134 replies to this topic

[Jeff B]

This thread is meant to compliment the discussions already presented in this thread concerning the Orion ST120 Achromat:

 

https://www.cloudyni...ion-120-review/

 

This purpose of this thread is to document some aspects of the mechanical build quality and optical performance not covered in Ed Ting's evaluation video.  This scope is NOT the sample used by Ed during his video.  Rather, it is a brand new sample obtained by me. 

 

The scope arrived today, was well packed and undamaged.  It appears to be a new, original sample, and not a repackaged return sample.  No nicks, dents, scrapes, scuffs or other cosmetic defects were noted.  Glass and coatings are very clean.

 

Focuser

 

The focuser is really very good, tight, no play, but easy to use and could handle my Denk bino-viewers well without slipping...at least at the basement temperatures.  It has about 2.5" of total draw tube travel.  The draw tube has two internal light baffles. (more on that later).  The visual back is OK, with two set screws, 90 degrees apart and secure 2" accessories well.  The 1.25" adapter is fine with a single set screw. 

 

The focuser collimation is excellent with the laser beam coming within a mm or so of the mask's center.  The beam stayed put when racking the focuser through its entire range of travel.  Excellent.

 

Lens Collimation

 

As judged by the reflections off of the back surfaces of the two elements, as seen in my Cheshire "eyepiece", the lens "collimation" was is quite literally "spot on".  As you see in the picture, the element reflections fall right on top of each other.  Impressive!

 

I have to say, this is outstanding mechanical build, especially for the money.

 

One thing I have noticed is that this sample seems to have basically a "zero" sized fully illuminated FOV, meaning the light baffles internal to the focuser and main tube allows only a fully illuminated "point" at focus, not a fully illuminated circle, at least with a two inch diagonal.   Now the aperture is only slowly masked as you move off of the center axis, but I find that a little disappointing, considering my main reason for getting this scope is for wide field, low power viewing and one of the advantages of a refractor is the ability to make it so that you can have a pretty hefty fully illuminated FOV.  .Fortunately, as I'm going to keep this scope, I will either remove and/or reposition the internal baffling to open up the fully illuminated FOV (but I suspect the inner diameter of the focuser draw tube will be the ultimate "choke point" for the size of the field illumination.  Repositioning the baffles will also let me see the full aperture during DPAC, and yes, I've already had a peak at that.....which is why I'm keeping the scope....but more on that later too.

 

Jeff

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[George Methvin]

Very nice write up Jeff I am impressed with this scope its a best buy for what you get, I really like mine plan to hold on to it for a long time.

[stevew]

Jeff, can we assume this is the F-5 version?

Looking forward to the DPAC test results.

[tony_spina]

Jeff,

Nice work. We look forward to more details 

[RLK1]

Nice overview.  Glad to know it's properly collimated.  The stock focuser does appear different from the one in the Ed Ting video so it appears that one may have swapped-out for a two speed unit.

[Mitrovarr]

Upgrading the focuser is an extremely popular thing to do.

[Sergey Stern]

One thing I have noticed is that this sample seems to have basically a "zero" sized fully illuminated FOV, ...

It is not just your sample. They are designed that way. So is also 80 F5 refractors from Skywatcher and Co. I have them both.

 

Sergey

[LDW47]

Nice overview.  Glad to know it's properly collimated.  The stock focuser does appear different from the one in the Ed Ting video so it appears that one may have swapped-out for a two speed unit.

I don’t think that focuser has changed in well over a decade, I have several ! I change them to 2” GSO Crayfords, easily done !

[peleuba]

One thing I have noticed is that this sample seems to have basically a "zero" sized fully illuminated FOV, meaning the light baffles internal to the focuser and main tube allows only a fully illuminated "point" at focus, not a fully illuminated circle, at least with a two inch diagonal.   Now the aperture is only slowly masked as you move off of the center axis, but I find that a little disappointing, considering my main reason for getting this scope is for wide field, low power viewing and one of the advantages of a refractor is the ability to make it so that you can have a pretty hefty fully illuminated FOV.

 

 

Nice!

 

I won't be able to test one for a bit as I have other lenses ahead of it.   The conventional wisdom (from the OP and others) on the previous thread indicated that testing would not yield any information that would be truly useful to current owners or prospective owners of an F/5 achromat.  Glad to see some of those folks are participating in your thread.  

