35 replies to this topic

[Scott in NC]

A fellow CN'er kindly let me test a few of his scopes a few months ago, but I've been so busy that I'm finally getting around to posting them.  So here they are, with great apologies to the scopes' owner for my delay!

 

First is an Astro-Tech AT111EDT.  This is a 111mm triplet with an FPL-51 ED element.

 

[Scott in NC]

AT111EDT

 

Green inside / outside focus:

 

 

Red inside / outside focus:

 

 

Blue inside / outside focus:

 

 

White inside / at / outside focus:

 

[Scott in NC]

The AT111EDT's optics appear to be best corrected for spherical aberration in red (with slight overcorrection), although green is not too far behind red.  There is a thin turned down edge, and possibly a very shallow central zone. Compared to other scopes I've seen, I doubt that this zone would cause much harm to the image.  Color correction looks very nice, as would be expected from an apo triplet, and I think pretty impressive considering that it uses FPL-51, rather than an element with even lower dispersion such as FPL-53, FCD-100, or fluorite.

[Scott in NC]

Next is a very nice Skywatcher 100ED, which is a 100mm f/9 doublet utilizing FPL-53 as the ED element.

 

[Scott in NC]

SW100ED

 

Green inside / outside focus:

 

 

Red inside / outside focus:

 

 

Blue inside / outside focus:

 

 

White inside / at / outside focus:

 

[Scott in NC]

This SW100ED appears to have pretty nice spherical correction in both green and red, and blue isn't that far off (some over correction in blue, but still quite minimal, and better than the AT111EDT posted above).  There may be a very thin turned down edge, but significantly thinner than on the AT111EDT, and a small central zone is noticeable in the white "at focus" image.  Color correction is very nice for a doublet, but if you compare it with the AT111EDT above you'll see that an f/9 doublet utilizing FPL-53 may still have slightly more color than an f/7 triplet utilizing FPL-51.

 

Overall this is a very nice scope, and the owner has told me that it's given him some very nice views (which I don't doubt based upon the test results posted here). 

[Scott in NC]

Next is a William Optics 66SD.  This is a fast f/5.9 doublet with an FPL-51 ED element.

 

[davidc135]

I expect these (first two) scopes have been returned to their owner but it would be interesting to see side by side tests between them with their small errors and the masked down, virtually perfect TSA120. Would any difference be noticed?

 

David

Edited by davidc135, 12 August 2023 - 01:13 PM.

[Jeff B]

You just gotta love those 100ED F9s.  I've tested three samples, one with the Celestron brand and two from Orion. They all look very similar to each other and this sample, right down to the overall color and minor tinting around the line shadows, sample attached.  Amazing quality and consistency over the decades of their existence. 

 

But also the images offer some more direct proof that DPAC done right is also very consistent no matter who does it. 

 

Again, well done Scott.

 

Jeff

Attached Thumbnails

• 

[Scott in NC]

William Optics SD66

 

Green inside / outside focus:

 

 

Red inside / outside focus:

 

 

Blue inside / outside focus:

 

 

White inside / at / outside focus:

 

[Scott in NC]

I expect these (first two) scopes have been returned to their owner but it would be interesting to see side by side tests between them with their small errors and the masked down, virtually perfect TSA120. Would any difference be noticed?

 

David

You're correct that I no longer have access to these scopes.  I drove a couple of hours to the owner's house to do the testing, and don't foresee repeating that trip anytime soon.  But you bring up a very good question, and one to which I don't really know the answer for sure.  My educated guess is that for high-magnification planetary viewing (especially of Jupiter and Saturn), you'd likely notice a slight difference in visibility of subtle features. You'd probably also notice less chromatic aberration on bright stars and Venus.  For low to medium power, I think most of us would be hard-pressed to notice any significant differences.  

[Scott in NC]

You just gotta love those 100ED F9s.  I've tested three samples, one with the Celestron brand and two from Orion. They all look very similar to each other and this sample, right down to the overall color and minor tinting around the line shadows, sample attached.  Amazing quality and consistency over the decades of their existence. 

 

But also the images offer some more direct proof that DPAC done right is also very consistent no matter who does it. 

 

Again, well done Scott.

 

Jeff

Thanks for sharing that image, Jeff.  Yes, very consistent indeed!

[Scott in NC]

The WO66SD shows mild under correction in green and red, and mild over correction in blue.  The polish appears to be reasonably smooth, without any detectable zones.  Color correction isn't quite as good as with the first two scopes, but isn't too far off from the SW100ED, and considering that this is a very fast f/5.9 FPL-51 doublet, I think it's quite acceptable.

[JeremySh]

The AT111EDT's optics appear to be best corrected for spherical aberration in red

Are you sure it wasn’t an SV? 
 

[Scott in NC]

LOL.  In the case of the AT111EDT, the term "best" is really splitting hairs.  The two colors are very nearly the same as far as correction for SA goes. I don't see any evidence of an intention to figure the lens for better correction in red vs. green.

