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SkyWatcher 150mm ED

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#1 tog

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Posted 16 September 2019 - 09:40 AM

I purchase a Skywatcher 150mm Evostar ED (1200mm fl/ f8) a couple of weeks back. Last night was clear and I got the scope out for its first light. It was a very bright sky with the moon one day past full so I thought this would be a good time to check the scope’s ability to deal with chromatic aberration, (CA). I didn’t use a moon filter.

Using a 17mm Orion Stratus eyepiece (68 degree apparent fov) I centered the moon and noticed CA along the small terminator (green). There was no CA along the limb, around craters, or anywhere else I could see. I changed the eyepiece to a 25mm plossl (52 degree fov) and did not notice any CA. A 10mm plossl (52 fov) gave another CA free view. I didn’t stay out long as the sky was becoming somewhat hazy and my test was limited to these focal length eyepieces.

I know this was a very short session with this scope and I plan on pulling an all-nighter as soon as the opportunity presents itself. I have a few questions: About the Stratus-Why does it show CA when the plossl did not?  What is a better test for CA? What eyepieces should I purchase for this scope? Thanks for any advice!


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#2 SeattleScott

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Posted 16 September 2019 - 11:53 AM

That doesn’t sound like CA. It sounds like lateral color in the Stratus. Not uncommon with wide angle eyepieces. The other eyepieces didn’t have a wide viewing angle.

CA is typically a blue halo. LC is typically a narrow yellow/green fringe. A better quality wide angle eyepiece may eliminate the LC.

I found LC was more of an off axis abberation. CA would be visible anywhere. If it goes away when you put it in the center of the field, then it is LC, not CA.
Scott

Edited by SeattleScott, 16 September 2019 - 12:01 PM.

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#3 areyoukiddingme

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Posted 16 September 2019 - 04:01 PM

Off the top of my head, I can't think of a wide field eyepiece that does not show that kind of lateral color. 

 

All Pentax XWs do, Leica zoom, Ethos, Morpheus.

 

I think better to aim at a bright star in the center of field to look for purple. But even so, if there was significant CA from the scope, that should have been seen as a purple smudge on the moon.



#4 salico

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Posted 16 September 2019 - 04:15 PM

my Naglers and Panos also show LC


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#5 Sol Robbins

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Posted 16 September 2019 - 04:26 PM

I agree that the color you see is probably lateral color from the eyepiece.

 

Some eyepieces will show that apple green color at the Moon's edge. Additionally, it might get gradually more prominent progressing from the center of the field of view to the edge of the field of view. Some eyepieces have shown this as a bluish linear edge. Sometimes, if you position your eye a little off center, you'll see more or no color at all.

 

For me Orthos show least of this.

 

Its not CA. CA will appear as a blur that's a bluish purple. 

 

Enjoy the new scope! I've been enjoying my big refractor for nearly 2 decades.


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#6 Jeff B

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Posted 16 September 2019 - 04:51 PM

More than likely lateral color as others have said.  I see it too, even in triplets.

 

Regarding use of the moon to judge "CA", be careful as the moon is so bright it stops down your pupil to below that of scope, which effectively reduces the scope's aperture at low power, which improves the subjective  "CA".

 

Use Vega and/or Altair.  Vega is a very severe test.

 

Jeff


Edited by Jeff B, 16 September 2019 - 04:52 PM.

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#7 CHASLX200

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Posted 16 September 2019 - 05:33 PM

There is not enough color to bother with.  Seems people worry or go looking for color. I am picky, but my SW150ED had no color that would bother me.


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#8 Bomber Bob

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Posted 16 September 2019 - 09:51 PM

I check / assess false color the same way as my star tests:  straight-through (No Diagonal), and with simple eyepiece designs -- Kellner / Orthoscopic / Plossl -- fewer lenses / internal faces limits glare, alignment issues, etc.

 

After 50 years of achromatics, I know where to look on The Moon to see CA, and it sounds like you checked the right places, and it also sounds like your S-W 150ED controls CA well.


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#9 CHASLX200

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Posted 17 September 2019 - 05:14 AM

You guys want color then take a peek thru a CR6 f/8.


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#10 Illinois

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Posted 17 September 2019 - 06:07 AM

I don't see CA in my SW150ED. Try better eyepiece like TeleVue and ES also good diagonal.
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#11 Jon_Doh

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Posted 17 September 2019 - 08:18 AM

If you want to check for CA remember the two V's, Vega and Venus.


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#12 daquad

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Posted 18 September 2019 - 06:51 PM

If you want to check for CA remember the two V's, Vega and Venus.

The problem with Venus is if it is not high enough in the sky then atmospheric dispersion gets in the way of assessing CA.


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#13 5ash

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Posted 28 September 2019 - 03:09 AM

Hi, have just joined this forum and have recently purchased a SW150ED . Am yet to test it out.will comment when I’ve had more time with it.


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#14 Illinois

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Posted 28 September 2019 - 06:14 AM

5ash....Welcome to CN! Few days ago, Jupiter and Saturn look great and sharp in my SW150ED at 218 power (ES 5.5 100 degree eyepiece) No CA! Saturn's band and Cassini Division in ring is easy to see and sharp!