 

It was not clear to me in what you said above - but was the choking of the aperture affecting autocollimation results?  I did a quick sketch on napkin and I am sure that it would as the edge of the lens would be clipped/vingetted in some of the wavelengths tested perhaps giving a better result then would be expected. 

 

I would also agree with you that clipping the aperture defeats the purpose of short, widefield, low power refractor.  In the past, manufacturers have purposely done this as masking down the aperture using a baffle or the focuser draw tube improves the overall color correction and negates any edge-of-lens anomalies.  I recall reading user's of such a telescope would describe it as a "well corrected achromat" having less color error then expected given the focal ratio and aperture.  

Edited by peleuba, 29 September 2020 - 10:16 AM.

[Mitrovarr]

If you guys want, you could do a flashlight test to get the unobstructed aperture. You coule also try modifying it to remove whatever is obstructing the light path and see if it improves or gets worse.

[howardcano]

One thing I have noticed is that this sample seems to have basically a "zero" sized fully illuminated FOV, meaning the light baffles internal to the focuser and main tube allows only a fully illuminated "point" at focus, not a fully illuminated circle, at least with a two inch diagonal.

Mine was like that also.  I added a 2.7" focuser to increase the fully-illuminated image circle.

[Jeff B]

Here is a quick DPAC showing the vignetting of the aperture in DPAC, green, inside of focus, first at full aperture, then stopped to 52mm using the dust cap center hole.

 

As you can see, the full aperture shot is ovalized which means an internal stop(s) is intruding into the light cone in DPAC.  With DPAC, the source LED and return beams are just a tad off axis, which means you are looking just a tad off axis.  Now the source LED and return beams are ~5mm center to center, straddling the middle of the Ronchi screen holder.  Normally, this is not an issue with imaging the full aperture but I have found that scopes designed to have basically a point focus, or a tiny circle that's fully illuminated will vignett a bit in DPAC because of the slight off-axis positions of the source and return beams.  For example, my MN's, Maks and some of my SCTs are notorious for this.

 

To prove the point, just look at the same inside of focus DPAC shot but with the aperture stopped to 52mm using the hole in the dew shield cap.  Nice and round (and look at those straight lines!! ). 

 

Fortunately, my ML 2.5" focuser with it's 2.5" travel draw tube is a direct plug in for the stock focuser.  I'll swap the focusers out and have another look.  

 

Later.

 

Jeff

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[peleuba]

 

To prove the point, just look at the same inside of focus DPAC shot but with the aperture stopped to 52mm using the hole in the dew shield cap.  Nice and round (and look at those straight lines!! ). 

 

 

Pretty good spherical correction with just very slight zone at the ~50% radius.  Not a lot to complain about in green. 

Edited by peleuba, 29 September 2020 - 10:57 AM.

[tony_spina]

Pretty good spherical correction with just very slight zone at the ~50% radius.  Not a lot to complain about in green. 

Agree.

 

I have said this many times that these scopes are well corrected spherically and that plays a big part in the sharp views this scope puts up

Edited by tony_spina, 29 September 2020 - 11:31 AM.

[RLK1]

Nice!

 

I won't be able to test one for a bit as I have other lenses ahead of it.   The conventional wisdom (from the OP and others) on the previous thread indicated that testing would not yield any information that would be truly useful to current owners or prospective owners of an F/5 achromat.  Glad to see some of those folks are participating in your thread.  

 

It was not clear to me in what you said above - but was the choking of the aperture affecting autocollimation results?  I did a quick sketch on napkin and I am sure that it would as the edge of the lens would be clipped/vingetted in some of the wavelengths tested perhaps giving a better result then would be expected. 

 

I would also agree with you that clipping the aperture defeats the purpose of short, widefield, low power refractor.  In the past, manufacturers have purposely done this as masking down the aperture using a baffle or the focuser draw tube improves the overall color correction and negates any edge-of-lens anomalies.  I recall reading user's of such a telescope would describe it as a "well corrected achromat" having less color error then expected given the focal ratio and aperture.  

 

"The conventional wisdom (from the OP and others) on the previous thread indicated that testing would not yield any information that would be truly useful to current owners or prospective owners of an F/5 achromat.  Glad to see some of those folks are participating in your thread." I didn't state that and, in fact, I posted a link to an interferometry report on one. I also noted my rationale for framing such a report relative to  the mechanics, optics, and actually using one for observing . The latter is more important to me than the former in this case.

