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Exoplanet transit w/o filter?

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#26 gregj888



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Posted 18 March 2023 - 10:21 PM

There are generally 2 aspects to filters:


First is to match a prescribed band pass containing specific wavelengths to be captured for specific purposes.  This is the reason for most of the photometric systems and why there are more than one.  These filters are often used in pairs or groups with their results ratio-ed to get the final data. Per another thread, you want to choose filters that tie to the research in these cases, IMO.


The second use is to provide a performance improvement not specifically worrying about the wavelengths.   The CBB seems to be one of these as are narrow band filters in most cases (as weird as that sounds).  I use a Sloan 'r or a long pass orange, red  or long pass NIR filters for speckle....  each used for performance reasons with total disregard of the actual wavelengths.  The key here is to understand what the filters are doing and use them to your advantage.


So what is the CBB filter doing with EXOs?  If you look at the filter response, the CBB cuts the blue and is being applied to all star types.  Blue scatters more than red and bends easier.    Seeing improves with the square of the wavelength, that is 800nm has 1/4 the seeing distortion of 400nm.  So clipping the blue likely give a little better/sharper dip.  The long pass v.s. a closed band pass gives as much signal as possible.  If you look at the literature, folks are using everything from photometric filters and long pass filters from the NIR to none.  I'll ask Tuesday at the NASA Exo-Planet Watch meeting, but suspect there isn't a preference.


My suggestion above is based on:

  Yellow long pass gives the most photons while limiting the blue scatter (and halo in some refractors).  #8 lets in the most, #12 looks close to the CBB and #15 cuts a little more as seeing degrades, #21 for   

  further  seeing reduction.  These LP filters are quite inexpensive.  If Light Pollution is bad, choose filters that cut the pollution bands or use bands that omit them.  I hate to say it this way, but there aren't any   

  rules here :-)

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#27 yuzameh



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Posted 18 March 2023 - 10:39 PM

I'd agree with that.  As far as I can tell the main interest is in transit timings, so the cleaner the signal to noise ratio to see these low amplitude dips the better.


If pros really want photometry they'll throw a big scope or even spectroscope at them, up to and including jwst.


On LP filters though I dunno what people do about that in the current days of blue weighted white LED street lights, from the above it seems a blue cutoff is useful,but after that I think the things are pretty much continuum so no special lines to block.

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#28 gregj888



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Posted 19 March 2023 - 12:44 AM

yuzameh,  Agree.  Many of the LEDs have a strong emission around 460 nm (InGaN) so you would want to block it for sure.  By 660nm or so the red end falls way off so that may be the place to start.  Those filters would be:


70  @ 650nm

89B @ 690 nm

88A @ 720 nm

87    @ 740nm


There are a bunch of others but watch the response curve of your sensor.  Something like #26 or #29 might be a good place to start (585, 600nm respectively), but the 89A would probably be my choice if I had LED lighting to contend with (and I might).

#29 GaryShaw



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Posted 19 March 2023 - 08:10 AM


In this topic, there’s been a lot of good info about what filters are, what they do and how they do it - clearly a lot of knowledge being offered for us all to enjoy and learn from.


Looking back at the Original Poster’s initial question, may I try summarizing a few key points may help him move forward?  First, there is no requirement to use any filter at all to do casual studies to see if he can detect an exoplanet transit. Just use the Swarthmore Transit Finder to select an upcoming transit that has a deep transit, say > 20 ppt, and go for it. He’ll need to first learn the basics of capturing the images and using AIJ or another appropriate application - this later will be the challenging aspect of the overall process. 

If he later wants to participate in helping the Pro’s refine transit center times, (Tc), and help confirm targets’ various parameters, he can join with Rob Zellem’s ‘Exoplanet-Watch’ folks, use their specified software ( Exotic ) and use whatever filters they require. If he wants to contribute as an individual via the AAVSO’s reporting portal, he can use a V filter ( preferred by AAVSO Exoplanet Section) or use a CBB filter and, with either of these, his observations will be of value when NASA ‘sweeps’ his observation report into their database and performs their QC check.


If he wants to use yellow, green, pink, orange or scarlet filters, he can do so for casual observing on his own but none of those will bring much, if any, value to the Pros or help with the need for growing our database on confirmed exoplanets. For that, he’ll need to work within a structured process.


Gathering, analyzing and reporting the data in a useful way, is both time-consuming and a bit challenging to learn for some of us. Following a few initial observations to get the hang of the process, and, if his interest in the work ‘blooms’, why not align with one of the established ‘global’ teams above and submit observations as part of that team?   I hope you go for it Steve.



#30 GoFish



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Posted 19 March 2023 - 09:52 AM

Agreed. This has been a very educational thread for me as the OP.  I’ve become very familiar with searching and screening for objects using the Swarthmore page, and plotting finder charts with reference stars using the AAVSO app page. 

I’ve quit worrying about filters, for now.  And I’m probably going to try using EXOTIC initially rather than jump into AIJ, at least to get started. 


Now, if the weather would cooperate…

#31 gregj888



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Posted 22 March 2023 - 02:49 PM

I asked today it the NASA Planet Watch meeting and the results were as expected.  They will take any observations, any filter to unfiltered:  no recommendations or .requirements.


Some of the transits give a dip that is pretty small so anything that improves the SNR is beneficial.


One participant mentioned that the CBB filter is a single glass (OA ?) and about $50 for a 5" x 5" piece. 


Exotic the NASA Exoplanet Watch software has a way to add filters and custom filters for the data reduction.


So my opinion, the discussion above holds.  The #8 or #12 are probably good in darker locations.  If you are plagued by LED street lights, a long pass filter that starts in the 620nm - 720nm range should be worth a try depending on your camera. Lots of hand waving here, see what you have and try it.  Calculate the SNR of a target star and see which filter gives you the best numbers.


I'll be adding a 6xx - 7xx LP filter to the FW as I have LED lighting encroaching.  Seems like a good option to have on hand.



Edited by gregj888, 22 March 2023 - 09:01 PM.

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