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Old 05-05-2019, 02:56 PM
benklerk
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Star Bloat Issue

I recently got my first APO and I got star bloat with my Blue Filter, haven't used LUM yet. Camera is ZWO 1600mm

1) Is it an Issue with the Skywatcher 72ED?
2) I have the ZWO new 1600mm LRGB filters, would this be cause?
3) How to fix it in pixinsight?
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  #2  
Old 05-05-2019, 03:27 PM
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Atmos (Colin)
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Being a semi fast doublet it won’t be fully corrected in the blue so this is what you’d expect
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Old 05-05-2019, 04:56 PM
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Merlin66 (Ken)
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Do the ZWO filters have any UV-IR coating?
How does the B image compare (bloat wise) with say the R image?
(I had to fit a UV-IR filter to my DMK cameras when I set them up with the 60mm f3.8 electronic finder)
Have you tried to refocus the B filter??
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Old 05-05-2019, 05:03 PM
benklerk
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Quote:
Originally Posted by Merlin66 View Post
Do the ZWO filters have any UV-IR coating?
How does the B image compare (bloat wise) with say the R image?
(I had to fit a UV-IR filter to my DMK cameras when I set them up with the 60mm f3.8 electronic finder)
Hi Ken

The Red image is fine, no bloat that I can see. I don't know if the Blue has UV-IR coating.
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Old 05-05-2019, 05:05 PM
CeratodusDuck (George)
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The Skywatcher 72Ed is an ED doublet - it is not apochromatic. Semi-apochromatic at best. Yes, you will get blue bloat with most doublets (some exceptions in exceptionally well corrected systems, but they cost a lot for doublets).


I do hope the dealer you bought it rom did not claim they are apochromatic. Merely having ED elements does NOT an apochromat make, nr does a triplet assure apochromatism (many Chinese triplets are not apochromatic)
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Old 05-05-2019, 05:13 PM
benklerk
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Quote:
Originally Posted by CeratodusDuck View Post
The Skywatcher 72Ed is an ED doublet - it is not apochromatic. Semi-apochromatic at best. Yes, you will get blue bloat with most doublets (some exceptions in exceptionally well corrected systems, but they cost a lot for doublets).


I do hope the dealer you bought it rom did not claim they are apochromatic. Merely having ED elements does NOT an apochromat make, nr does a triplet assure apochromatism (many Chinese triplets are not apochromatic)
My dealer did claim that is was a apochromatic refractor . Its even on their website.
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Old 05-05-2019, 05:16 PM
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I think there is objective pinching in your image too - something peculiar about the artefacts around Lambda Centaurus
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Old 05-05-2019, 05:23 PM
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I have a few (!!) ED80 scopes which I use for spectroscopy...
I find by using a narrowband B type filter and refocusing I can get smaller star images on the spectroscope slit.
I have some good spectra going down to 3700A, so getting a non bloat blue image is not impossible with an ED.
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Old 05-05-2019, 05:27 PM
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It's a doublet that's faster than F/6, it's going to have reasonable correction but it isn't going to be up to the standard of any F/6 triplet so you will get bloat.
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Old 05-05-2019, 05:34 PM
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It's a doublet that's faster than F/6, it's going to have reasonable correction but it isn't going to be up to the standard of any F/6 triplet so you will get bloat.
I agree there Colin. What George said is pretty true in most respects as well.

Having said that, I have seen some horrendous bloat in many North Group/Explore Scientific/Meade triplets.
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Old 05-05-2019, 05:34 PM
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Quote:
Originally Posted by benklerk View Post
My dealer did claim that is was a apochromatic refractor . Its even on their website.
Which dealer?
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Old 05-05-2019, 05:39 PM
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Quote:
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I agree there Colin. What George said is pretty true in most respects as well.

Having said that, I have seen some horrendous bloat in many North Group/Explore Scientific/Meade triplets.
I completely agree with you there, even those are F/7 I believe. They have the colour correction of a doublet but also have the price of one so you see getting what you pay for.
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Old 05-05-2019, 05:44 PM
benklerk
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Quote:
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Which dealer?
Andrews Communcations
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Old 05-05-2019, 05:50 PM
benklerk
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Looks like I'll be doing NB wide field until I get my obs built.

But I'll use the 72ED as my new guidescope to replace my cheap ZWO one later on for my GSO 6 RC.

Ive been looking. Skywatcher Espirt or William optics as a good wide field refractor. Im looking at one to match my 1600mm.
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Old 05-05-2019, 05:53 PM
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Quote:
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I think there is objective pinching in your image too - something peculiar about the artefacts around Lambda Centaurus
The artifacts is from the 1600mm, it doesn't have AR coating on the senor so you get microlensing around bright stars.
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Old 11-05-2019, 08:43 AM
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I totally agree with what have been said about colour correction and microlensing.

Here is an example of a similar diffraction pattern caused by microlensing with the same sensor (from Jon Rista's website: https://i.imgur.com/2ntiE8T.jpg

And my attempt at LRGB imaging with a 102mm ED doublet: https://www.astrobin.com/full/105505/0/

Same target, same equipment, but narrowband: https://www.astrobin.com/full/105131/C/

Also, 71mm aperture diffracts light more significantly than larger apertures and this will show as less-pinpointy (read: bloated) stars, unless larger pixels are utilised and we err on the side of under-sampling.

To sum up, IMHO, unless one does wide field under-sampled LRGB imaging, narrowband, from my experience, yields more pleasing results with a small ED refractor than RGB.
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Old 11-05-2019, 01:17 PM
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To be fair to Andrews, Skywatcher describe it as an Apo.

I am using an OSC camera but I Just had a look at a sample sub and I reckon the green is showing the biggest stars in my case.

Last edited by The_bluester; 11-05-2019 at 09:15 PM.
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Old 11-05-2019, 08:19 PM
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Quote:
Originally Posted by benklerk View Post
My dealer did claim that is was a apochromatic refractor . Its even on their website.
That's not an APO quality image.

I have, well, a lot of APO's (I'm a lucky guy) and they do not do that to the Blue...or any other channel...bloat is what APO's simply don't do.

Not fit for purpose? Maybe.

Australian consumer law is pretty clear on that....and you are entitled to to replacement or refund.

But only your satisfaction is guaranteed...and you might struggle to find a APO...i.e. by definition, colour-free refractor at the same price-point.
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Old 11-05-2019, 09:29 PM
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I think the honest answer is that the description of "APO" capabilities is being/ has been depreciated.
"near enough is good enough"
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Old 12-05-2019, 10:23 AM
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Quote:
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I think the honest answer is that the description of "APO" capabilities is being/ has been depreciated.
"near enough is good enough"
While I doubt the ACCC will do anything (a remarkably useless organisation IMHO) you could take the respective manufacturers to task. Meade tried to cash in on the RC bandwagon some years ago and pass their modified SCT's off as Ritchey's. They got sued in The US, and despite some creative posturing, Meade lost the case. Meade were forced to rebadge the product as their ACF range.

Perhaps so called APO manufacturers need the same lesson.

Last edited by Peter Ward; 12-05-2019 at 11:17 AM.
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