[peleuba]

"The conventional wisdom (from the OP and others) on the previous thread indicated that testing would not yield any information that would be truly useful to current owners or prospective owners of an F/5 achromat.  Glad to see some of those folks are participating in your thread." I didn't state that and, in fact, I posted a link to an interferometry report on one. I also noted my rationale for framing such a report relative to  the mechanics, optics, and actually using one for observing . The latter is more important to me than the former in this case.

 

 

You wrote in the following direct quote:   Do you really need to know the type of glass in the objective?How many baffles? The length of a dew cap? The amount of camera weight its standard focuser can support?  And what about those optical tests? Strehl ? Interferometry? On a single sample of one mass-produced RFT? How helpful is that?"   (emphasis mine)

 

This quote, which I took in context, made by you is particularly unambiguous as it pertains to your feelings on testing.   I was complimenting you for showing up in this thread even though you find such tests on a single sample of an F/5 Achromat to be meaningless.  Sometimes its best just to take the compliment, yes?

 

Jeff started this thread on testing precisely because its interesting and helpful to those of us who have optical benches and test telescopes  Moreover, its if not helpful to you, there are other folks who are interested in understanding both relative performance under the stars and absolute performance on a test bench.

 

As soon as I am able, I will test the ST120 to see if Jeff's results compare to a second sample.  At $250 new, I may purchase additional samples when they come in to Orion on 10/6. 

 

In any event, this already taking a deeper dive then the one you started vis-a-vis performance of the optic.  If your just wanting to bicker, as before PM me.  

 

Regards.

Edited by peleuba, 29 September 2020 - 12:17 PM.

[RLK1]

You wrote in the following direct quote:   Do you really need to know the type of glass in the objective?How many baffles? The length of a dew cap? The amount of camera weight its standard focuser can support?  And what about those optical tests? Strehl ? Interferometry? On a single sample of one mass-produced RFT? How helpful is that?"   (emphasis mine)

 

This quote, which I took in context, made by you is particularly unambiguous as it pertains to your feelings on testing.   I was complimenting you for showing up in this thread even though you find such tests on a single sample of an F/5 Achromat to be meaningless.  Sometimes its best just to take the compliment, yes?

 

Jeff started this thread on testing precisely because its interesting and helpful to those of us who have optical benches and test telescopes  Moreover, its if not helpful to you, there are other folks who are interested in understanding both relative performance under the stars and absolute performance on a test bench.

 

As soon as I am able, I will test the ST120 to see if Jeff's results compare to a second sample.  At $250 new, I may purchase additional samples when they come in to Orion on 10/6. 

 

In any event, this already taking a deeper dive then the one you started vis-a-vis performance of the optic.  If your just wanting to bicker, as before PM me.  

 

Regards.

No, I don't want to bicker. I noted those tests because you were initially vague in what tests you had been referring  to in that thread.  And, no, I don't think they're relevant in a sample size of one. And that was clearly borne out in the interferometry report that I had linked to in that thread...

[Jeff B]

So I swapped out the stock focuser for my ML 2.5" draw tube I.D. focuser with a 3" travel currently mounted on my SW 120ED.  Fit like a glove.  And the collimation of the focuser was, again, spot on as you see in the photos.  It certainly looks slick.  

 

And NO vignetting in DPAC.  In fact the fully illuminated FOV now seems to be about 10mm or so, limited by the main tube internal baffles (two of them) and, again, with a gradual cutoff as you move laterally outward from there.  

 

DPAC will follow.

 

Jeff

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[Jon Isaacs]

Just add my two cents:

 

I like tests and reviews of scopes like the 120 mm F/5.  Reviewing an Astro-Physics refractor is probably fun but basically an exercise in nitpicking.  You know it's going to be great.

 

Scopes like this, an ED-80, there's a lot of room for both the good and the bad, a lot to find out.

 

 

Jon

[Jeff B]

And now the DPAC results...and the crowd goes wild.       

