49 replies to this topic

[Jeff B]

Specifically, Paul from Indy's AP102 f/8 Starfire (Pre-ED) 1989: Serial Number SF4855

 

I wish I had taken more shots as this sample because it is just beautiful.  Instead you get the subterranean DPAC bunker image full of odd stuff and peeling wallpaint.

 

Oh well.

 

More to follow.

 

Jeff

 

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[Stellar1]

Wow!  that’s a holy grail type refractor right there, worth a kidney

[MarMax]

Come on . .  you could at least put a nice bottle of bourbon or scotch somewhere on the table for ambience.

[Kitfox]

Scott, you have to run my 130 f/6 to compare the two…780 vs 816mm, same hands on the glass…a couple of decades in between. 

 

TBH, this one is probably the one I worry about the least. Something tells me 

Edited by Kitfox, 23 February 2023 - 09:40 PM.

[Scott in NC]

Jeff…we’re all eagerly awaiting the Ronchigrams.

[Kitfox]

“Keeping me way ay ayting…”

[Jeff B]

Ok, ok, here are some results.   

 

First the inside/outside of focus white light Master Images.  Again, just place your cursor over the image to read the title.

 

Right away you can tell that overall, the objective is mildly over corrected.  Roland was accused by some folks back then of slightly overcorrecting his lenses on purpose, something he denied and got rather testy about.  In a falling temperature environment, an overcorrected objective can have an advantage though.  As an objective cools from the edge inwards, that adds a bit of undercorrection to the system, which can compensate to a degree for an objective's mild overcorrection.  And I've experienced just that with most Asian sourced achromats(which as a group, tend to be overcorrected in green), and my old AP178 F9, pre-ED Starfire, which is also mildly overcorrected in green.   But I've also had other similar vintage scopes from AP that were neutrally to slightly undercorrected.  Chalk it up to sample variation. 

 

You also see a bit of blue fringing at the line tips and a slight overall warm tone to the image, both of which point to a mild miss in the blue.   But this is a well color corrected objective, certainly for visual use, especially considering there is no ED glass insight.  Roland told me a couple of times, the color correction for these vintage lenses is similar to an achromat with 3X to 4X the focal ratio.  My observations with my scopes of this vintage suggest better than that so I think he's being conservative.  

 

Jeff

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[davidc135]

How does the test look in green light? Is it an illusion but the green parts of the above seem straighter? Mind you, both yellow and blue bend the same way. 

 

 David

Edited by davidc135, 24 February 2023 - 03:43 AM.

[Jeff B]

Here are the color montages derived from the masters by using software to isolate the color channels.

 

You can see the color focus error in blue (relative to green/yellow) as more lines coming in from the sides inside of focus, and fewer, outside of focus.  The spherochromatism in blue is also easily seen as the more strongly curved lines on each side of focus, again relative to green and especially the orange/red, which is where the best correction for this objective is.   

 

Now if I look carefully at the green image with the circled areas, I see a bit of a "compound correction", specifically, the outer, maybe, 30% of the line's span is slightly more overcorrected than the middle with a smooth transition between them.  This suggests to me some mild, smoothly transitioning, broad zoning.    

 

Which is exactly what I see in the green at-focus image (with hyped up contrast).   The other feature I see is how smooooth the overall figure is.  The objective is completely polished out leaving the "figure errors" (zoning).

 

Jeff

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[Jeff B]

So, just how "bad" is the figure error in green.

 

Not that bad really, in fact, pretty good.  I use Maciek's DPAC simulation program to get an idea of what the Strehl of an optic might be, assuming pure under/overcorrection are the only figure errors, no abrupt turned edges, sharp zones, kinks or large scale roughness.  This objective to me is pretty close to that condition, so I ran the program for both the inside and outside of focus green images.  In a perfect universe, I should get the same result on each side of focus.....

 

Inside of focus the simulation gave ~.945 and .969 for outside of focus with conic constants of -1.75 and -1.55 respectively.   Pretty good agreement really.  There is no point in simulating orange or red as I can tell "by inspection" they will hanging out between .98 - .99, especially the red.

 

Jeff

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Edited by Jeff B, 24 February 2023 - 01:04 PM.

[Jeff B]

So, I also took at crack at using Maciek's simulation using the blue light images.

