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Old 17-09-2013, 08:01 PM
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cometcatcher (Kevin)
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Do achromatic refractors still suffer CA with RGB imaging?

So we were having a discussion over here http://www.iceinspace.com.au/forum/s...wpost&t=112179 and the question was asked if achro refractors can be cured of CA by RGB imaging.

I know achros can do narrow band without CA but what about RGB (with good quality filters not GSO ones )?
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  #2  
Old 17-09-2013, 09:52 PM
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Should be ok as long as you refocus for each filter.
Geoff
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Old 18-09-2013, 05:02 PM
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Yes you must re-focus between filters, and that will be a pita unless you automate. As for the results - depending on the quality of the glass the blue image will be bloated compared to the R and G even at best focus. You can compensate for this by softening the R and G - that is trade sharpness for colour - then use an L frame but the results will not be as good as with an APO.
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Old 18-09-2013, 05:43 PM
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Quote:
Originally Posted by JohnH View Post
Yes you must re-focus between filters, and that will be a pita unless you automate. As for the results - depending on the quality of the glass the blue image will be bloated compared to the R and G even at best focus. You can compensate for this by softening the R and G - that is trade sharpness for colour - then use an L frame but the results will not be as good as with an APO.
You cant shoot luminance with an Achro, this is its main downfall imo. As for individual rgb frames I cant see why they wouldn't work. The thing is that you are just going to reduce the bloating of each colour by reducing the wavelength range that gets to the sensor, the individual colour filters let through a range of wavelengths and these will still come to focus at different points.
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Old 18-09-2013, 05:57 PM
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Originally Posted by Peter.M View Post
You cant shoot luminance with an Achro...
Thereby upholding the no-such-thing-as-a-free-lunch principle
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Old 18-09-2013, 06:45 PM
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I've noticed the blue is bloated even with best focus.

No free lunch with an APO either, as they are more expensive to purchase in the first place. Reflectors have free lunch, oh wait they have coma.

Looks like every system has a catch somewhere.
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Old 18-09-2013, 07:21 PM
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Personally would have thought that if you go to the expense of buying good filters and a mono camera putting to put it on an Achro would be a waste, I mean a good set of filters will set you back the cost of a cheap APO. I would rather no name filters and at least a semiapo
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Old 18-09-2013, 07:45 PM
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Yes I agree. The question was hypothetical.
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Old 19-09-2013, 06:20 AM
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Kevin,
The "blue bloat" may be due to UV. Depends on your camera sensitivity. A UV cut filter will improve the results.
Luminance and re-focussing are the main issues.
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Old 19-09-2013, 07:38 AM
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Okay Ken thanks.
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Old 11-12-2014, 06:29 AM
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I have always wondered about refractors for imaging.
It looks like results are worse if you take luminance.

It seems that you can only take RGB for the sharpest shots?

It seems also that Takahashi has a solution as discussed on another thread here.
http://www.iceinspace.com.au/forum/s....php?p=1138642

- a 5 element design:
http://www.astromart.com/classifieds...fied_id=870387

http://www.kyoei-tokyo.jp/shopdetail/000000006021/

cheers
Allan
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Old 11-12-2014, 07:25 AM
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Quote:
Originally Posted by cometcatcher View Post
.... Reflectors have free lunch, oh wait they have coma.

Looks like every system has a catch somewhere.
Actually coma is easily managed and at low cost. Imaging with a f/5 reflector it is not an issue with a Baader MPCC stuck on the front of your camera. It is true that short f number reflectors become more problematic but really do you need a f3.9 with collimation and coma issues which only gain you slightly shorter subs?

I have tried to image with my iStar 127mm f/12 acrho refractor, which has far less of a CA problem than short ones. But honestly, a reflector is much easier with shorter exposures, and true colour rendition. You can also manage diffraction spikes by going to a curved spiders of they bother you. Frankly diffraction spikes don't bother me.

The comments above relate to DSO imaging, for high magnification planetary I would prefer a good ED APO refractor for improved contrast - despite the longer exposure required at higher f numbers.

Last edited by glend; 11-12-2014 at 07:52 AM.
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Old 11-12-2014, 07:27 AM
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Quote:
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Should be ok as long as you refocus for each filter.
Geoff
Long time ago I had an Orion ED80 so a doublet. Very neat optics. Narrowband was great. RGB with an OSC wasn't so good. Blues were pretty out of focus when red was in focus and green close to focus.

For this reason I bought an APO triplet. That sorted it out. Everything was sharp and in focus. As Geoff said, you need to refocus for each filter if you use a mono camera. I'd also venture to say that LUM will still be more bloated with a doublet because LUM contains all the colors so it's logical to assume an APO would give you a sharper LUM?
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Old 11-12-2014, 08:26 AM
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You can get an Aries Chromacorr which corrects at the eyepiece end thus requiring much smaller lenses. I don't know how workable they are as part of the problem is not just colour aberration but other aberrations that are not corrected for in non APO lenses. But I did read some good reviews of them at one stage. I believe there was a 2nd version at least of these.

Greg.
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Old 11-12-2014, 09:08 AM
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I tried the RGB experiment with my SW120 (imaging at f/4). Like Marc, I found that blue was the most problematic colour - it really caused bloating (it had UV cut), even with careful attention to focusing. Green was sharp, and red was a little hazy. This is compared to nice, sharp images with even 12nm narrowband filters.

So overall, I considered the achro RGB experiment a failure, and the NB a success.
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Old 11-12-2014, 09:55 AM
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By their very nature, typical achromatic refractors would be no good with most types of imaging, including RGB. As noted already, the blue end of the spectrum is the most problematic. Achromats fail to keep blue in a tight line, the blue spectrum is spread out, with violet end the least tight, or more scattered. No matter of blue filters or violet minus filters will help here as the entire blue spectrum is spread out from the start.

Then, depending on the individual optical arrangement, eg, 80mm, 120mm, etc, focal length/focal ratio, the glass used, the number of elements, other colours of the spectrum will respond differently too, some blurry others pin sharp. This too will determine the ultimate quality of APO's too, and this has been noted here too.

The achromatic refractors that work best for imaging are oil spaced long focal ratio instruments, and can be doublet or triplet. These are able to control spectrum scatter very well, as well as high quality APO's. But these instruments today are rare as they are typically larger aperture too, making them both monstrous in size and price. Smaller, high quality APO's have made these redundant.
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Old 11-12-2014, 06:55 PM
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I agree with these posts. An achro's real use is narrowband which is the same for some camera lenses - they are best for narrowband where the lack of colour corrections is not an issue as only a very narrow band of the spectrum is being imaged at a time.

Greg.
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Old 11-12-2014, 07:29 PM
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Quote:
Originally Posted by gregbradley View Post
I agree with these posts. An achro's real use is narrowband which is the same for some camera lenses - they are best for narrowband where the lack of colour corrections is not an issue as only a very narrow band of the spectrum is being imaged at a time.

Greg.

Hi Greg,
Even with your APO TEC 180 -
did you ever find that the Luminance wasn't as sharp as the RGB?

cheers
Allan
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