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Sharpstar 61 EDPH II APO Chromatic Abberation?

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#101 cedric_d

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Posted 08 April 2021 - 03:23 PM

The scope+reducer spot diagram has a blue ring. While the scope alone does not.

Could this be an explanation ?

 

Scope spot diagram

94edph-spot-diagram.jpg

 

Scope + reducer spot diagram

94edphreducer-spot-diagram.jpg


Edited by cedric_d, 08 April 2021 - 03:23 PM.


#102 TareqPhoto

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Posted 08 April 2021 - 03:55 PM

The scope+reducer spot diagram has a blue ring. While the scope alone does not.

Could this be an explanation ?

 

Scope spot diagram

attachicon.gif94edph-spot-diagram.jpg

 

Scope + reducer spot diagram

attachicon.gif94edphreducer-spot-diagram.jpg

Not color corrected reducer?



#103 Barologuy

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Posted 08 April 2021 - 03:55 PM

The scope+reducer spot diagram has a blue ring. While the scope alone does not.
Could this be an explanation ?

Scope spot diagram
94edph-spot-diagram.jpg

Scope + reducer spot diagram
94edphreducer-spot-diagram.jpg

Ding ding ding this is why the focal reducers are causing problems for people. The glass can only be so good and when you don't sue the best glass possible, you are forced to use more and more elements in your reducer to correct the light. This explains why the initial one looks fine, but the reducer sends it all over the place. Better off slapping a field flattener on these SharpStar scopes and just living with their native f/ and focal lengths.


Edit: This happens with all scopes, assuming the color correction isn't applied in the reducer. The color is already pretty good, and then it's getting sent through more glass which has to be corrected *again*

Edited by Barologuy, 08 April 2021 - 03:58 PM.

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#104 cedric_d

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Posted 08 April 2021 - 04:02 PM

I don't understand why Sharpstar would make good APO scopes and ruin them with bad "dedicated" reducers ?



#105 TareqPhoto

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Posted 08 April 2021 - 04:24 PM

Ding ding ding this is why the focal reducers are causing problems for people. The glass can only be so good and when you don't sue the best glass possible, you are forced to use more and more elements in your reducer to correct the light. This explains why the initial one looks fine, but the reducer sends it all over the place. Better off slapping a field flattener on these SharpStar scopes and just living with their native f/ and focal lengths.


Edit: This happens with all scopes, assuming the color correction isn't applied in the reducer. The color is already pretty good, and then it's getting sent through more glass which has to be corrected *again*

 

 

I don't understand why Sharpstar would make good APO scopes and ruin them with bad "dedicated" reducers ?

The question is, what if we tested another reducer with not Sharpstar scope but it is an APO triplet and didn't see halo or CA, will that be good reducer for Sharpstar scope then?

 

I mean, do you care about CA or flattened stars by edges at best?



#106 Barologuy

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Posted 08 April 2021 - 11:38 PM

The question is, what if we tested another reducer with not Sharpstar scope but it is an APO triplet and didn't see halo or CA, will that be good reducer for Sharpstar scope then?

I mean, do you care about CA or flattened stars by edges at best?


This will show up on likely any scope near these price points, and regardless even if it wasn't in the scope, the depth of focus is so shallow it's likely the best images will come from computer controlled focused that can determine best focus, as opposed to the focuser that ships with the unit. It's a good focuser, but it's not a computer.

Case in point I visited a telescope manufacturer in my city yesterday and they are working with a focus plane of 1.7 microns for f/1.7 on a 1.3m scope. That's extreme, but given these setups the likely errors come from ourselves. It has CA, but if it didn't have CA, how many of us have really been able to get it in perfect focus? Ideally all of us, though I'm not sure. The Focuser on the scope is clearly stepped.

#107 UKalwayscloudy

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Posted 09 April 2021 - 05:17 AM

Sharpstar's website does not have the pair of spot diags for both without and with the reducer, just the with, for the specific 61EDPHII scope we are discussing. Can't find it on this thread so here it is. It's dated May 15 2020 but I don't recall seeing it before. 

Attached Thumbnails

  • SPOT - 1.png


#108 TareqPhoto

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Posted 09 April 2021 - 08:32 AM

This will show up on likely any scope near these price points, and regardless even if it wasn't in the scope, the depth of focus is so shallow it's likely the best images will come from computer controlled focused that can determine best focus, as opposed to the focuser that ships with the unit. It's a good focuser, but it's not a computer.

Case in point I visited a telescope manufacturer in my city yesterday and they are working with a focus plane of 1.7 microns for f/1.7 on a 1.3m scope. That's extreme, but given these setups the likely errors come from ourselves. It has CA, but if it didn't have CA, how many of us have really been able to get it in perfect focus? Ideally all of us, though I'm not sure. The Focuser on the scope is clearly stepped.

Yesterday i saw someone used this Sharpstar 61II scope with WO reducer for narrowbanding image and i didn't see CA, but i can't judge from one image i think.



#109 cedric_d

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Posted 09 April 2021 - 12:32 PM

Blue chromatic aberration shouldn't be a problem for narrowband imaging because the lowest wavelenght is OIII=500 nm.

RGB with a standard IR/UV filter (like Astronomik L2) starts at 400 nm. L3 starts at 420 nm.


