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Sharpstar 61 EDPH II APO Chromatic Abberation?

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#1 Barologuy

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Posted 10 October 2020 - 11:00 AM

I finally had the opportunity to setup everything and start testing the go-to, which seems to work relatively well. However, my photos of the Pleiades have these purple halos around them and I can't help but wonder if I got a dud scope, or if it really matters and I can edit this out and correct at a later point. I posted here because I want to know if this is specifically a fault with the telescope and I do not have any eye pieces to just pop into the scope to check myself.

This is just a 30 second exposure and it seems to be visible in other stars, not red ones or orange ones, but blue ones specifically.

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  • IMG_20201010_095620.jpg

Edited by Barologuy, 10 October 2020 - 11:20 AM.


#2 sg6

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Posted 10 October 2020 - 11:25 AM

Well looks like CA, mainly as it is a halo. Scope is a triplet and in that respect the 3 primaries should be at the same focal plane. had it been a doublet may have espected.

 

I suspect the "problem" is that it is still fast even for a triplet, 61mm and f/5.5 is I suggest pushing the glass.

 

Very strange question it is the triplet not the earlier doublet ?

If it is the doublet then at 61 and f/5.5 then expect CA. Basically too fast for sense.

 

Couple of points:
Sharpstrar refuse to name the ED glass in use, seems strange. Most if they use FPL-53 or a match are keen to state it.

 

One retailer here has on their site:

The SharpStar 61EDPH provides a very good observation and imaging system thanks to its fast focal ratio. The unique three-element design incorporates a special ED lens, which helps to control chromatic aberration very well, makes the stars look sharp, and the image more vivid.

I noted that it says "helps control chromatic aberration very well".

"Very well" is not no CA.

The above comes from the 61EDPH II page, not possibly the doublet and is stated as a triplet.

 

Being distrustful I would say they are not using the best glass they could and have with that produced a refractor that is too fast for the glass being used. The result being the presence of CA.

 

For all scopes "fast" is not good, either refractor or reflector. But it seems if Scope A is faster then Scope B people almost blindly buy Scope A.



#3 Barologuy

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Posted 10 October 2020 - 11:31 AM

Well looks like CA, mainly as it is a halo. Scope is a triplet and in that respect the 3 primaries should be at the same focal plane. had it been a doublet may have espected.

I suspect the "problem" is that it is still fast even for a triplet, 61mm and f/5.5 is I suggest pushing the glass.

Very strange question it is the triplet not the earlier doublet ?
If it is the doublet then at 61 and f/5.5 then expect CA. Basically too fast for sense.

Couple of points:
Sharpstrar refuse to name the ED glass in use, seems strange. Most if they use FPL-53 or a match are keen to state it.

One retailer here has on their site:
I noted that it says "helps control chromatic aberration very well".
"Very well" is not no CA.
The above comes from the 61EDPH II page, not possibly the doublet and is stated as a triplet.

Being distrustful I would say they are not using the best glass they could and have with that produced a refractor that is too fast for the glass being used. The result being the presence of CA.

For all scopes "fast" is not good, either refractor or reflector. But it seems if Scope A is faster then Scope B people almost blindly buy Scope A.



Thank you,

This is the triplet, with a 0.8 focal reducer and field flattener... Maybe that's part of the problem as well?

#4 junomike

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Posted 10 October 2020 - 11:48 AM

"undisclosed glass" means if could be FPL-51 (or similar) which IMO might not be CA-perfect at such a fast F-Ratio.



#5 sg6

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Posted 10 October 2020 - 11:59 AM

The amount of glass will not really help, it makes 3 at the front and 3 at the rear, and the rear isn't far from the front.

 

I have never been drawn  to fast optics, I have 1 scope at 5.9 my triplet and as that is a WO I seriously suspect it is really 6. They seem to claim 1mm extra width in places so manage 5.9 whereas others end up at 6. The ZS61 and AT60 are examples. Marketting I expect and as said people buy the faster, even if it is the same.

 

WO used to have fast refractors then one day they made them slower - like f/7 and f/7.5, great news as far as I was concerned. Sensible ratios and on triplets.

 

As said my suspicion is they have reduced costs on the glass, knowing that people examine the glass type so will not say, and have gone a little too far on having a fast scope.

 

Here Skywatcher stopped stating the glass, people stopped buying Skywatchers, Skywatcher have decided to state the glass used (if it is FPL-53).

 

It may be your objective and another scope may be better - could you request an exchange?

Just thinking it could be one that passed but only just passed the quality checks.



#6 Barologuy

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Posted 10 October 2020 - 12:22 PM

The amount of glass will not really help, it makes 3 at the front and 3 at the rear, and the rear isn't far from the front.

I have never been drawn to fast optics, I have 1 scope at 5.9 my triplet and as that is a WO I seriously suspect it is really 6. They seem to claim 1mm extra width in places so manage 5.9 whereas others end up at 6. The ZS61 and AT60 are examples. Marketting I expect and as said people buy the faster, even if it is the same.

WO used to have fast refractors then one day they made them slower - like f/7 and f/7.5, great news as far as I was concerned. Sensible ratios and on triplets.

As said my suspicion is they have reduced costs on the glass, knowing that people examine the glass type so will not say, and have gone a little too far on having a fast scope.

