30 replies to this topic

[GoFish]

Would it be a waste of time to try capturing a transit using only my UV/IR cut filter? I’m leery of spending the money on a CBB filter before I know if I have the skill and patience for exoplanet research. 
 

For context, I would probably initially experiment using an ASI178MM camera, CDK17 OTA, derotator, and L500 mount in alt-az config. And no guiding (the mount and derotator track extremely well). 
 

Worth a try, or waste of time?

[Jaimo!]

What is the value you place on your time?  If you've got the gear and think you can pull it off, why not try?  All you have to loose is an evening.

 

[Tapio]

Well, if people have done it with DSLR then I'm sure you can do it.

After all it's about differential photometry.

[rutherfordt]

The filter you use doesn't really matter since you are typically just trying to detect the transit itself-- ingress, midpoint, and egress.  Those will all show up in any filter.  I typically observe transits using an "r" filter but have had students sometimes use a clear filter (luminance) when a high signal/noise ratio on the target was needed.  Your signal/noise ratio should be at least 100, preferably higher and as long as you can achieve that then the filter doesn't matter (unless you have some reason for needing a particular one).

[pvdv]

I have detected transits with a dslr (5D MkII) and a tele lens (300mm 2.8 stopped at 3.2) on the GG channel. Detection is really, really, easy (depending on what you aim for obviously). It would have been ok with a 100mm macro as well. It is a really cool concept, it sounds a bit hard, but in fact it is extremely easy. I own a CBB - not worth the expense imho. 

There's a glut of exo-planets and exo-planets candidates and what is somewhat missing currently are more eyes. The field is extremely probabilistic at its core, something that many people forget, and this constantly leads to surprises/revisions (such as https://phys.org/new...t-vulcan.html).

It is amusing to see how what is essentially the maximum likelihood of a set of degenerate parameters can lead to amazing press releases...







 

[GaryShaw]

Hi

You don’t need any filter to just experiment with detecting the transit.

 

The more important questions are how do you plan to calibrate (adding the BJD_TDB time base), analyze the images and apply the model fit to the data in order to show whether a transit was detected? AIJ would be the standard approach but you’ll need to commit time in order to learn how to do it correctly. The AAVSO Exoplanet Section has a terrific Manual that covers the entire process. There are also video tutorials to help out. This is quite a bit more involved than just taking the images and analyzing them as you would in doing photometry for a variable star. 
 

For a helpful overview on what’s involved, I’d suggest you go to Patriot Astro YouTube Channel and watch the first video Chad does to introduce the process. 

Gary

[GoFish]

Thanks for your thoughts, folks. 
 

I don’t mind spending some time on the effort, even if it ends up going nowhere. The best possible outcome would be exoplanet detection “catching on” with the members of our club and maybe even having an intern or local student get involved. We have a very nice observatory available. 
 

Worst case I spend some time learning it ain’t for me 

 

So far, I am following Dennis Conti’s online best practices guide. Therefore my plan would be to use AIJ as the guide describes. My filter question came up because the guide recommends using one. 

[GaryShaw]

 

So far, I am following Dennis Conti’s online best practices guide. Therefore my plan would be to use AIJ as the guide describes. My filter question came up because the guide recommends using one. 

Perfect.

 

For initial testing and exploration into exoplanet science, a filter really isn’t necessary. The recommended filter for routine Exoplanet transit detection and reporting on ‘confirmed’ exoplanets, is the photometric ‘V’ filter. If you have it, go ahead and use it, but going without a filter at this early stage would shorten your required exposure times and increase your imaging cadence. More images generally equates to more data and an enhanced ‘fitting’ of the data once you pull it into AIJ.

 

regards,

Gary

[GoFish]

On a related topic: I am testing the whole process of ID’ing target stars using the Swarthmore.edu site. It’s working for me, but it gives me targets like “XO-6 b” and RA/Dec coordinates.

 

Of course I can always just type in the coordinates, but the software I use (Cartes du Ciel, SkySafari, PWI4, etc) likes the SAO or HD designations. What’s the trick for getting a reference to a target like XO-6 in a more familiar (to me) catalog?

[GaryShaw]

Hi

The target references like the one you mention above, all refer to the satellite or ground based instrument/initiative responsible for first identifying the target. It's reasonable to use these as they are the common references, or 'language', used by the exoplanet community. I've found it easiest to simply enter the target RA and Dec coordinates.

 

The Swarthmore Transit Finder Results pages will offer you links to Simbad and Aladin if you want to cross check other 'identifiers'.

Gary

Edited by GaryShaw, 10 March 2023 - 10:54 AM.

[GoFish]

Got it. Thanks.

[pvdv]

NINA (https://nighttime-imaging.eu/) has a nice plugin that will

- interface transparently with the various transit/ephemeris databases

- slew your scope to the correct location, center the target and create a reference image.

