686 replies to this topic

[John Huntley]

Yep. I agree with that and time will tell.  Need to remember that a 7" versus a 6" gathers 52% more light and a bit more resolution.

 

But this scope is actually cheaper versus a SW Esprit and other 150mm scopes.....

 

 

But s it cheaper when the additional mounting needs of a 185mm are taken into account ?

 

If you already have a mount with the capacity then that's not an issue but if additional mount investment is required then the financial equation changes.

[lylver]

Or at least one of the spacers has been flipped? Assuming they have that kind of profile, flipping a spacer or both can change the spacing.

 

lens spacer.jpg

On the contrary, i think adding around 0.4 to 0.6mm between the two rear lenses would recenter spherical aberration.

Doing this in the first front lenses would be much slower (may help for 1/3rd or 1/4th effect)

Edited by lylver, 01 February 2024 - 07:05 PM.

[Cbaxter]

Or at least one of the spacers has been flipped? Assuming they have that kind of profile, flipping a spacer or both can change the spacing.

lens spacer.jpg

No, it is the design of the objective. Others have well explained it in this thread already but it is evident from the DPAC results that it is nulled in red and the green and blue error are consistent with that.

[Joe G]

I think 200mm APO is a 200mm f4 wide field small telescope used for AP. I have the 185mm version.

Yes.  That is true.

[Joe G]

I would agree that just like the SVX 180, it seems this scope must have been figured in red, but I am not an optics expert.  Certainly the DPAC results are on the disappointing side of things.  Maciek's strehl calculations as well.

 

But, I am going to lay out some comments from Rick Runcie's post about his SVX 180 with my comments in red:

 

First off I observed Sirius at approximately 300x.  It was split easily and showed no color as I remember. It showed similar diffraction rings on either side of focus. The one side was sharper and more distinct. The side that was not as distinct was not too far off from the other and the rings were apparent just a little softer. To my eye the first diffraction ring appeared similar on both sides of focus. This was done without a green filter and the seeing at that altitude Sirius was was not as steady as I would have liked.

 

Splitting Sirius is no easy feat.  You need very steady seeing and good optics.

 

Next up was Saturn. Saturn looked beautiful. I noticed immediately that three belts were easily seen not just the large darker belt that you can see in just about every size telescope, but two above it and then the cap. These belts were more apparent and easier to see than in my 6-in lzos refractor or my 10-in zambuto reflector. The image was very sharp and contrasty. It took magnification well and the Cassini division was a wide etched division and even very apparent at the very narrowest part of the Rings as a thin line.

 

So this does not sound terrible.  He says Saturn looked better than in his LZOS 6" refractor and a much larger aperture Zambuto mirror.

 

Jupiter as well did not disappoint, but this is where I noticed the red and blue fringe on either side of the planet. Jupiter itself was very detailed. I could note some smaller ovals in the polar region as well as a plethora of festoons between the two main belts that were very obviously blue in color. The belts were all different shades of pastel colors and I observed including all the minor divisions easily 8 to 10 if not more. The color fringes on either side of the planet again blue on one side and red on the other were somewhat of an annoyance to me. I very rarely if ever saw these fringes in my other telescopes.

 

So this is where the scope runs into trouble.  My first light did not indicate this but the seeing was not excellent.  Time will tell.  As I have said I thought, largely because of the lesser grade of ED glass used versus the SVX 180, that Jupiter might be compromised.  The tradeoff for me is I hoped it would do better on DSOs versus my SW 150 triplet APO with FPL 53 glass.  I am fortunate that I can keep both scopes.  The SW 150 DPACs very well.

 

Mars, my favorite planet to observe during favorable oppositions showed a lot of features ie the polar Hood, several Albedo dark features with small extensions and a cloud Haze on the limb. The limb was a little compromised because of the annoying blue and red Fringe on the edge of it, similar to Jupiter.

 

I swung back to Jupiter and was enjoying the view, but again the color fringes were impacting it even though the moons that were visible of Jupiter show clearly different size discs. I could see the fringes on the larger moons like Ganymede which was a shame because I could pick out some Albedo features on it.

 

The telescope did put up sharp contrasty images when the seeing allowed. I was dismayed by the color fringes because I was not used to seeing this while observing the planets. For the next couple weeks I observed every opportunity I could to test the telescope. More than not I saw these color

 

I have owned a few 8" reflectors that I have used both for visual and imaging including one with a premium Royce mirror.  I have preferred using my SW 150 to 8" reflectors on GEMs.  I have a couple of excellent Dobs too.  I can't argue the price/performance tradeoff of an 8" reflector for imaging, even compared to a premium well corrected refractor.

 

Finally, like Rick Runcie, if I paid close to $20,000 I'd expect a premium optic optimized in green light.  But this scope wasn't $20,000.  Too bad Askar didn't read Paul Leuba's thread about Rick's scope. 