Edited by Illinois, 28 September 2019 - 06:16 AM.

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#15 tog

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Posted 28 September 2019 - 12:00 PM

Saturn and Jupiter are too low from my viewing area. I need to pack up and head to higher ground.



#16 daquad

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Posted 28 September 2019 - 12:28 PM

Saturn and Jupiter are too low from my viewing area. I need to pack up and head to higher ground.

Higher ground won't help.  You need a lower latitude.  I have the same problem.


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#17 Tyson M

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Posted 28 September 2019 - 02:49 PM

Hi, have just joined this forum and have recently purchased a SW150ED . Am yet to test it out.will comment when I’ve had more time with it.

You're going to love it!  Welcome to the forum!



#18 5ash

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Posted 29 September 2019 - 12:28 AM

That doesn’t sound like CA. It sounds like lateral color in the Stratus. Not uncommon with wide angle eyepieces. The other eyepieces didn’t have a wide viewing angle.

CA is typically a blue halo. LC is typically a narrow yellow/green fringe. A better quality wide angle eyepiece may eliminate the LC.

I found LC was more of an off axis abberation. CA would be visible anywhere. If it goes away when you put it in the center of the field, then it is LC, not CA.
Scott

  Hi, I’m not familiar with the term LC ? I understand the physics of CA and would like to understand what LC is.

                            Regards philip



#19 5ash

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Posted 29 September 2019 - 12:36 AM

             Am hopefully going to use a camera tonight to test out my SW 150ED. My best eye was damaged in a cataract operation 14 years ago and I can’t visually test it to my satisfaction. Anyway removing the variable  contributions an eyepiece makes to the image , should make for a better test.



#20 5ash

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Posted 29 September 2019 - 07:04 AM

               Have just come in after taking some pictures of Jupiter and Saturn with the SW150ED. Jupiter and Saturn were high in the sky early evening but seeing was average , observed Antares in and out of focus and it was all over the place. However did manage to get several captures of Jupiter and Saturn using an ZWOASI120MC-S and GSO 2x 2” Barlow. Stacked 1500 frames out of 15,000 using autostacker and tweaked the pictures in registax 6. I’m not a planetary imager so the results are as they are but pleasing for the seeing conditions. Am pleased with this part of my testing out the telescope. Notice a slightly discernible  bluish edge to the Jupiter picture.

                The pictures are in the 5ash album

                                                         Philip


Edited by 5ash, 29 September 2019 - 08:09 AM.

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#21 Galicapernistein

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Posted 29 September 2019 - 12:06 PM

               Have just come in after taking some pictures of Jupiter and Saturn with the SW150ED. Jupiter and Saturn were high in the sky early evening but seeing was average , observed Antares in and out of focus and it was all over the place. However did manage to get several captures of Jupiter and Saturn using an ZWOASI120MC-S and GSO 2x 2” Barlow. Stacked 1500 frames out of 15,000 using autostacker and tweaked the pictures in registax 6. I’m not a planetary imager so the results are as they are but pleasing for the seeing conditions. Am pleased with this part of my testing out the telescope. Notice a slightly discernible  bluish edge to the Jupiter picture.

                The pictures are in the 5ash album

                                                         Philip

I envy anyone who can view Jupiter and Saturn high in the sky right now. 



#22 j.gardavsky

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Posted 29 September 2019 - 02:23 PM

LC, CA

 

An easy test is to use an UHC filter with enough transmission in red, like the Astronomik,

and to aim the scope with the EP and filter mounted against some bright stars.

 

When the stars makes a blue and red "double" away from the center field, and a symmetric double on the other side of the center field, then you have LC (lateral color chromatic aberration).

 

Lots of the multilens eyepieces flip the blue chromatic aberration of the achromatic refractor away from the red CA and away from the green sweet spot focus. This results in the blue hue around bright stars, Moon, and some bright planets. This is an ugly form of the CA during the observing sessions.

You can eventually return this flip inserting a Barlow.

 

Some eyepieces designed in past for the achromatic optics correct to some extent the blue CA,

but much less the red CA. And again, differently on different refractors.

 

Even if I have run some tests in past to find out the EPs harmonizing with my refractor,

there is no all-purpose recipe, sorry

 

JG


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#23 5ash

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Posted 30 September 2019 - 08:10 AM

Thanks JG for the information and how to test for LC , will take it on board.

                       Thanks, philip


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#24 aa6ww

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Posted 01 October 2019 - 02:52 PM

Vega is straight up right now in the early evening. It should be awesome in your scope with no issues with "fake news" color.

...Ralph

#25 5ash

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Posted 01 October 2019 - 06:17 PM

Vega is straight up right now in the early evening. It should be awesome in your scope with no issues with "fake news" color.

...Ralph

Hi Ralph,

                Unfottunately Vega is setting early here in Oz . My eyepiece collection is not very exciting and they will have some issue with CA ,that’s why I’m testing this scope with a camera. What was your impression of the pictures of Jupiter and Saturn taken through the scope ( 5ash gallery))

                                              Philip


Edited by 5ash, 01 October 2019 - 06:19 PM.



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