- determine the optimal exposure so you stay in the linearity range of your sensor

- capture the data

One tutorial here - the plugin has improved since
https://www.youtube....h?v=dN_s_4HjSZU

I usually don't acquire my data with Nina (old habits die hard) but it is extremely convenient as a way to see all the transits available at a given time from your location, visualize orientation, elevation and meridian crossings, etc...

[GoFish]

NINA (https://nighttime-imaging.eu/) has a nice plugin that will

- interface transparently with the various transit/ephemeris databases

- slew your scope to the correct location, center the target and create a reference image.

- determine the optimal exposure so you stay in the linearity range of your sensor

- capture the data

One tutorial here - the plugin has improved since
https://www.youtube....h?v=dN_s_4HjSZU

I usually don't acquire my data with Nina (old habits die hard) but it is extremely convenient as a way to see all the transits available at a given time from your location, visualize orientation, elevation and meridian crossings, etc...

That NINA plug in looks fantastic. Thanks for the tip. 

[GaryShaw]

Here’s a link BELOS to the Transit Finder that most folks use list and evaluate transits at their observing site.

By selecting one of the 3 data bases at the top, entering your location, and selecting a date range, you can get a list of all the transits happening in your skies over the date range you specify. It’s far easier to use than the NINA pluggin but the pluggin does have the advantage of showing the transit happening in the context of your custom horizon - if you have set up one in NINA. 
 

https://astro.swarthmore.edu/transits/

 

good luck

Gary

[gregj888]

There doesn't seem to be anything special about a CBB filter.  It a 500nm long pass filter--

http://brucegary.net...DIX H v9918.htm

 

This is almost identical to a #12 yellow filter  (~ $50) ... which is often included  in the inexpensive filter sets.

https://www.cloudyni...netary-filters/

 

Assuming the goal of cutting the blue is to help with scatter and light bending around the exoplanet, you could also use a #21 orange filter if you have or can find one of those.  $31 for both on Amazon plus 3 others. depending on the size.

 

Just a thought...

[GaryShaw]

Hi Greg

Thank you for providing the link to the Bruce Gary filter discussion. It was Bruce’s final paragraph (quoted below) that initially convinced me to acquire and use the CBB filter. 
 

His analysis presents the case for that filter as well suited for amateur astronomers doing work on confirmed exoplanets . More recently, a resource for determining the ‘limb darkening’ coefficients for use with the CBB filter has been published and is available as an online resource. 
 

All this is great but, for the OP, who is interested at this point in just testing out the process of observing and detecting an exoplanet transit, I’d still maintain that using ‘no filter’ would be perfectly reasonable. This approach defers the cost of a filter until the OP decides whether to pursue this line of work and, more importantly, it results in a higher SNR which will likely help with his initial attempt at transit detection.

 

If he decides to pursue exoplanet work, he’ll likely want to acquire the standard photometric ‘V’ filter to start off as recommended to me by the AAVSO. 
regards

Gary

 

“To my knowledge this is the first report of results from an observing session designed specifically to identify optimum filter choices for exoplanet light curve observing. There may be flaws in my procedure, and I am open to comments on an improved observing protocol or an improved image analysis protocol. I welcome others to conduct their own “filter playoff” observations, and share them with the community of amateur exoplanet observers. Until others confirm what I have found it is fair to characterize my results as merely “suggestive.” The suggestion, to be explicit, is that the overall best filter choice is CBB-band.”

 

 

[GoFish]

I’m afraid I see a new barrier on the horizon. The FOV (0.14° x 0.1°) of my tentative imaging rig (CDK17, ASI178MM) will probably be too small for platesolving. 
 

The pointing model of the L500 mount will put me on target, so no worries there. But I’m not clear on whether the light curve analysis relies on platesolving?

Edited by GoFish, 12 March 2023 - 10:35 AM.

[GoFish]

Uh-oh. I think I’ve hit a deal breaker. 
 

The target star I picked for tonight, Kelt-23 A b, doesn’t seem to have any reference stars within the tiny FOV (8’ x 6’) that my proposed rig will show me. I’m guessing that will be a common theme if I try to use the 17” reflector with the small sensor on the 178MM. 
 

What would be the technical hurdles of using a OSC instead? I don’t have access to a mono camera with a larger sensor than the 178MM. 

[GaryShaw]

You don’t need reference stars in the sense that AAVSO uses them for variable star photometry. As long as you have a few stars of similar ‘flux’ in your FOV, you’ll be fine for exoplanet analyses. Recall that absolute magnitude isn’t the issue with exoplanet transit analysis - it’s all about relative flux. As long as the comp stars are not themselves variables you’re fine. The issue can get a bit more complex later but for testing out the process a few stars is all you need. Even one star in the FOV allows for analysis to proceed. 
regards

Gary

[gregj888]

Gary, Bruce was comparing the CBB  filter to the normal photometric filters.  The 2 filters I mentioned are broadband like the CCB or NIR filters.

