8 replies to this topic

[Jeff B]

This is another DPAC comparison without and with an Aries Chromacor.

 

This time the scope is an older AP 178 F9, pre-ED Starfire with the Chromacor I, model "U1".   The model U1 means the unit applies ~1/8 wave of undercorrection and is meant to be used with an objective that is mildly over-corrected in green light.  The model U1 was chosen here as the objective on this scope displays very mild overcorrection in green with DPAC and, indeed, in DPAC, the model U1 was the "best fit" for this objective compared to my other Chromacors 

 

Chromacors were originally designed to be used with the numerous Synta 6" F8 achromats out there, reducing their level of "CA" to that of something like a 6" F30 -F36 or so achromats.  However, they can be used with scopes "off design" and Valery (of Aries) suggested I try them out with some of the older AP scopes and, specifically, the old Meade 7" F9 ED doublet (I did, the CA was reduced but the lens still sucked). 

 

I recently tried a Chromacor I, "N" with my older "Phoenix" triplet and it worked very well in DPAC, especially in blue light:

 

https://www.cloudyni...-6-f10-triplet/

 

So I did the same thing with my AP 178 F9 Starfire and got similar results, a very nice improvement, especially in blue again.

 

I'm presenting the images in a more compact form that I find makes it very easy for me to see differences. 

 

Also, in these types of comparisons, I'm going to stop saying on which side of the green focus the red and blue focuses fall, especially with triplets (pretty easy with most achromats though), as I've found focus positions in triplets for the various colors can meander around the green focus position, depending on the design and sample to sample variation.  The best I'll comment on is to say if the red and blue focus positions seem to fall closer to (or further away from) the green position when using the Chromacor.

 

Jeff

 

 

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[Jeff B]

Up first is a side-by-side, without/with the Chromacor comparison in white light, first inside of focus, then outside of focus.  Hover the cursor over the images to read their titles.

 

As you can see, there is less blue fringing around the bands that happens in the images when the Chromacor is installed.  That's because, as you will see later, that the blue is being brought in closer to the green/red focus positions. 

 

So the Chromacor works.

 

But look a little closer.  Notice how the blue fringing around the outer bands in the "Base" images are symmetrical about the middle band and they "flip" sides on the band as you switch from inside/outside of focus.  Now in the images with the chromacor installed, notice that while the blue fringing is reduced, it's no longer symmetrical about the middle and it has switched sides on the bands that show it.  I believe this is catching out a bit of the off axis lateral color that Chromacors introduce in exchange for better correction on axis.  I say this because in my DPAC set up, my source LED, as well as my view of the return image, are always just a bit off axis.   So it makes sense to me.   But the better overall correction is blue is easy to see.

 

Jeff

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Edited by Jeff B, 03 March 2020 - 04:39 PM.

[Jeff B]

Here is the inside of focus color comparisons.  The "base" images are on top.  As you can see, the blue focus with the Chromacor is much closer to the green/red focuses (which are very close to each other as this is how Roland made these older "visual" triplets).  It seems the spherical correction in blue is nicely improved as well and a tad better in green too.

 

Jeff

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Edited by Jeff B, 03 March 2020 - 04:40 PM.

[Jeff B]

Now the outside of focus comparisons.  Same story really.

 

Now I need to get some good weather to have a real look and see if star testing and lunar viewing backs up what I see in DPAC when adding the Chromacor to this lens.  That would be a nice icing for this AP cake as it is already a very sharp visual scope.   

 

Jeff

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[junomike]

Now the outside of focus comparisons.  Same story really.

 

Now I need to get some good weather to have a real look and see if star testing and lunar viewing backs up what I see in DPAC when adding the Chromacor to this lens.  That would be a nice icing for this AP cake as it is already a very sharp visual scope.   

 

Jeff

Interersting to see what happens.  IMO my Aries 0 - 1 made well corrected Triplets worse but Achros/ED Doublets better. 

In my Meade 178ED the Aries 0 - 1 essentially makes it an Apo (as opposed to ED).
 

[Jeff B]

Hey Mike so your experiences mirror...I mean reflect...I mean refract....no that does not sound right here either here in a refractor forum.

 

Ok, they are similar to mine.

 

Here's one last comparison I did which is similar to one I just posted in the Chromacor/Phoenix thread.

 

All are with the Chromacor U1 installed.  The top row is inside of focus, the bottom row outside of focus.  Taken on it's own, this is a very good lens.  There is some spherochromatism seen, especially in blue, best correction in yellow, just like Roland says it should be, with a zone in the middle and overall, maybe just a hair overcorrected.....maybe.  Very nice.

 

To give some perspective on the spherochromatism, attached are also the blue, green and red DPAC shots, taken outside of focus from my TEC 160ED, using individual LEDs (which is code to ignore what you may want to call focus differences between the colors).  Compare them to the bottom row.   There is spherochromatism, but I can tell you, the 160ED is a very sharp, visually, color free scope to me.  

 

Observing with the old AP and the Chromacor should be very interesting!  

 

Jeff

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Edited by Jeff B, 03 March 2020 - 07:22 PM.

[precaud]

But look a little closer.  Notice how the blue fringing around the outer bands in the "Base" images are symmetrical about the middle band and they "flip" sides on the band as you switch from inside/outside of focus.  Now in the images with the chromacor installed, notice that while the blue fringing is reduced, it's no longer symmetrical about the middle and it has switched sides on the bands that show it.  I believe this is catching out a bit of the off axis lateral color that Chromacors introduce in exchange for better correction on axis.  I say this because in my DPAC set up, my source LED, as well as my view of the return image, are always just a bit off axis.   So it makes sense to me.   But the better overall correction is blue is easy to see.

 

Jeff

Nice tests you're performing, Jeff, very interesting stuff.

 

I'm not convinced that the LED offset is the cause, or that fringing should be symmetrical around the middle band in all instances. If you have 4 bands of one color overlaying 3 bands of another, there is no way it would be symmetrical. The only ways they would be symmetrical is with an odd number of bands, and with the middle one perfectly centered. Or with an even number of bands in both colors with a white band inthe center. Make sense?

 

It really just shows how difficult it is to interpret the original "white" images. You really have to break the colors out to see what's happening.

Edited by precaud, 06 March 2020 - 09:36 AM.

[Jeff B]

Thanks John!

 

Yeah, the lateral color thing is speculation on my part really, a shot at explaining what I see when inserting my Chromacors into the light path with several of my scopes.  It would be cool to see a color simulation of what lateral color looks like in DPAC though.

 

Jeff

[precaud]

I think you're doing the right thing. It's just that not many conclusions can be drawn from the white light photo. We can see there's a problem, but to understand it requires breaking the colors out. At least that's been my experience.