Here Skywatcher stopped stating the glass, people stopped buying Skywatchers, Skywatcher have decided to state the glass used (if it is FPL-53).

It may be your objective and another scope may be better - could you request an exchange?
Just thinking it could be one that passed but only just passed the quality checks.


I emailed Sharpstar sales with an image and the retailer I purchased it from, and I'm waiting to hear back from either one. I can definitely remedy it in post processing, but I have to agree that my next scope is going to be FPL-53 stated, and I'll probably go for a larger f ratio as well. I'd rather have a sharp image, accurate even if it takes longer, rather than edit color out of an image in post.

#7 Cometeer

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Posted 10 October 2020 - 01:11 PM

Any filters used? What camera was this taken with. Do you have a raw sub?

#8 telfish

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Posted 10 October 2020 - 01:26 PM

The Pleiades has a lot of blue dust around the stars. Try on another target without any nebulosity.



#9 Barologuy

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Posted 10 October 2020 - 01:54 PM

Any filters used? What camera was this taken with. Do you have a raw sub?


No filters, Canon M50, I do have a raw sub I can post, I just don't know where to upload to.

#10 Barologuy

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Posted 10 October 2020 - 01:54 PM

The Pleiades has a lot of blue dust around the stars. Try on another target without any nebulosity.



Alright, will do.

#11 alphatripleplus

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Posted 10 October 2020 - 03:39 PM

M52, M39 or M29 might be good open cluster targets.



#12 junomike

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Posted 10 October 2020 - 03:40 PM

The Pleiades has a lot of blue dust around the stars. Try on another target without any nebulosity.

IMO It's not blue dust or nebulosity.  It's CA...



#13 mmalik

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Posted 10 October 2020 - 04:40 PM

Did you buy it new or used? May try swapping (...returning) for a CAT or upcoming RAD. Regards



#14 Barologuy

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Posted 10 October 2020 - 05:54 PM

Did you buy it new or used? May try swapping (...returning) for a CAT or upcoming RAD. Regards


Purchased it new.

#15 Barologuy

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Posted 10 October 2020 - 05:59 PM

So attached is Pleiades after PS, looks okay, but the halos are still noticeable, just gray.

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Edited by Barologuy, 10 October 2020 - 06:02 PM.


#16 Barologuy

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Posted 10 October 2020 - 06:06 PM

And M52 without processing...

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#17 mmalik

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Posted 10 October 2020 - 07:08 PM

For comparison sake a single unguided 3min exposure with CAT; 150% zoom to a small section of the image. Click image for the complete RAW. Regards

 

 

Note: 3min unguided, ISO2000, a7S/SkyGuider/PoleMaster/SpaceCat/D2/WOBase/RRS/Feisol; RAW...

 

post-205769-0-39109900-1595878729_thumb.


Edited by mmalik, 10 October 2020 - 07:21 PM.


#18 Barologuy

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Posted 10 October 2020 - 07:17 PM

Looking at your image Malik when downloaded it appears your red peak is in front of the blue and green, and on my basic images this trend is reversed.



#19 mmalik

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Posted 10 October 2020 - 07:31 PM

That probably would explain the difference. Are you using any light pollution filter; mine was taken with D2... and 'Removed LPF-2' camera. Try one (pollution filter) if you are not already to rule out external factors. Regards

 

 

Note: CAT RAW below

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Edited by mmalik, 10 October 2020 - 07:41 PM.


#20 Barologuy

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Posted 10 October 2020 - 07:57 PM

Alright, I'll give it a shot!



#21 anat

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Posted 10 October 2020 - 08:12 PM

I find similar CA with my 61edphII and ASI533mc (with Astrodon L filter). I don't have the CA issue with Redcat51.

41d88091be1a6ec6913cb0d90d6c278b.jpg

Edited by anat, 10 October 2020 - 08:16 PM.


#22 Barologuy

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Posted 10 October 2020 - 08:35 PM

I find similar CA with my 61edphII and ASI533mc (with Astrodon L filter). I don't have the CA issue with Redcat51.

 

I think this answers it then, this is probably inherent to the scope itself.



#23 Bowlerhat

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Posted 10 October 2020 - 08:40 PM

It is a fast scope, especially after getting faster with reducer.

However, these kind of triplet doesn't mean the three ED lens as corrector, usually just one element. And sharpstar doesn't really disclose their glass..but fpl-51 is different than fpl-53.

 

I have the 72 sharpstar ED which shows much CA compared to skywatcher 72ED. Difference is while skywatcher didn't disclose their glass either, many has speculated it's FPL 53, and many reports high power usage is possible. I've pushed 168x with the skywatcher ED and it has very little CA while sharpstar has way more noticeable CA. The two scopes are similar in specs, with similar focal length and f ratio.



#24 F.Meiresonne

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Posted 11 October 2020 - 01:34 PM

Very strange this is...

 

See below a picture taken with the doublet version...i can't see color, so i am very surprised to that much CA in the triplet here..

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#25 anat

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Posted 11 October 2020 - 02:23 PM

Very strange this is...

See below a picture taken with the doublet version...i can't see color, so i am very surprised to that much CA in the triplet here..

What camera and filter did you used? How long was the exposure?

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