[Borodog]

Red and blue fringing on either side is not CA. It's atmospheric dispersion and it isn't the scope's fault. Atmospheric despersion correctors are cheap (maybe too cheap, for the ZWO ones, which have a poor reputation for quality control).

[Borodog]

Also, being able to see atmospheric dispersion visually is a testament to how sharp the scope is. Uncorrected, atmospheric dispersion in my 11" SCT accounts for a change in the COGs of the red and blue channels relative to the green of order 1 pixel at critical sampling, depending on altitude. That's 0.15".

[peleuba]

 

Finally, like Rick Runcie, if I paid close to $20,000 I'd expect a premium optic optimized in green light.  But this scope wasn't $20,000.  Too bad Askar didn't read Paul Leuba's thread about Rick's scope. 

 

 

Joe, 

 

Truth be told, I am seriously considering getting the Askar 185 based on your testing and feedback.  I would use mine as a permanent setup on a pier in my back yard kept under a 365 type of cover.   It would take the place of a Celestron SCT.  The optics in the Askar 185 are superior to what one finds in a commercial C8, C9¼ or C11 for not much more money.  Combined with no collimation required and a faster cool down, there is little downside to this. 

 

The thing to take away from your DPAC photos and the analysis that some of us provided is this:  The scope is working as its designed.  There are no badly rolled edges nor spurious spherical aberration that cannot be accounted for by the design.  And the color correction looks really nice.  If its free of astigmatism, at this price, its a winner.

Edited by peleuba, 01 February 2024 - 04:59 PM.

[BWAZ]

Now we've got Paul's endorsement.   You got me seriouly thinking about getting one as well. 

[peleuba]

Now we've got Paul's endorsement.   You got me seriouly thinking about getting one as well. 

[Joe G]

Joe, 

 

Truth be told, I am seriously considering getting the Askar 185 based on your testing and feedback.  I would use mine as a permanent setup on a pier in my back yard kept under a 365 type of cover.   It would take the place of a Celestron SCT.  The optics in the Askar 185 are superior to what one finds in a commercial C8, C9¼ or C11 for not much more money.  Combined with no collimation required and a faster cool down, there is little downside to this. 

 

The thing to take away from your DPAC photos and the analysis that some of us provided is this:  The scope is working as its designed.  There are no badly rolled edges nor spurious spherical aberration that cannot be accounted for by the design.  And the color correction looks really nice.  If its free of astigmatism, at this price, its a winner.

Ha, at least we will get another DPAC sample.

[Jeff B]

Joe, some folks have mentioned stopping the Askar down to 160mm and retesting it.

 

That would be interesting but I don't think it would "help" the overall correction as, to me, the outer 50% span is the best corrected portion of the objective.  But it would be cool to see what you get.  If anything, it would also be interesting to stick a, maybe, 1.5" paper central obstruction over the objective with thread and retest.  That would be interesting too.  Both would be fun (for me at least) to make such comparisons visually to see any sharpness differences.

 

I'm really curious as to what your Espirit comparisons show when seeing eventually allows a high power comparison with the Askar.  

 

Jeff

[Joe G]

Joe, some folks have mentioned stopping the Askar down to 160mm and retesting it.

 

That would be interesting but I don't think it would "help" the overall correction as, to me, the outer 50% span is the best corrected portion of the objective.  But it would be cool to see what you get.  If anything, it would also be interesting to stick a, maybe, 1.5" paper central obstruction over the objective with thread and retest.  That would be interesting too.  Both would be fun (for me at least) to make such comparisons visually to see any sharpness differences.

 

I'm really curious as to what your Espirit comparisons show when seeing eventually allows a high power comparison with the Askar.  

 

Jeff

I'll try that this weekend while it is raining.

[davidc135]

Interesting to see the test results of further examples. As price goes down so sample variation may go up. The best correction in red may (or may not) be due to chance just as the slight extra central error is.

 

David

Edited by davidc135, 01 February 2024 - 06:31 PM.

[SandyHouTex]

I would agree that just like the SVX 180, it seems this scope must have been figured in red, but I am not an optics expert.  Certainly the DPAC results are on the disappointing side of things.  Maciek's strehl calculations as well.

 

But, I am going to lay out some comments from Rick Runcie's post about his SVX 180 with my comments in red:

 

First off I observed Sirius at approximately 300x.  It was split easily and showed no color as I remember. It showed similar diffraction rings on either side of focus. The one side was sharper and more distinct. The side that was not as distinct was not too far off from the other and the rings were apparent just a little softer. To my eye the first diffraction ring appeared similar on both sides of focus. This was done without a green filter and the seeing at that altitude Sirius was was not as steady as I would have liked.

