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Old 10-10-2014, 09:41 PM
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Reducing blue bloat/flare/halo

Finally acquired a Pentax SDUF II f/4 100mm astrograph (the fore-runner to the modern Vixen VSD 100 f/3.8 system, which is 5 elements instead of the Pentax's 4). Should keep me entertained for a while...hopefully.

Being a 4 element Petzval, with an SD element (Super Extra-low Dispersion glass), it is well corrected throughout the spectrum - especially red - EXCEPT for blue. Blue in images acquired with these show blue flare/bloat/halo.

Will a Baader semi-apo filter help in IMAGING (I don't care visually) to reduce the blue menace?

I ordered one from myastroshop anyway -guess it can't hurt to try!
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Old 10-10-2014, 11:27 PM
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Quote:
Originally Posted by LewisM View Post

Will a Baader semi-apo filter help in IMAGING (I don't care visually) to reduce the blue menace?
Not by much Lewis. See this post. http://www.iceinspace.com.au/forum/s...9&postcount=11
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Old 11-10-2014, 01:02 PM
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Lewis,

I'm facing the same problem with my TEC140 and Trius combination. Mind you with my G2-8300 camera I don't have an issue. But, the Trius with the extra sensivity in blue/violet/UV is a problem. I'm using a Baader blue filter which transmits to 380 nm.

So far I have tried the Hutech IDAS LP (cuts around 410 nm) and the Baader Semi Apo. The Hutech certainly helps and the Baader helps more. So far my tests are very preliminary but I believe the overall colour balance is a bit better using the Hutech and the Hutech transmits somewhat more light in blue and other frequencies. But stars are much tighter with the Baader. I think the Semi-Apo is probably overkill. The Baader Fringe might be just as good and not cut as much light at other freq. compared to the Semi-Apo. I also have two filters coming from Astronomik (two mounted Schott filters (GG420 and GG435), and the Baader Fringe Killer. Once I have all the filters it should be straight forward to test for the best. I will have all of them in a single filter wheel and can do direct comparisons.

Your refractor probably has a different correction to the TEC140 so I have no idea if my results will help you. But stay tuned!

Peter

Also preliminary, but in blue using the Hutech or Baader it looks like overall exposure would need to increase by 40%+ to reach the same level of saturation.
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Old 11-10-2014, 01:35 PM
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Show us a pic Lewis so we can gauge the extent of the problem.
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Old 11-10-2014, 02:09 PM
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might consider experimenting with a Wratten 2E - it is probably very similar to the 420 filter that Peter is getting, but is a gelatin photographic filter. Have never dealt with the supplier, but was the only Aus source I could find. http://www.photovideo.com.au/100mm-w...%20Wratten%202

Last edited by Shiraz; 11-10-2014 at 03:08 PM.
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Old 11-10-2014, 09:58 PM
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This is from the new VSD100, but the SDUF gives similar results with maybe a tiny bit more bloat/flare in blue but better reds

http://www.teleskop-express.de/shop/...o-ophiuchi.jpg
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Old 12-10-2014, 02:44 AM
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Looks alright to me?
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Old 12-10-2014, 07:22 AM
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Lewis,

If that is your example of blue bloat I think we are really talking about different things. I don't see exactly what you are talking about. As best I know you are probably looking at the blue halos around the blue stars. I think this is entirely normal as those blue stars are saturated and won't colour in the center - only the fringe will colour and certainly any saturated star is going to be larger.

Have you done your own images? Do you see blue stars with bloat? What blue filter and where does it cut? If you read my earlier thread on the problem you can see the issue I'm having. Is yours similar? On my images blue stars, even unsaturated, are out of focus looking. Adding a cut filter makes the stars come into focus much better, but only with the Trius camera. With my KAF8300 CCD I don't see enough of the issue to even care.

Peter
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Old 12-10-2014, 09:23 AM
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Peter,

The unit has not arrived as yet, but going by all the reviews and discussion out there, getting ahead of myself. Even the new Vixen VSD remake is criticised in the latest Sky at Night review for blue star bloat, not coming to focus with this system, and the SDUF ancestor did/does the SAME thing.

S@N continued to say the OSC/DSLR images show blue defocus, but NB H-a and OIII were exacting.

I will try it when I get it TOMORROW (), but seeing I use an FLI ML 8300OSC, I have a little trepidation about the blue. Time will tell. Was hoping to be fore-armed, after being fore-warned
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Old 12-10-2014, 10:05 AM
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isn't an apo optical system meant to bring visible light to focus at a common focal point?
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Old 12-10-2014, 10:22 AM
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Lewis, the solution is blowing in the wind my friend:

"Sell"

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Old 12-10-2014, 02:34 PM
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Oh, you want to buy it Andrew? Yours for $5000 and a bottle of Ouzo

Richard, indeed, but these seem to be 2/3rd's apochromatic Better than semi-apo I guess

Even the FSQ106 doesn't QUITE reach perfection - it too seems to show a fair bit of blue flaring (less apparent in the fluorite versions).
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Old 12-10-2014, 02:43 PM
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Mate, do what I did.
Buy yourself a TOA and be done with it.
You won't need another scope !

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Old 12-10-2014, 03:23 PM
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TSA-102 in Sydney Customs already - go pick it up for me please Andrew
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Old 12-10-2014, 06:19 PM
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Keep all these imperfections coming chaps, as a cat fancier who dreams of the mythical perfect refractor it's good to see the grass isn't necessarioy greener...
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Old 12-10-2014, 07:12 PM
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No filter will give you the perfection you are after Lewis.
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Old 12-10-2014, 07:57 PM
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Quote:
Originally Posted by rmuhlack View Post
isn't an apo optical system meant to bring visible light to focus at a common focal point?
An achromat brings two wavelengths to focus at the same point. An apochromat brings three wavelengths to focus at the same point. The laws of Physics prevent a refractor from being perfect, alas, but you can get closer at long f/ratios. At short f/ratios even an apo has noticeable shortcomings. We do out best to fix those in PixInsight

Cheers,
Rick.
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