[Victory Pete]

Can you show what is involved in doing these tests as far as the procedure and equipment needed?

[Alan S]

Can you show what is involved in doing these tests as far as the procedure and equipment needed?

Pete,

 

There are several threads and numerous posts on CN with this information already.  Try a search on CN forums for ‘how to DPAC’ as I recall a thread titled something along those lines.

 

Personally, I’d recommend you continue your foray into learning how to use your new scope and imaging system and not go down a new rabbit hole…one that is a completely different facet of this hobby.

[m0bius]

DPAC is a bit of a misnomer...

 

If you're really curious, also search for: Ronchi autocollimation

Edited by m0bius, 13 August 2023 - 02:20 PM.

[Wildetelescope]

The AT111EDT's optics appear to be best corrected for spherical aberration in red (with slight overcorrection), although green is not too far behind red.  There is a thin turned down edge, and possibly a very shallow central zone. Compared to other scopes I've seen, I doubt that this zone would cause much harm to the image.  Color correction looks very nice, as would be expected from an apo triplet, and I think pretty impressive considering that it uses FPL-51, rather than an element with even lower dispersion such as FPL-53, FCD-100, or fluorite.

A good thing about these tests is that they provide some insight into questions of design and glass choice.   The Astro tech EDT scopes have consistently been shown to be pretty darn good scopes at their price point.  The 115 in particular has a Stellar(pun intended) reputation:-).  Yes, you can get better, but you will spend substantially more;   Folks starting out should buy these scopes with confidence.  I have been very impressed with the AT refractors that I have purchased or have had the opportunity to use for extended periods.  Kudos to Mike and the folks at AT for the work and time they have put into developing the relationships with the sources of these optics to insure that what they get is of excellent quality.  

 

JMD

[Scott in NC]

DPAC is a bit of a misnomer...

 

If you're really curious, also search for: Ronchi autocollimation

Yes, indeed. I’m just sticking with the common parlance, even if part of the name is redundant.

[peleuba]

Can you show what is involved in doing these tests as far as the procedure and equipment needed?

 

 

These will get you started and after reading them, you may know more then some telescope makers...

 

https://www.cloudyni...your-refractor/

 

https://www.cloudyni...fractor-optics/

 

=======

 

Also, do a forum search in Refractors on the following:

 

Bench Test of and DPAC Challenge   This is how Jeff B and I have chosen to name our respective testing threads here in the Refractors forum.

[Victory Pete]

These will get you started and after reading them, you may know more then some telescope makers...

 

https://www.cloudyni...your-refractor/

 

https://www.cloudyni...fractor-optics/

 

=======

 

Also, do a forum search in Refractors on the following:

 

Bench Test of and DPAC Challenge   This is how Jeff B and I have chosen to name our respective testing threads here in the Refractors forum.

Okay thanks, I will take look. I am looking forward to definitely getting into this. I love technical analysis and tweaking things. I have a few expensive strobe tuners, analog and digital, and spend more time tuning my guitars than playing them, which by the way is a very complicated subject, equal temperament tuning I mean. My workshop is ready to go, I just need the equipment and knowledge.

[peleuba]

Okay thanks, I will take look. I am looking forward to definitely getting into this. I love technical analysis and tweaking things. I have a few expensive strobe tuners, analog and digital, and spend more time tuning my guitars than playing them, which by the way is a very complicated subject, equal temperament tuning I mean. My workshop is ready to go, I just need the equipment and knowledge.

 

Let me know if your going to Cherry Springs this Fall.   

 

Yes, like tuning a guitar, its the nuances that are most interesting.   No telescope looks the same on the test bench.  I expect that you'll find it similar to guitars in a round-a-bout sort of way.

[Victory Pete]

Let me know if your going to Cherry Springs this Fall.   

 

Yes, like tuning a guitar, its the nuances that are most interesting.   No telescope looks the same on the test bench.  I expect that you'll find it similar to guitars in a round-a-bout sort of way.

https://youtu.be/QOrrNM4md8c

[Alrakis]

Okay thanks, I will take look. I am looking forward to definitely getting into this. I love technical analysis and tweaking things. I have a few expensive strobe tuners, analog and digital, and spend more time tuning my guitars than playing them, which by the way is a very complicated subject, equal temperament tuning I mean. My workshop is ready to go, I just need the equipment and knowledge.

The most expensive piece of equipment that you need is the flat of sufficient size. You would want a flat about an inch larger than the optic you are testing. The flat doesn't have to be expressly flat (as in better than 1/x wave flat), just a regular, smooth surface. Other than the flat, you need a light source (or sources, maybe green and white), a ronchi grating (133 LPI is a good start), something 3d printed to hold the light source and ronchi grating against (and in) the visual back, and a camera.

 

As an alternative to the flat (especially if you are test very large optics) there is a method to use a large pan with heavy oil. The oil acts light a mirror and will take the basic curvature of the earth as a slight convex curve with an edge that has a concave curve. 

 

https://www.cloudyni...llimation-test/

 

Chris