 

Splitting Sirius is no easy feat.  You need very steady seeing and good optics.

 

Next up was Saturn. Saturn looked beautiful. I noticed immediately that three belts were easily seen not just the large darker belt that you can see in just about every size telescope, but two above it and then the cap. These belts were more apparent and easier to see than in my 6-in lzos refractor or my 10-in zambuto reflector. The image was very sharp and contrasty. It took magnification well and the Cassini division was a wide etched division and even very apparent at the very narrowest part of the Rings as a thin line.

 

So this does not sound terrible.  He says Saturn looked better than in his LZOS 6" refractor and a much larger aperture Zambuto mirror.

 

Jupiter as well did not disappoint, but this is where I noticed the red and blue fringe on either side of the planet. Jupiter itself was very detailed. I could note some smaller ovals in the polar region as well as a plethora of festoons between the two main belts that were very obviously blue in color. The belts were all different shades of pastel colors and I observed including all the minor divisions easily 8 to 10 if not more. The color fringes on either side of the planet again blue on one side and red on the other were somewhat of an annoyance to me. I very rarely if ever saw these fringes in my other telescopes.

 

So this is where the scope runs into trouble.  My first light did not indicate this but the seeing was not excellent.  Time will tell.  As I have said I thought, largely because of the lesser grade of ED glass used versus the SVX 180, that Jupiter might be compromised.  The tradeoff for me is I hoped it would do better on DSOs versus my SW 150 triplet APO with FPL 53 glass.  I am fortunate that I can keep both scopes.  The SW 150 DPACs very well.

 

Mars, my favorite planet to observe during favorable oppositions showed a lot of features ie the polar Hood, several Albedo dark features with small extensions and a cloud Haze on the limb. The limb was a little compromised because of the annoying blue and red Fringe on the edge of it, similar to Jupiter.

 

I swung back to Jupiter and was enjoying the view, but again the color fringes were impacting it even though the moons that were visible of Jupiter show clearly different size discs. I could see the fringes on the larger moons like Ganymede which was a shame because I could pick out some Albedo features on it.

 

The telescope did put up sharp contrasty images when the seeing allowed. I was dismayed by the color fringes because I was not used to seeing this while observing the planets. For the next couple weeks I observed every opportunity I could to test the telescope. More than not I saw these color

 

I have owned a few 8" reflectors that I have used both for visual and imaging including one with a premium Royce mirror.  I have preferred using my SW 150 to 8" reflectors on GEMs.  I have a couple of excellent Dobs too.  I can't argue the price/performance tradeoff of an 8" reflector for imaging, even compared to a premium well corrected refractor.

 

Finally, like Rick Runcie, if I paid close to $20,000 I'd expect a premium optic optimized in green light.  But this scope wasn't $20,000.  Too bad Askar didn't read Paul Leuba's thread about Rick's scope. 

What I would like to know is why in the world are the Chinese figuring their large refractors in red?

[peleuba]

What I would like to know is why in the world are the Chinese figuring their large refractors in red?

 

Its very possible when the objective is matched to the sub-aperture focal reducer/corrector the null is shifted to a more neutral position.   My sense is that this scope is primarily and astrograph to be accessorized with the reducer/corrector.  In other words, the nominal correction might have to be in red because the reducer with its own lenses pushes the null toward green.

 

We would have to see the design of the corrector/reducer to be sure.  But this is one possibility.

 

Another possibility is that it was done on purpose because CCD/CCMOS cameras are very sensitive to the red and deep red parts of the spectrum.

Edited by peleuba, 01 February 2024 - 08:14 PM.

[Joe G]

It would be interesting to know what Askar's thoughts are in regards to figuring towards red.

 

But for the gurus.  I understand the human eye is more sensitive to green light.  But we are not looking at jungles with these scopes, but the sky. 

 

Given that there is not a lot of green in the sky and stars tend to be red, white or blue, does it still make sense to optimize the scope for green?  I understand that we use red lights at night because of that insensitivity.  I also understand green is in the middle of the visual spectrum.

 

Thoughts?

[Cbaxter]

It would be interesting to know what Askar's thoughts are in regards to figuring towards red.

But for the gurus. I understand the human eye is more sensitive to green light. But we are not looking at jungles with these scopes, but the sky.

Given that there is not a lot of green in the sky and stars tend to be red, white or blue, does it still make sense to optimize the scope for green? I understand that we use red lights at night because of that insensitivity. I also understand green is in the middle of the visual spectrum.

Thoughts?

My understanding is that green being in the middle of the visual spectrum meant nulling in green gave the best balance above and below it, red and blue, on the spectrum. I suppose that may depend on the optical prescription and may not necessarily be the rule.