 

The inside/outside Strehls were  .536/.703.  So, a bigger variation but, as I expected, well below .80 for blue.

 

Jeff

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[RichA]

Specifically, Paul from Indy's AP102 f/8 Starfire (Pre-ED) 1989: Serial Number SF4855

 

I wish I had taken more shots as this sample because it is just beautiful.  Instead you get the subterranean DPAC bunker image full of odd stuff and peeling wallpaint.

 

Oh well.

 

More to follow.

 

Jeff

Very nice result.  Nothing like a good refractor to show smooth bands as opposed to some of the scopes that show bands that looked like they were chewed on by some animal.

[Jeff B]

I am actually struck by how similar this 4" pre-ED Starfire measures in DPAC compared to my old AP 178 F9 Starfire of similar vintage.

 

Here is a comparison of their inside of focus color montages (no orange image for the 178).  

 

Almost identical really except for a bit more of a miss in the blue.  The 178 also has an overall mild over correction in green with best correction in red.   

 

At focus in green, the 178 is also completely polished out with a similar zone.  

 

I would have liked to actually look through the 102 Starfire.  I imagine it is a very sharp scope, perhaps a touch "warm" in color tone as my old 178 Starfire is extremely sharp, a touch warm and measures very similarly to the 102.   

 

Also, I noted no signs of astigmatism with the 102.  There was no rotation/clocking of the middle ronchi line as I swept through focus in DPAC.  Paul says it gives a great star test and is very sharp.

 

This is a really nice sample of Roland's earlier, pre-ED and pre-interferometer, work.

 

Jeff

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[Jeff B]

Very nice result.  Nothing like a good refractor to show smooth bands as opposed to some of the scopes that show bands that looked like they were chewed on by some animal.

Or struck by lightening.

[Scott in NC]

Nice, Jeff!  Do you have any examples to illustrate how astigmatism would look, or is that more of a dynamic feature that shows up better visually when running in and out of focus rather than showing up well on a single photographic image?

[Jon Isaacs]

Jeff:

 

Thank you for sharing your efforts..  The elephant in the room:  How does this compare to a Synta ED-100?

 

 

(Tom's Orion ED-100)

 

Jon

[Scott in NC]

Jeff:

 

Thank you for sharing your efforts..  The elephant in the room:  How does this compare to a Synta ED-100?

 

 

 

(Tom's Orion ED-100)

 

Jon

Jon, 
Jeff did that here: https://www.cloudyni...-100ed-doublet/

[Jon Isaacs]

Jon, 
Jeff did that here: https://www.cloudyni...-100ed-doublet/

 

 

Scott:

 

I knew that Jeff, Paul and Paul's nameless friend (who has been responsible for many well known astro-photos) have tested Synta ED-100's with consistently excellent results.. 

 

I just wanted someone else to point it out.

 

Jon

[sqrlman]

I had one of these. I would call it the worst of the Starfire line. The scope is oversized. It looks like a 5" but it's only a 4". Performance is behind the Celestron 102F. Astronomy magazine found that out when they did their test of the 4" refractors.. I agree completely. I had a Celestron102F for many years. Much better than the Starfire 102 that I briefly owned.

 

Steve

[Jeff B]

But it's also nice to see side-by-sides too....

 

 

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[Jeff B]

Also in purple with a Newt for comparison...

 

 

 

 

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[Kitfox]

That Celestron is a Vixen fluorite, correct?  That isn’t fighting fair. 

[Jeff B]

That Celestron is a Vixen fluorite, correct?  That isn’t fighting fair. 

Don't know.  My sample C100ED (-R?) is fairly recent, 2010-is.

[Lagrange]

That Celestron is a Vixen fluorite, correct?  That isn’t fighting fair. 

The one Jeff B tested is ED glass (FPL53?) rather than fluorite, and it's only a doublet whereas the A-P is a triplet, albeit one without ED glass so the comparison shouldn't be unfair to either scope.

 

Post #19 mentions the review of 4" refractors done by Astronomy magazine (in 1990? - I should have a copy somewhere) which put the Celestron Vixen fluorite doublet as the winner for image quality, ahead of the A-P Starfire which in turn was better than the 94mm Brandon and the TeleVue Genesis f/5.

[Kitfox]

Sorry, sqrlman’s post mentioned a C102F. I transposed that to Jeff’s post.