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Old 17-01-2017, 04:35 PM
Gordo (Daniel)
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Chromatic Abberation - Celestron Nexstar 127 SLT

Hi! Long time lurker, first time poster.

For Christmas I was gifted a Celestron 127 SLT, my first telescope. Setting it up on our balcony (Inner West Sydney), terrible light pollution, I was pretty amazed to be fairly clearly make out M42 with the supplied 25mm eyepiece.
Looking for another challenge, I attempted to resolve some double stars with the supplied 9mm plossl.
However, whenever I get close to focus I get a substantial amount of Chromatic Aberration which makes it impossible to focus, much less resolve any doubles
I'm guessing this may be due to the kit eyepiece? Or would it be lack of experience/aperture, or all of the above?
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Old 17-01-2017, 10:17 PM
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doppler (Rick)
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Hi Daniel, it wouldn't be chromatic aberration as such but just a lot of atmospheric turbulence or "bad seeing". You will find that some nights are better than others for viewing the stars. Your scope and eyepieces are quite up to the task, you just need practice and perseverance.
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Old 17-01-2017, 10:39 PM
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AstroJunk (Jonathan)
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Agreed. It is not unusual for telescopes to be limited by turbulence in the atmosphere - it's what makes the stars twinkle, and it gets worse the closer to the horizon you look.

On a lovely still night that telescope will come good and provide great images at 167x with that 9mm eyepiece. Get up early and enjoy Jupiter. Often the atmosphere is more stable then too.
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Old 18-01-2017, 04:30 PM
Gordo (Daniel)
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Thanks for the replies! I hadn't thought about the seeing conditions, that would definitely be a factor, is it usual in bad seeing for colours to diffract differently? I didn't mention it in the OP, but I was seeing the blues separate from the rest of the image as I approached focus
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Old 18-01-2017, 06:26 PM
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doppler (Rick)
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Seeing that you have a Mak Cassegrain, it should be virtually free from Chromatic aberration which is more of an achromatic refractor problem.
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Old 18-01-2017, 07:36 PM
Wavytone
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Gordo, depending on where you are I'd suggest bring the scope to an observing night of the NSAS at Terrey Hills (see their website for details) and the experienced guys will sort out whats going on. You'll probably learn heaps too. This is arguably the best site in metropolitan Sydney, bar none.
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Old 19-01-2017, 06:03 AM
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sil (Steve)
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I have the 102 SLT and got the same chromatic aberration. it IS the included eyepieces which are appallingly bad but the OTA itself responds well with good eyepieces. In my case I used some Baader Hyperions. Doubles with the BH 8mm are crisp and easily separated. Beyond 8mm though seeing conditions quickly make it hard to focus well.
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Old 19-01-2017, 05:13 PM
Gordo (Daniel)
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Thanks again for the replies!

It's refreshing to know someone else had a similar issue and was able to narrow it down to the provided eyepiece. As an amateur photographer I understand the importance of having good 'glass', so was intending to on 'upgrading' anyway - I guess I'll be doing that sooner for my high powered eyepiece

Thanks for the info Wavytone, hopfully I can make it out there sometime soon
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Old 23-01-2017, 06:43 AM
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sil (Steve)
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The "good glass" is in the eyepieces. And every cheap telescope I've tried a better eyepiece on has improved the viewing sharpness and contrast dramatically. Tripods and eyepieces included in a telescope package are where they save money most noticeably.
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Old 24-01-2017, 05:32 AM
AEAJR (Ed)
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The Celestron site does not say what kind of eyepieces are included with the scope. I am assuming they are some kind of Kellner eyepieces.
http://www.celestron.com/browse-shop...ized-telescope

As for colors. The lower to the horizon you are viewing the more atmosphere you are looking through. Anything less than 30 degrees above the horizon is going through a lot of air. Take a look at things above 45 degrees and see if you see less of this "chromatic aberration".
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Old 24-01-2017, 12:25 PM
Gordo (Daniel)
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Thanks again for the replies

Unfortunately the weather here has been tewrrible all year, so I've only had a few chances to take the scope out, but learning more each time.
Last night I took the scope out and learnt what bad seeing looks like
The previous night I took the scope out and had some pretty decent viewing. With the help of the Goto mount I was able to learn about and view 145 Canis Majoris quite clearly with both eyepeices.
It was only when returning to Rigil, which was higher up, that I encountered the same issue again - so I'm thinking possibly the brightness of the target has some bearing on this...

Regardless I'm looking to get a new eyepiece in the very near future (most likely TV 8mm Delos), so I'm intrigued to see how much difference there is the next chance I get to take the scope out
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Old 24-01-2017, 10:34 PM
bigjoe (JOSEPH)
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Re colours

Quote:
Originally Posted by Gordo View Post
Hi! Long time lurker, first time poster.

For Christmas I was gifted a Celestron 127 SLT, my first telescope. Setting it up on our balcony (Inner West Sydney), terrible light pollution, I was pretty amazed to be fairly clearly make out M42 with the supplied 25mm eyepiece.
Looking for another challenge, I attempted to resolve some double stars with the supplied 9mm plossl.
However, whenever I get close to focus I get a substantial amount of Chromatic Aberration which makes it impossible to focus, much less resolve any doubles
I'm guessing this may be due to the kit eyepiece? Or would it be lack of experience/aperture, or all of the above?
Hi Gordo .I concur with Ed- People often use too much power on objects low on the horizon. Things look so much better above 45* its not funny!
Your ep should be passable (prob 3 elements) but a proper plossl or ortho would beat it handily. The 127 Mak is a little cracker usually; they are bulletproof, so seeing and a better ep will help greatly
Cheers bigjoe.
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