green light, I saw no meaningful coma or astigmatism.  Spherical correction was actually rather good with the outer first two diffraction rings close to focus being reasonably uniform in thickness and brightness and similar to each other on each side of focus.  However, that central zone was having an obvious effect on the inner rings.  At focus, the airy disk was fairly round and surrounded by a couple of fainter rings but the view was a little "hairy" .  But it certainly didn't suck., actually, I was surprised about how nice it was.  Without the green filters, well......why bother as I will never use it anywhere near this magnification at full aperture.

 

Stopped to 80mm this became a completely different and better scope.  Still no coma or astigmatism but the diffraction patterns in green were notably more "solid" and distinct at each side of, and at, focus than at full aperture and spherical correction seemed entirely neutral and consistent with the DPAC images in green.  The central zone was still visible but much better integrated into the image, especially the inner rings on each side of focus.  The airy disk at focus was round, sharp, and surrounded 360 by one faint ring.  Very nice.  Sans filters, the extra unfocused color was obvious but the diffraction patterns were still sharp and easy to see.

 

So now it's time to get this scope out under some real stars.

 

Jeff 

 

 

Attached Thumbnails

  • Star Test A.jpg
  • Star Test B.jpg
  • Star Test C.jpg

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#34 Mitrovarr

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Posted 30 September 2020 - 02:00 PM

Perhaps you can frame that under practical terms for us, meaning Orion states that with a 25 plossl eyepiece, "this telescope serves up a sprawling 2.1° swath of sky!" So, is the baffling vignetting/reducing the field illumination and visibly compromising the operating parameter of the scope?


An effect like this would be subtle and hard to see, you'd be losing some aperture in the center and more at the edge. It certainly does not cut off, not even with a 30mm ES82.

#35 Jeff B

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Posted 30 September 2020 - 02:01 PM

Perhaps you can frame that under practical terms for us, meaning Orion states that with a 25 plossl eyepiece, "this telescope serves up a sprawling 2.1° swath of sky!" So, is the baffling vignetting/reducing the field illumination and visibly compromising the operating parameter of the scope? 

Orion states that with a 25 plossl eyepiece, "this telescope serves up a sprawling 2.1° swath of sky!"  This is a true statement regardless of aperture or focal ratio for a 600mm focal length scope.

 

So, is the baffling vignetting/reducing the field illumination.   Relative to the center, yes, field illumination will drop steadily as you move off of the center axis towards the field outer edges.  

 

and visibly compromising the operating parameter of the scope?   Good question as to the visibility of the light drop off towards the FOV's outer edge as it is gradual and uniform.  "Compromised" means setting standards that are lower than acceptable.  I suspect, based upon my own observations with other refractors and Maks with similarly very small fully illuminated FOV's that the light drop off would go largely unnoticed.   Also, as this scope has been available for decades and thousands sold, if it were an overtly obvious issue, there would be tons of bashing and carping about it on the net and here on CN's.  I have not noticed that, even with imagers.  So is it "compromised" ?  I guess that depends on if it bugs you or not.  

 

Jeff


Edited by Jeff B, 30 September 2020 - 02:03 PM.


#36 Mitrovarr

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Posted 30 September 2020 - 02:07 PM

I mean, I know the ST120 puts up satisfying wide field images, from using my own. But if obtaining better field illumination via a 2.5" focuser or moving baffles provides a noticeable improvement at the eyepiece, it is notable as a possible upgrade.

#37 RLK1

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Posted 30 September 2020 - 02:14 PM

Orion states that with a 25 plossl eyepiece, "this telescope serves up a sprawling 2.1° swath of sky!"  This is a true statement regardless of aperture or focal ratio for a 600mm focal length scope.

 

So, is the baffling vignetting/reducing the field illumination.   Relative to the center, yes, field illumination will drop steadily as you move off of the center axis towards the field outer edges.  

