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LRGB using (U)BVRc(Ic) filters

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#1 Lucullus

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Posted 07 September 2015 - 08:35 AM

Hi there,

 

I am thinking of buying a filter set and need your advice. What I would like to do is, of course, pretty pictures - so LRGB filters is needed. But I like the idea of doing science as well once in a while - so UBVRcIc or SDSS filters are the way to go. With LRGB you can't do reliable science as with the latter. But the other way round might be possible. That's my question to you. How can you do it and is it the better way to go when buying a photometric filter set if science and pretty pictures are the goal?

Am I right with the assumption that UBVRcIc is still the most common today, but SDSS (and the like) is the future as they are manufactured having the CCD sensitivity in mind instead of the old photographic glass plates? What does this mean for the future of photometry: I guess one needs to apply SDSS to UBVRcIc conversions and the other way round to compare them, right? How future proof is it to get an UBVRcIc set now? What can be done with the UBVRcIc that is not possible with the SDSS?

 

Thank you for your insights. I am very interested in your information.

 

Lucas



#2 A VAN ZEGVELD

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Posted 07 September 2015 - 08:57 AM

If you want to do "reliable science" you will need a CCD-camera that is real LINEAIR.

If that is not the case, it is no use to try to do "reliable science" and you better go for pretty pictures.



#3 Lucullus

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Posted 07 September 2015 - 11:42 AM

Well, if that is your definition then no "reliable science" could be done with CCDs using anti-blooming gates, ABG. But in fact, this is possible as long as one stays in the linear, non-saturated ADU range. Thus, let's specify the topic with "reliable science in the CCD response of non-saturation", which limits the scientific usefullness of any CCD to its linear behaviour. Let's stick to the filter discussion with agreement to this background assumption.


Edited by Lucullus, 07 September 2015 - 01:06 PM.


#4 David Ault

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Posted 07 September 2015 - 01:44 PM

Lucas, I did a presentation on The Astro Imaging Channel talking about using data from scientific sources and making pretty pictures.  In the presentation I talk about a few ways to combine data to get an RGB-like output image but really you can combine them however you see fit (I use PixInsight for processing so all the math is done from that perspective).  If you want to play with this before investing I would download some of the data from the Sloan Digital Sky Survey.  I would definitely go with the Sloan filters over the others as the transmission is much higher and more of the scientific community are using them now.

 

Regards,
David



#5 Lucullus

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Posted 07 September 2015 - 03:12 PM

Thank you David. Are there advantages of the UBVRcIc that the Sloan filters cannot offer?



#6 David Ault

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Posted 07 September 2015 - 04:00 PM

The only advantage I can think of is if you want to participate in a study where the other contributors are all using UBVRcIc.  Even then I think there are generally accepted conversions between the data sets.  Because the Sloan filters have such steep cut-offs it does, theoretically, make building a pseudo RGB image more difficult.  In practice I haven't really found that to be the case though and some of the disadvantages, like the very poor transmission of the B filter definitely outweigh any potential advantages I see.  I know a couple other people that when they got a new camera and were considering filters, opted for the Sloan set and have been very pleased with them.

 

As an aside, I know of several institutions that use ABG cameras for scientific research.  Most manufactures set the gain so that you reach the max ADU while still in the linear range of the sensor.  NABG cameras do offer a Q.E. gain though.

 

Regards,

David


Edited by David Ault, 07 September 2015 - 07:27 PM.


#7 freestar8n

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Posted 07 September 2015 - 04:48 PM

Hi-

I have been working with Sloan filters for some time and occasionally post i'r'g' images mapped to RGB. I calibrate each channel using AMASS Sloan magnitudes for field stars - and UCAC4 provides plenty to use. This allows for a much more detailed calibration than using a single G2V star.

Sloan filters have advantages and disadvantages over normal imaging RGB filters. The advantage is that the i'r'g' spans a wider part of the spectrum and pulls in signal from the infra-red. They also split into well-defined non-overlapping spectral bands so there is no cross contamination of signal into two channels. Finally they can be calibrated accurately using available sloan magnitudes - so there is no conversion from sloan to BVI magnitudes.

A disadvantage is that imaging rgb filters have a notch or gap in the spectral coverage designed to block light pollution. Sloan filters - and other photometric filters - would not have this gap - so they would possibly be more noisy. Another issue that I don't think is important is that some filter sets are designed to be "balanced" so the exposure can be the same in each channel. But this is done just by blocking out part of the spectrum received by some of the filters - to reduce its signal. This makes no sense to me since the "signal" may have an unknown spectrum - and either way you can "balance" just by taking fewer sub-exposures in a given channel.

For nebulosity, Sloan i'r'g' images will have both Nii and H-alpha emission in the green - which is a very different "look" from the way they are normally presented. But it shouldn't be too odd since H-alpha is mapped to green in the Hubble palette - and in fact the eye would be most responsive to the H-Beta in the green.

The main reason I do i'r'g' is that I can get calibrated, deterministic colors that have physical meaning. For stars you can immediately relate them to the H-R diagram, and for galaxies it allows direct comparison of one galaxy's color to another. Two things that show up are the number of faint, very red foreground stars that show prominently in red - while in normal rgb images they would not look very different. Another is that distant galaxies are recognizable by a fairly consistent yellow-orange color.

Example i'r'g' images are at the top of: http://www.astrogeek...ages/index.html . The m83 image shows the benefit of Sloan filters in distinguishing the features near the nucleus - showing the color difference between the Hii regions in green and the stellar nucleus in yellow orange.

I think Sloan measurements are more common, but BVI measurements are still being made. I mainly like the fact that i'r'g' breaks the spectrum into well defined pieces - but the overlapping curves of BVI may be a better fit to visual response - and would allow more blending of hues since emission lines can appear in two channels at the same time.

Frank

#8 Lucullus

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Posted 16 September 2015 - 11:45 AM

I found that the MPG/ESO 2.2m telescope images presented on www.eso.org are often done using BVR filters: http://www.eso.org/p...dv=&facility=15

As they look great I think BVR filters instead of RGB would be just fine. The images look as beautiful as those with LRGB. So, I'd probably go with BVR and the respective SDSS filters for pretty pictures and photometry instead of LRGB and BVR or SDSS.



#9 Lucullus

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Posted 17 September 2015 - 08:32 PM

Would it be possible at all to use photometric broadband filters with any success at a f/10 Fraunhofer refractor?


Edited by Lucullus, 17 September 2015 - 08:32 PM.



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