Do you have the flattener for this scope and if so, is it possible or meaningful to look at the ronchi autocollimation results with the flattener attached? The reason I ask is because I have a TS Optics astrograph that is a triplet with a pre-attached corrector that can be removed and a visualback attached. I got a star test and autocollimation report with the scope and noticed that the test indicated that the corrector was attached. When I inquired about why the triplet wasn't tested by itself, without the corrector, TS told me that basically it was because the scope was designed primarily as an astrograph to be used with the corrector for "best results" so that is how they tested it.

Edited by Cbaxter, 01 February 2024 - 10:18 PM.

[Joe G]

I don't have the flattener yet.  Agena does not have them.  I have ordered the flattener but not the reducer/flattener.

 

To test the scope with the flattener???  I will ave to figure out how to do that.  If someone has suggestions please let me know.

 

I think that this scope might have been designed as an astrograph, but that is speculation.

[Bomber Bob]

From SHARPSTAR/ASKAR:   The large aperture of 185mm gives it an inherent advantage in terms of light-gathering power, resolution, brightness, and contrast, allowing the photographer to gather more light and present more details and brighter objects in the image.  Along with more light coming into the astrograph, the limiting magnitude can reach 13.1.

 

https://www.sharpsta...ducts_1/93.html

 

There ya go - designed for digital imaging, rather than primarily visual use... [Stated in the Very First Paragraph]

 

FYI:  My 2 ED Doublets - APM 152ED F8 & AT-102ED F7 - had very good (& nearly identical) DPAC patterns in Green.  Both were excellent near-zero CA visual refractors.

Edited by Bomber Bob, 01 February 2024 - 10:40 PM.

[Joe G]

From SHARPSTAR/ASKAR:   The large aperture of 185mm gives it an inherent advantage in terms of light-gathering power, resolution, brightness, and contrast, allowing the photographer to gather more light and present more details and brighter objects in the image.  Along with more light coming into the astrograph, the limiting magnitude can reach 13.1.

 

https://www.sharpsta...ducts_1/93.html

 

There ya go - designed for digital imaging, rather than primarily visual use... [Stated in the Very First Paragraph]

 

FYI:  My 2 ED Doublets - APM 152ED F8 & AT-102ED F7 - had very good (& nearly identical) DPAC patterns in Green.  Both were excellent near-zero CA visual refractors.

I get it.  Vic of SV said the same thing, if I recall in the "thread," but then said StellarVue now utilizes DPAC to figure closer to green.

 

I bought the the Lunt version of your APM scope and gave it to my son.  I did DPAC it and it looks nice.  It does not do as well on the planets as my Esprit though.  But it is a very nice scope.

 

Edited by Joe G, 01 February 2024 - 11:07 PM.

[Bomber Bob]

The APM was somewhat better than the AT (a Real Bargain, anyway!).  That is, the APM was reliably sharp at 80x per inch in 8 / 10 or better seeing - showed albedo patterns on Ganymede.  The AT was a 65x per inch instrument - and a blast as a near-RFT.

 

My 4 TAKs - 2 APOs, an S-APO triplet, & achro doublet - all had near-perfect DPACs in Green.  Though I sold it, I still use photos of my FC-50's pattern as a reference pattern for other refractors.  Rail-sharp & very thin bars. 

 

My DPAC rig can test in Red, Blue, Green, or White (I just manually switch LED connectors on a circuit board), and uses a 133 LPI screen.  After dozens of tests, I've found that DPAC doesn't lie.  Great pattern on the rig, great Views at the eyepiece.

[SandyHouTex]

Its very possible when the objective is matched to the sub-aperture focal reducer/corrector the null is shifted to a more neutral position.   My sense is that this scope is primarily and astrograph to be accessorized with the reducer/corrector.  In other words, the nominal correction might have to be in red because the reducer with its own lenses pushes the null toward green.

 

We would have to see the design of the corrector/reducer to be sure.  But this is one possibility.

 

Another possibility is that it was done on purpose because CCD/CCMOS cameras are very sensitive to the red and deep red parts of the spectrum.

Well thankfully Tak doesn't do that.  And from the DPACs I've seen, neither does TEC or AP.

Edited by SandyHouTex, 02 February 2024 - 12:05 AM.

[SandyHouTex]

It would be interesting to know what Askar's thoughts are in regards to figuring towards red.

 

But for the gurus.  I understand the human eye is more sensitive to green light.  But we are not looking at jungles with these scopes, but the sky. 

 

Given that there is not a lot of green in the sky and stars tend to be red, white or blue, does it still make sense to optimize the scope for green?  I understand that we use red lights at night because of that insensitivity.  I also understand green is in the middle of the visual spectrum.

 

Thoughts?

Our eyes are much more sensitive to green then red or blue, both photopically (well lit) and scotopically (poorly lit).

[Joe G]

I understand that, but we are not looking through our telescopes at green martians???