 

and visibly compromising the operating parameter of the scope?   Good question as to the visibility of the light drop off towards the FOV's outer edge as it is gradual and uniform.  "Compromised" means setting standards that are lower than acceptable.  I suspect, based upon my own observations with other refractors and Maks with similarly very small fully illuminated FOV's that the light drop off would go largely unnoticed.   Also, as this scope has been available for decades and thousands sold, if it were an overtly obvious issue, there would be tons of bashing and carping about it on the net and here on CN's.  I have not noticed that, even with imagers.  So is it "compromised" ?  I guess that depends on if it bugs you or not.  

 

Jeff

Ok, thanks, and that's along the lines of what I was thinking. Do you agree with Wolfgang Rohr that the maximum magnification would be around 125X for the scope or do you think you can push it further without breakdown in the image with your particular sample?



#38 peleuba

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Posted 30 September 2020 - 02:36 PM

Ok, thanks, and that's along the lines of what I was thinking. Do you agree with Wolfgang Rohr that the maximum magnification would be around 125X for the scope or do you think you can push it further without breakdown in the image with your particular sample?

 

Pardon me for jumping in, but I think max magnification would at some point depend on the object.  If its a double star where you are looking for the split and unconcerned about optical "purity", you could push it, perhaps beyond its design threshold.  Of course, deep sky objects like M31, M33 and other largish objects would look terrific at 125X or below.

 

While I don't look at W. Rohr's website routinely as I sometimes don't agree with his conclusions, 125X may be where the color error becomes obtrusive to him  on bright point source objects like Vega or Jupiter/Saturn.  And even without seeing this sample, I would agree with this conclusion.   

 

Color error becoming obtrusive in an achromat has nothing to do with optical quality and everything to do with aperture and focal ratio.


Edited by peleuba, 30 September 2020 - 02:38 PM.


#39 LDW47

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Posted 30 September 2020 - 03:01 PM

I mean, I know the ST120 puts up satisfying wide field images, from using my own. But if obtaining better field illumination via a 2.5" focuser or moving baffles provides a noticeable improvement at the eyepiece, it is notable as a possible upgrade.

What is the cost of a good 2.5” focuser vs the overall cost of the scope, maybe a quick cost / benefit analyses, anyone of you experts ?



#40 peleuba

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Posted 30 September 2020 - 03:07 PM

So I set up for the indoor star test.  I used my TEC 160ED as a beam collimator for my artificial star.  The TEC has exceptional correction in green.  I also used my "leaning tower of barlows", which were a couple of high quality Celestron Silver Top 2x barlows stacked with a high quality Televue 2.5X barlow.  I chose these barlows as they are "parafocal" with eyepieces.  The barlows are used to create a point source for the star.  The artificial star is then inserted into the whole stack.  Both scopes are lined up.   Kinda scary looking in the pictures.

 

 

I have used something similar as a collimator.  What size is the native artificial star in microns?  Each Barlow you place in front of it will reduce (demagnify) the star.  In this case, its 2X, then 2X again, then finally 2.5X.  You want the highest quality Barlow last in line.  All the aberrations in the collimator are additive to what is already in the telescope under test.  But I am certain its at a low level.  My TEC160ED was an honest ⅛ wave in green.  Also, you'll want to evaluate the star with a complex eyepiece like a Delite, Radian, Nagler T6 etc.  Plossls and Orthos will add their own spherical aberration.  In this case, more glass is better.

 

Nice job with this evaluation.


Edited by peleuba, 30 September 2020 - 03:16 PM.


#41 peleuba

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Posted 30 September 2020 - 03:23 PM

What is the cost of a good 2.5” focuser vs the overall cost of the scope, maybe a quick cost / benefit analyses, anyone of you experts ?

 

Moonlite prices:  $445 plus $95 for a flange.  You can't compare the cost of the focuser to the cost of the package as delivered stock by Orion as the telescope would be worth more with the Moonlite.  By how much - you'll have to decide for yourself.  On paper, if you have a good lens, it may be worth it.   But its difficult to place intrinsic value on items that are for enjoyment.  It boils down to value systems and priorities.  Yours's are not the same as mine and mine are not the same as the next guy.  Such is the value placed on goods that are used to pursue a hobby.  


