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??
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.
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)
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.
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.
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.
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.
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.
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
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.
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.