Edited by cedric_d, 09 April 2021 - 12:34 PM.


#110 TareqPhoto

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Posted 09 April 2021 - 12:58 PM

Blue chromatic aberration shouldn't be a problem for narrowband imaging because the lowest wavelenght is OIII=500 nm.

RGB with a standard IR/UV filter (like Astronomik L2) starts at 400 nm. L3 starts at 420 nm.

Right, so that blue halo or CA is only from RGB filters or color camera then



#111 cedric_d

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Posted 09 April 2021 - 01:13 PM

This is just a theory because i don't do narrowband imaging :)

I ordrered a L3 though, it should confirm if increasing the lowest wavelenght helps.



#112 TareqPhoto

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Posted 09 April 2021 - 03:00 PM

This is just a theory because i don't do narrowband imaging smile.gif

I ordrered a L3 though, it should confirm if increasing the lowest wavelenght helps.

Let us know when you do, i am still waiting my scope to be delivered but after i finish clear the payment, very soon hopefully, then i will test it to see if i see any CA or halo or whatever issues, if there is that blue halo or CA then most likely i will buy either L3 or one of those LP filters starting after 400mm.



#113 cedric_d

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Posted 09 April 2021 - 03:18 PM

I plan to test the scope without filter / with L2 / with L3. I'll come back when it's done.


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#114 TareqPhoto

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Posted 09 April 2021 - 04:22 PM

I plan to test the scope without filter / with L2 / with L3. I'll come back when it's done.

Good luck and clear sky waytogo.gif



#115 Barologuy

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Posted 09 April 2021 - 06:51 PM

I shot Bodes with a Baader Moon and Skyglow last night. Using the reducer... -- no CA!

Edit: Very little CA.

Edited by Barologuy, 10 April 2021 - 07:54 AM.

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#116 MartinPond

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Posted 09 April 2021 - 11:48 PM

I see issues both with and without, at blue.

But.....the scale is really blown up.

Inside the blue exclamation without reducer,

I believe there is a hidden blue spot.

With reducer, there is a blue ring but I think the inner blue is

  being hidden.

 

I don't know if you can avoid a 2-peak blue whatever the scope is at that F-ratio,

 at that scale.



#117 UKalwayscloudy

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Posted 15 April 2021 - 04:26 AM

I shot Bodes with a Baader Moon and Skyglow last night. Using the reducer... -- no CA!

Edit: Very little CA

That;'s interesting. Sounds like we might be homing in on the need for a modest UV cut to bring things into the domain of being manageable with a light room correction. 



#118 UKalwayscloudy

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Posted 15 April 2021 - 04:27 AM

I plan to test the scope without filter / with L2 / with L3. I'll come back when it's done.

That will be really useful information. 



#119 leemajors

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Posted 20 April 2021 - 04:36 PM

I plan to test the scope without filter / with L2 / with L3. I'll come back when it's done.

Waiting in anticipation for the results, I use my CLS ccd filter at the moment and that completely eliminates the CA blue fringing as it cuts all blue below 450nm, but its also a rather strong filter for things like galaxies and IFN that contain yellow, so id rather use an L3 if it cuts the blue enough. On paper it seams to cut at exactly 420nm the same as my standard Baader lum filter, but im hoping its just a tad stronger than the graphs show, as its designed just for this purpose.

I also tried my Baader Fringe killer and it i proves things a lot but not quite as well as the CLS, there was still a negligible amount of Ca on the brightest stars and they were also slightly softer than with the CLS.

i was going to order an L3 to try but might as well wait for your results now.

lee



#120 cedric_d

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Posted 21 April 2021 - 02:11 AM

I started some tests with L2 / L3 and without filters.

First results here : https://www.cloudyni...orrection-issue



#121 leemajors

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Posted 26 April 2021 - 09:03 AM

I started some tests with L2 / L3 and without filters.

First results here : https://www.cloudyni...orrection-issue

I ended up buying the L3 as I would have always wondered otherwise, but unfortunately it was no better than my existing Baader Lum filter, as I suspected from the Graphs which show almost identical blocking point of 420nm. So its on its way back for refund.

So the only broadband filter that ive found is usable with this scope/ reducer is my Astronomik CLS ccd filter, without that the blue fringing is apparent.

I intend to do some more tests without the reducer to confirm its the reducer and not the scope itself, that won't really help as I bought it with the intention of always using the reducer for the wider FOV. Its fine in Narrowband of course, so even just using it for that its worth the money. just a bit disappointed with the fringing issue for Broadband targets.

CLS  solves it totally, but is also such a strong filter I think it blocks the faint stuff like integrated flux nebula and dark nebulas.



#122 leemajors

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Posted 26 April 2021 - 11:55 AM

Baader Lum UV/IR filter

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  • BaaderLum.jpg


#123 leemajors

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Posted 26 April 2021 - 11:55 AM

Astronomik L3

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  • L3.jpg


#124 leemajors

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Posted 26 April 2021 - 11:56 AM

Baader Fringe Killer

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  • Fringe.jpg


#125 leemajors

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Posted 26 April 2021 - 11:57 AM

Astronomik CLS ccd

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  • CLS.jpg



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