#21  
Old 12-05-2019, 01:43 PM
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LewisM
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William Optics got roasted over their fraudulent claim of the FLT line of scopes being fluorite (they most assuredly are not) and have had to revise their advertising.

The main issue is an urban myth that ED glass = apochromatic, which it most certainly DOES NOT. The ED=apochromat needs to die the death it deserves, but manufacturers and retailers perpetuate this falsehood ad infinitum.Its got to the stage almost when any refractor is being labelled “apo”.

Caveat emptor is do research and know a modicum of the physics of optics.
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  #22  
Old 12-05-2019, 03:30 PM
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The_bluester (Paul)
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Or alternately go in with open eyes (I did) knowing that a scope costing $500 is not going to perform the same as one costing five times as much.

In my case it is being used with an OSC cam and performs quite acceptably, and most important for my purposes, I could afford to lash out for it guilt free.
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  #23  
Old 12-05-2019, 04:08 PM
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Merlin66 (Ken)
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Guys,
In "Telescopes, Eyepieces, Astrographs" by Hallock-Smith, Ceragioli and Berry
Chapter 6.2 details the design and construction of a TWO element APO (three wavelength corrected, Strehl +/_ 0.9 or better) using an ED element....

FPL53, Ohara flour-crown (the same as used in the SW ED series) combined with Schott's zinc crown ZKN7 as a flint element.
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  #24  
Old 12-05-2019, 04:42 PM
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Atmos (Colin)
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Quote:
Originally Posted by Merlin66 View Post
Guys,
In "Telescopes, Eyepieces, Astrographs" by Hallock-Smith, Ceragioli and Berry
Chapter 6.2 details the design and construction of a TWO element APO (three wavelength corrected, Strehl +/_ 0.9 or better) using an ED element....

FPL53, Ohara flour-crown (the same as used in the SW ED series) combined with Schott's zinc crown ZKN7 as a flint element.
What's the focal ratio though?
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  #25  
Old 12-05-2019, 05:10 PM
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Merlin66 (Ken)
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Colin,
That was based on an f15 example.
(Bearing in mind the three element oil/ spaced designs also given, ar f15, f10 and reach the "acceptable" limits at f8.
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  #26  
Old 12-05-2019, 08:32 PM
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What's the focal ratio though?
Touche' !
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  #27  
Old 02-06-2019, 03:08 PM
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Yeah welcome to the world of APO's and pretend APO's for marketing reasons.

There is a reason the top tier APO makers are so expensive. Good APOs are hard to make.

So a $500 scope was never going to cut the mustard although the old Orion ED80 was pretty good for the money.

As a general rule in astro buying I think its better to have less but what you have is high quality. You keep the items and get the enjoyment out of the hobby.

The cheaper item route may be forced on us by economics but it is usually more expensive long term as you are never fully satisfied, sell it at a considerable loss and end up getting the better gear later anyway.

Not an easy policy to implement though.

Greg.
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  #28  
Old 03-06-2019, 11:29 PM
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cometcatcher (Kevin)
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Hi Ben. I have the same problem as you but with a slightly different setup. I have a Kson ED80 and ZWO ASI 183mm Pro. With blue filter it is badly bloated. The red and green filters are fine. These scopes are not fully APO quality I'm afraid.

I'm in the process of experimenting with additional filters. So far I have found the Baader Semi-Apo filter stacked over the blue filter to help a lot. I will keep experimenting to see how it turns out.
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  #29  
Old 04-06-2019, 09:25 AM
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Just a thought with the ED72 based on something I recently observed. I have not had a chance to re test yet. I was getting some odd bloating with mine (With an OSC camera) on bright stars, most noticeably the redder end of the spectrum.

While it was still in summer, I had one night that cooled rapidly and had really heavy dew and the scope dewed up really badly afte I went to bed (I had not got a heater on it at the time, who would think you would need it with a string of 30 degree days?) and it appears that it managed to wick moisture in between the objective elements (Air spaced)

It might well be why I was seeing what looked like tilt in some of the corners as there was a nearly complete ring of liquid water between the elements around the edge so parts of the lens were air spaced as they should be and parts had liquid separating them. I used it last week and even with a heater it dewed up and I realised that there was condensation AND liquid in between the elements.

I figured I knew what Skywatcher's response to it would be warranty wise with obvious water between the objective elements so I took a breath and disassembled the objective to dry and clean it yesterday. I might go again, I can see a tiny bit of haze still there after cleaning with cotton balls and isopropyl alcohol (Even the Bintel magic juice was leaving a residue)

If I go again I will go cotton balls and isopropyl then a rinse off with plenty of distilled water and let them air dry, but it is pretty good so I might not bother. Any marks are really obvious when you have the lens cell out as each lens is in focus when viewed from the other side so any smudges or stray wisps of cotton ball are really visible. If flats deal with it I will leave it alone. if anyone is thinking it, it is hard to see the two elements being indexed together, there is an O ring under the locking ring and the first half a turn or so as you loosen it (And again when tightening it) rotate the first element under the tension of the O ring so indexing the two would be a tricky affair, there were no obvious index marks either.