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#42 LDW47

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Posted 30 September 2020 - 03:32 PM

Moonlite prices:  $445 plus $95 for a flange.  You can't compare the cost of the focuser to the cost of the package as delivered stock by Orion as the telescope would be worth more with the Moonlite.  By how much - you'll have to decide for yourself.  On paper, if you have a good lens, it may be worth it.   But its difficult to place intrinsic value on items that are for enjoyment.  It boils down to value systems and priorities.  Yours's are not the same as mine and mine are not the same as the next guy.  Such is the value placed on goods that are used to pursue a hobby.  

I agree with you, I only mentioned it as in the past some made a big issue of an accessory being worth far more than say the scope even though for many, including myself that isn’t a deciding factor. Some think it is atrocious to spend on an add on worth those prices, so I just wondered if it was in this thread based on comments ? Interesting thats all !


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#43 tony_spina

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Posted 30 September 2020 - 06:52 PM

Jeff,

Great work, keep it up

 

Are you planning on removing the field stops or repositioning them so as to not cut off the light cone as you mentioned is the case?



#44 Mitrovarr

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Posted 30 September 2020 - 08:38 PM

Moonlite prices: $445 plus $95 for a flange. You can't compare the cost of the focuser to the cost of the package as delivered stock by Orion as the telescope would be worth more with the Moonlite. By how much - you'll have to decide for yourself. On paper, if you have a good lens, it may be worth it. But its difficult to place intrinsic value on items that are for enjoyment. It boils down to value systems and priorities. Yours's are not the same as mine and mine are not the same as the next guy. Such is the value placed on goods that are used to pursue a hobby.


The moonlight is maybe overkill but there might be cheaper options, like a cheaper 2.5" focuser somewhere or maybe removing baffles and shortening the drawtube of a 2" focuser.

#45 tony_spina

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Posted 30 September 2020 - 09:09 PM

Most folks like myself who changed the focuser used the ubiquitous GSO 2" focuser. 


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#46 Binojunky

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Posted 01 October 2020 - 09:30 AM

Most folks like myself who changed the focuser used the ubiquitous GSO 2" focuser. 

It makes a big difference, though in my case the stock focuser worked fine for visual, but as its probably the last scope I will buy (you can stop laughing lol.gif ) I decided to do the focuser upgrade with a two speed GSO unit from Agena, Dave,



#47 Jeff B

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Posted 01 October 2020 - 11:27 AM

I got a really good deal on my used ML 2.5" focuser which already had the SW/Synta adapter so it was plug-and-play.  The ML is normally on my SW 120ED but it takes me all of 5 minutes to swap out focusers so when I use the 120ST, I'll just plug the ML focuser in and view away.  Besides, I think it looks great on the "little" ST.

 

Jeff 

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#48 tony_spina

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Posted 01 October 2020 - 12:12 PM

Jeff,

Are you planning on doing a flashlight test with the original focuser and ML 2.5 focuser,  and with and without the field stops?



#49 LDW47

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Posted 01 October 2020 - 12:13 PM

It makes a big difference, though in my case the stock focuser worked fine for visual, but as its probably the last scope I will buy (you can stop laughing lol.gif ) I decided to do the focuser upgrade with a two speed GSO unit from Agena, Dave,

Your last scope ?? Really, lol !



#50 LDW47

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Posted 01 October 2020 - 12:14 PM

I got a really good deal on my used ML 2.5" focuser which already had the SW/Synta adapter so it was plug-and-play.  The ML is normally on my SW 120ED but it takes me all of 5 minutes to swap out focusers so when I use the 120ST, I'll just plug the ML focuser in and view away.  Besides, I think it looks great on the "little" ST.

 

Jeff 

.  

But don’t you find that a pain in the a** ?




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