 

All shots were taken with my "white" LED (made up of individual R,G,and B mini LEDs) with the exception of the null shots, which were done with a green LED.  Using the white shots, the individual Blue, Green and Red color channels were then isolated with photo processing software and knitted together in a series.  This is now my "standard process".  One nice thing about using a single "white" LED is that I can then get a glimpse as to how close the focus positions of the three colors are relative to each other, which also gives a clue as to the "color balance" of the system chosen by the designer(s).  With a fast achromat these differences are readily seen because....well...after all....it's a fast achromat.

 

On top are the "inside" of focus shots.  In the middle are the outside of focus shots, and at the bottom, the null shots.  Place your cursor over each picture to read their titles.

 

Not bad but certainly not perfect either, but I would say it's at least diffraction limited overall in green.  As Paul pointed out, there is a well define, somewhat abrupt zone in the middle 50% of the lens, but, as you can see in the 2" aperture shot above, that "zone" is very well corrected in green. 

 

At full aperture, The lens seems to be slightly undercorrected in green (the centers of the lines bulge in towards the middle inside of focus and bulge out towards the edge outside of focus), though it's hard to tell because of the effect of that center zone.  Kinda looks like two lenses stacked on top of each other.

 

The edge appears to be very good.  

 

You can easily see spherochromatism at work too, with the blue being overcorrected and the red the flip of that, undercorrected, which is normal for such a fast refractor.

 

Like all of the China sourced lenses I've tested, this one shows that the red and green colors are fairly close to each other in focus positions, while the blue focus has been "let go" a bit.  You see that by looking at the number of lines displayed for each color and how they are placed across the face of the lens.  Red and green are very similar, blue, not so much.

 

Note the well defined zone in the green null images.  One thing I've learned from many experts here, particularly the ATM'ers, is that Rochi nulls can be extremely revealing, especially in DPAC.  Done carefully, it can easily resolve stuff down to 1/50 wave.  So don't point your finger at the hyper-contrast null image and gasp with bug eyes "UNCLEAN".  It looks a lot worse than it really is.  But it's sill there.

 

One thing not show but I check for in DPAC, (and will later check for with an indoor star test in green) is astigmatism.  In DPAC, this displays itself as a rotational "clocking" of the Ronchi lines as you sweep through from one side of focus to the other.  Specifically I position a single Ronchi line right smack in the middle, and keep it there as I approach focus. If the line starts to lean and rotate say to the left from vertical close to focus and then rotates back towards vertical from the right on the other side of focus, there is some astigmatism in the system.   I did not see any astigmatism with this lens in DPAC.  That's good news.

 

I'll next post the indoor star test results in green light.  I'll be looking for overall spherical correction, astigmatism, and coma.

 

Jeff

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Edited by Jeff B, 29 September 2020 - 02:23 PM.

[aa6ww]

Between the 80 F/5, 100 F/5  120 F/5 and 150 F/5, the  ST120 F/5 is the best of the bunch for me, because its still a small scope requiring a small mount and has very nice optics. Its a great comet  hunter on a moments notice also.

 

...Ralph

[peleuba]

Not bad but certainly not perfect either, but I would say it's at least diffraction limited overall in green.  As Paul pointed out, there is a well define, somewhat abrupt zone in the middle 50% of the lens, but, as you can see in the 2" aperture shot above, that "zone" is very well corrected in green. 

 

BIG SNIP

 

I'll next post the indoor star test results in green light.  I'll be looking for overall spherical correction, astigmatism, and coma.

 

I go wild for all of these tests...   Simply excellent job, Jeff.  

 

I will admit, I am a little surprised to see how prominent the zone is - I could barely detect it in the preliminary images you posted.  In second set of images, I estimate it at about  ¼ wave deep/high in green.  (its depth/height will vary in other wavelengths as the length of the wave will be different then in green).  But as you demonstrate, good null tells the a more complete story of the surface condition.    

 

Terrific on the star test.  Can't wait to see it.  Did you obtain the tester I suggested?

Edited by peleuba, 29 September 2020 - 04:30 PM.

[RLK1]

Thanks for taking the time and effort to do the eval and sharing it with us. I'm looking forward to the star test. 