Hopefully I see some clear sky to test it properly soon, with more heat to ensure it does not do it again!

Last edited by The_bluester; 04-06-2019 at 10:51 AM.
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  #30  
Old 04-06-2019, 10:15 AM
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This is my star bloat with the blue filter. It didn't seem as bad with the DSLR but the Mono cam seems to make it look worse. At F5.5 my optics are not fully corrected even though it's supposed to be an ED scope.
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  #31  
Old 04-06-2019, 10:26 AM
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Quote:
Originally Posted by cometcatcher View Post
Hi Ben. I have the same problem as you but with a slightly different setup. I have a Kson ED80 and ZWO ASI 183mm Pro. With blue filter it is badly bloated. The red and green filters are fine. These scopes are not fully APO quality I'm afraid.

I'm in the process of experimenting with additional filters. So far I have found the Baader Semi-Apo filter stacked over the blue filter to help a lot. I will keep experimenting to see how it turns out.
Hi Kevin

I have stopped doing LRGB on my ED72 but it works doing NB. HA,OIII and SII are all fine and don't have any star bloat. Its just the blue and lum filter for me.
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  #32  
Old 04-06-2019, 12:47 PM
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Quote:
Originally Posted by cometcatcher View Post
Hi Ben. I have the same problem as you but with a slightly different setup. I have a Kson ED80 and ZWO ASI 183mm Pro. With blue filter it is badly bloated. The red and green filters are fine. These scopes are not fully APO quality I'm afraid.

I'm in the process of experimenting with additional filters. So far I have found the Baader Semi-Apo filter stacked over the blue filter to help a lot. I will keep experimenting to see how it turns out.
You’re also using a VERY demanding camera for colour correction. Tiny pixels and very high QE stress even some of the better triplets on the market.
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  #33  
Old 05-06-2019, 09:53 AM
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You’re also using a VERY demanding camera for colour correction. Tiny pixels and very high QE stress even some of the better triplets on the market.
My next camera will be less demanding. Actually I'm having really good success with the Baader filter double stacked for RGB so I'll settle for that combo with the ED scope.
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  #34  
Old 05-06-2019, 01:46 PM
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Out of curiosity, I will plate solve the two images showing bloat so I can aim mine the same (When I next see some clear sky) and try for some comparison images with the OSC.
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  #35  
Old 05-06-2019, 07:23 PM
gb44 (Glenn)
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Kevin - a couple of questions, sorry if I'm missing the obvious

Kevin,
Does the blue channel bloat more than R or G given that you achieved perfect focus with each colour filter?

How much of a difference is there between focus for each filter? Does the Bahtinov mask work for each filter?

I ask as I have no experience with filters and am considering the difficulties with narrowband stuff .

Cheers
GlennB
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  #36  
Old 06-06-2019, 01:11 PM
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Kevin,
Does the blue channel bloat more than R or G given that you achieved perfect focus with each colour filter?

How much of a difference is there between focus for each filter? Does the Bahtinov mask work for each filter?

I ask as I have no experience with filters and am considering the difficulties with narrowband stuff .

Cheers
GlennB
Hi Glen, yes the blue filter bloats quite badly even though it's individually focused. I too thought I could just get away with focus for each filter, but blue bloats just the same.

There is little to no difference between focus for each filter. They are parfocal so normally don't need much, if any refocusing at all. But I did it anyway.

Yes the Bahtinov mask works for each filter.

This is just R/G/B not narrow band. I think why double stacking worked with my blue filter is that the Baader Semi-Apo clips the blue channel by 50%, thereby making it narrower than normal.
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  #37  
Old 09-06-2019, 03:11 PM
gb44 (Glenn)
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bloat isnt halo - Astronomik filter a solution?

I wondered if this might help.

https://www.astronomik.com/en/news/a..._halo_problem/

Here they discuss filter quality...might be worth considering?

Cheers
GlennB
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  #38  
Old 01-07-2019, 07:33 PM
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Coming back to this one, I was reading up and found a recommendation to use an IR cut filter with the camera I have so I gave one a try, and it made a massive difference to image quality. I have done a bit of trawling and found that the ZWO "IR" cut filter is a 400 to 700nm band pass just like they "UV-IR" cut camera windows thet sell.


Have you tried the blue with a UV or IR-UV cut filter in place?

Last edited by The_bluester; 02-07-2019 at 02:24 PM.
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  #39  
Old 02-07-2019, 02:17 PM
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The issue is probably short wavelength blue which is a bit out of focus compared to the "blue" focus point. Astronomik specifically designed their Deep Sky blue filter to deal with this issue. Compare the 2 different types of spectrums between the older Type 2 and Deep Sky sets. On my TEC140 the Deep Sky set clearly improved blue bloat.
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  #40  
Old 02-07-2019, 03:48 PM
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It just came to mind as I was looking at this thread again and noted that the passband on the ZWO LRGB filter set blue filter appears to start at around 375-380nm where the filter I have cuts at 400nm so any out of focus or scattering in the UV end would be reduced.
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