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Post by nobody on Oct 25, 2015 7:56:33 GMT -5
Hi I have already opened up this question with Mikey Cee on CN. Though there might be non-members of CN here and the information might be even more useful here under the iStar anastigmat banner. How does the very different CA correction of Rx objectives respond to Baader's Fringe Killer and others of their ilk? I noticed a narrow blue-green fringe on the lunar limb in first tests but none of the violet wash in the shadows common to normal achromats. A Yellow No8 had little or no obvious effect. I haven't tried my Fringe Killer yet. Perhaps other filter's would be more beneficial? A CA-ratio of 2.3 is marginally under-corrected for a 180mm, 7" f/12 [f/16 R35 amended] focal ratio by Sidgwick's standards. Any thoughts?  |
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Post by Ales - iStar Optical on Oct 25, 2015 14:45:25 GMT -5
Wonderful posts Nobody! Thank you so much for your continuous support of iStar Scope Club and for posting some very good questions and comments.
Regarding this filter issue. Let me ask our optician directly. He suggested couple of filters in the past but I never followed thru with this info and testing (shame on me...). So I will post his response as soon as he sends it to me.
cheers,
Ales
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Post by nobody on Oct 26, 2015 1:11:16 GMT -5
Wonderful posts Nobody! Thank you so much for your continuous support of iStar Scope Club and for posting some very good questions and comments. Regarding this filter issue. Let me ask our optician directly. He suggested couple of filters in the past but I never followed thru with this info and testing (shame on me...). So I will post his response as soon as he sends it to me. cheers, Ales Hello Ales Thanks for your kind response. I'll look forwards to the expert's advice. I have a Fringe Killer but conditions were too poor to try it last night. There are a number of other filter options with different cut and pass characteristics. I should emphasize that the 180/12 R35 presents no serious CA visually. Just a very narrow blue-green fringe on the limb of a very low Moon. Besides, I have always considered that CA is seeing condition dependent. There is usually room for improvement when squeezing every last drop of detail and contrast out of an objective lens. If a particular filter proves to be beneficial it can be fitted in the star diagonal to save time swapping it between eyepieces. Regards Chris |
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rgm
Full Member
Posts: 65
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Post by rgm on Oct 26, 2015 6:31:35 GMT -5
I have a 2" filter wheel on my R30. I like to use filters mainly for double stars. I use the Lumicon Minus Violet, #8 yellow, Fringe Killer and the Baader 496. The Minus Violet does a nice job of cleaning things up a little bit without changing the overall view, keeps it natural. The 496 is excellent for getting rid of everything, but does leave a yellow tint. I do not use the Fringe Killer that often, but it is there to experiment with when necessary. I find the #8 and Fringe Killer to be in the middle and both leave a yellow tint.
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Post by nobody on Oct 27, 2015 13:23:40 GMT -5
I have a 2" filter wheel on my R30. I like to use filters mainly for double stars. I use the Lumicon Minus Violet, #8 yellow, Fringe Killer and the Baader 496. The Minus Violet does a nice job of cleaning things up a little bit without changing the overall view, keeps it natural. The 496 is excellent for getting rid of everything, but does leave a yellow tint. I do not use the Fringe Killer that often, but it is there to experiment with when necessary. I find the #8 and Fringe Killer to be in the middle and both leave a yellow tint. Thanks, that useful information. I haven't tried a 496 but wonder whether it doesn't transmit the precise wavelengths where the blue-green fringe occurs? I tried my Fringe Killer on the [nearly] full moon last night but it seemed to have little or no effect either visually nor while taking my handheld afocal snaps. I have posted four further images on my blog to show progress at higher viewing altitudes. [35 degrees at 23.00 CET.@55n] fullerscopes.blogspot.dk/2015/10/7-f12-istar-refractor-24-early-results.html This image is with the Fringe Killer on a 20mm Plossl EP. I think you will agree that there is very little false colour to worry about here. |
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Post by mikey cee on Oct 27, 2015 18:14:54 GMT -5
Moon shots are getting to be old hat with showing the false color routine that's gone on all too long...sorry. Everywhere I see it on a constant level. I can get a color free shot of any moon phase with any dslr or cell phone that shows ZERO color fringing! Moon shots prove absolutely nothing in this area! Mike
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Post by nobody on Oct 28, 2015 1:32:39 GMT -5
Moon shots are getting to be old hat with showing the false color routine that's gone on all too long...sorry. Everywhere I see it on a constant level. I can get a color free shot of any moon phase with any dslr or cell phone that shows ZERO color fringing! Moon shots prove absolutely nothing in this area! Mike Hi Mike I feel I must disagree with you unless I have missed the precise point of your argument. Are you suggesting that it is the camera at the eyepiece which produces, or reduces, false colour in afocal lunar images? Should I dump the [specially chosen, short zoom] compact camera and invest in a DSLR or costly camera 'phone to improve my images? I started taking handheld moon shots with my 6" f/8 achromat and it certainly showed plenty of false colour at times. The practice also made an interesting hobby in trying to obtain the sharpest shot using only a handheld compact camera. It proved equally useful in trying to capture the planets, Sun and transits. [More successfully on the Sun and transits.] "Old hat" is certainly not how I would describe a very accessible means of astro photography. One that literally anybody with a telescope [or even binoculars] and a camera can try for themselves. The result is an image, or images, which can be easily shared using equipment that already exists without a huge investment. Even an altazimuth mounting and almost any type of telescope will do if the user is patient and perseveres. Since the camera cannot lie [until the image is digitally modified] it [probably] gives a better representation of false colour than the human eye. It is well established that, with age, the human eye lens yellows. Which probably explains the majority of disagreement over false colour. One man's "APO-like correction" is another's "purple rain." |
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Post by mikey cee on Oct 28, 2015 13:09:50 GMT -5
I simply meant that with the correct exposure you can "hide" CA very easily either intentionally or accidentally, nothing more than that. I've done it myself and I know. The point is you can either blow the color way out there or under expose to show none. Because by this very nature people can and then doubt whether a person's scope really performs as they say it does. This is in no way impugning your credibility as I myself would fall under the same scrutiny. It is just the nature of the beast....lunar photography concerning CA. Mike
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