While I'm on the fence as to acquiring one for myself at this time, these reports here and on the web are quite encouraging. On the one hand, they appear to be quite a bargain for what you get--I mean I have eyepieces that cost more than the OTA--but there are a couple of things that are holding me back. 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. And, given it dimensions, it's definitely not a grab and go scope. I think the Orion offering would do fine with the latter circumstance. What bothers me more though, is that Orion fails to respond to my query regarding the focuser, as mentioned in the Ed Ting review thread. I've sent them another query again earlier today but with no reply. Basically, I asked them if any modifications of the focuser have been made in response to the reviews on their website as well as on the web. I think they could have sent a generic response to the effect of, "While we feel the focuser performs to our specifications, if you do experience an issue with your unit, we would be happy to address for you." Instead, I can't get the courtesy of a reply and I find that unacceptable. It's tantamount to telling me they don't give a darn whether they get my business, or not...

[turtle86]

And now the DPAC results...and the crowd goes wild.       

 

All shots were taken with my "white" LED (made up of individual R,G,and B mini LEDs) with the exception of the null shots, which were done with a green LED.  Using the white shots, the individual Blue, Green and Red color channels were then isolated with photo processing software and knitted together in a series.  This is now my "standard process".  One nice thing about using a single "white" LED is that I can then get a glimpse as to how close the focus positions of the three colors are relative to each other, which also gives a clue as to the "color balance" of the system chosen by the designer(s).  With a fast achromat these differences are readily seen because....well...after all....it's a fast achromat.

 

On top are the "inside" of focus shots.  In the middle are the outside of focus shots, and at the bottom, the null shots.  Place your cursor over each picture to read their titles.

 

Not bad but certainly not perfect either, but I would say it's at least diffraction limited overall in green.  As Paul pointed out, there is a well define, somewhat abrupt zone in the middle 50% of the lens, but, as you can see in the 2" aperture shot above, that "zone" is very well corrected in green. 

 

At full aperture, The lens seems to be slightly undercorrected in green (the centers of the lines bulge in towards the middle inside of focus and bulge out towards the edge outside of focus), though it's hard to tell because of the effect of that center zone.  Kinda looks like two lenses stacked on top of each other.

 

The edge appears to be very good.  

 

You can easily see spherochromatism at work too, with the blue being overcorrected and the red the flip of that, undercorrected, which is normal for such a fast refractor.

 

Like all of the China sourced lenses I've tested, this one shows that the red and green colors are fairly close to each other in focus positions, while the blue focus has been "let go" a bit.  You see that by looking at the number of lines displayed for each color and how they are placed across the face of the lens.  Red and green are very similar, blue, not so much.

 

Note the well defined zone in the green null images.  One thing I've learned from many experts here, particularly the ATM'ers, is that Rochi nulls can be extremely revealing, especially in DPAC.  Done carefully, it can easily resolve stuff down to 1/50 wave.  So don't point your finger at the hyper-contrast null image and gasp with bug eyes "UNCLEAN".  It looks a lot worse than it really is.  But it's sill there.

 

One thing not show but I check for in DPAC, (and will later check for with an indoor star test in green) is astigmatism.  In DPAC, this displays itself as a rotational "clocking" of the Ronchi lines as you sweep through from one side of focus to the other.  Specifically I position a single Ronchi line right smack in the middle, and keep it there as I approach focus. If the line starts to lean and rotate say to the left from vertical close to focus and then rotates back towards vertical from the right on the other side of focus, there is some astigmatism in the system.   I did not see any astigmatism with this lens in DPAC.  That's good news.

 

I'll next post the indoor star test results in green light.  I'll be looking for overall spherical correction, astigmatism, and coma.

 

Jeff

 

Great work on this!  

[Bean614]

Thanks for taking the time and effort to do the eval and sharing it with us. I'm looking forward to the star test. 

While I'm on the fence as to acquiring one for myself at this time, these reports here and on the web are quite encouraging. On the one hand, they appear to be quite a bargain for what you get--I mean I have eyepieces that cost more than the OTA--but there are a couple of things that are holding me back. 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. And, given it dimensions, it's definitely not a grab and go scope. I think the Orion offering would do fine with the latter circumstance. What bothers me more though, is that Orion fails to respond to my query regarding the focuser, as mentioned in the Ed Ting review thread. I've sent them another query again earlier today but with no reply. Basically, I asked them if any modifications of the focuser have been made in response to the reviews on their website as well as on the web. I think they could have sent a generic response to the effect of, "While we feel the focuser performs to our specifications, if you do experience an issue with your unit, we would be happy to address for you." Instead, I can't get the courtesy of a reply and I find that unacceptable. It's tantamount to telling me they don't give a darn whether they get my business, or not...

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