 

The #12 filter is so close to CBB it may fall within manufacturing tolerances...  and they are cheap.    If your sky's are fairly dark and stable I would try the #12 filter.  If not I would also try the #21 and see which one gave me the best SNR on the dip... 

 

Jim, your small sensor may restrict your targets but you still should have plenty.

[*skyguy*]

Gary, Bruce was comparing the CBB  filter to the normal photometric filters.  The 2 filters I mentioned are broadband like the CCB or NIR filters.

 

The #12 filter is so close to CBB it may fall within manufacturing tolerances...  and they are cheap.    If your sky's are fairly dark and stable I would try the #12 filter.  If not I would also try the #21 and see which one gave me the best SNR on the dip... 

 

The Baader 495 Longpass filter (1.25"-$53, 2"-$79) looks like it might be a good substitute for the expensive Astrodon CBB filter.

 

https://agenaastro.c...-1-2458301.html

 

This filter also does a great job minimizing chromatic aberrations in fast achromatic refractors.

 

I have one and plan to try it out the next time I'm out collecting exoplanet data ... if the weather ever clears up.

 

Edited by *skyguy*, 13 March 2023 - 03:14 PM.

[GaryShaw]

SOMEHOW THIS POST FROM ABOVE WAS REPOSTED….

 

Hi Greg

Thank you for providing the link to the Bruce Gary filter discussion. It was Bruce’s final paragraph (quoted below) that initially convinced me to acquire and use the CBB filter. 
 

His analysis presents the case for that filter as well suited for amateur astronomers doing work on confirmed exoplanets . More recently, a resource for determining the ‘limb darkening’ coefficients for use with the CBB filter has been published and is available as an online resource. 
 

All this is great but, for the OP, who is interested at this point in just testing out the process of observing and detecting an exoplanet transit, I’d still maintain that using ‘no filter’ would be perfectly reasonable. This approach defers the cost of a filter until the OP decides whether to pursue this line of work and, more importantly, it results in a higher SNR which will likely help with his initial attempt at transit detection.

 

If he decides to pursue exoplanet work, he’ll likely want to acquire the standard photometric ‘V’ filter to start off as recommended to me by the AAVSO. 
regards

Gary

 

“To my knowledge this is the first report of results from an observing session designed specifically to identify optimum filter choices for exoplanet light curve observing. There may be flaws in my procedure, and I am open to comments on an improved observing protocol or an improved image analysis protocol. I welcome others to conduct their own “filter playoff” observations, and share them with the community of amateur exoplanet observers. Until others confirm what I have found it is fair to characterize my results as merely “suggestive.” The suggestion, to be explicit, is that the overall best filter choice is CBB-band.”

Edited by GaryShaw, 13 March 2023 - 09:07 AM.

[GoFish]

You don’t need reference stars in the sense that AAVSO uses them for variable star photometry. As long as you have a few stars of similar ‘flux’ in your FOV, you’ll be fine for exoplanet analyses. Recall that absolute magnitude isn’t the issue with exoplanet transit analysis - it’s all about relative flux. As long as the comp stars are not themselves variables you’re fine. The issue can get a bit more complex later but for testing out the process a few stars is all you need. Even one star in the FOV allows for analysis to proceed. 
regards

Gary

So, I have found the AAVSO Variable Star Plotter page. By using the “G” option for FOV I get a fairly representative picture of what I’ll see with my rig.

 

So far, each of the target stars I’ve investigated for possible observation in the coming nights has had at least one AAVSO reference in the field. Some have had 3 or more. 
 

[pvdv]

What would be the technical hurdles of using a OSC instead? I don’t have access to a mono camera with a larger sensor than the 178MM. 

The result will not be as good as a mono camera with our without specific filter, but for something like Kelt exoplanets, it still should be very easy.
I use the OSC (ASI2600mc) now and then with the c9.25, simply because I don't want to mess with my reducer/oag train: the results are quite OK in terms of transit timing. In those cases, I split R/G/B and only use the G channel (I read somewhere SNR would be better that way, I haven't really compared myself)

On Tres 1b I obtained 0.144 Rp/R* (literature is 0.136) and OC +/- 2.5mins (I was using 300sec exposures, could not expect better). It was kind of a worst case scenario: osc, of which only 50% of the signal is used, with a "slow" instrument. Still, the (easy) transit was very obvious.

Nothing worth reporting obviously but a good opportunity to test a different approaches and practice/fine tune various workflows. I have so few clear nights I often forget what I have to do from one transit to the next ;-)

[twoc]

I've used a #8 filter to capture a few transits and noticed an improvement in systematics vs no filter, particularly at high airmasses. 50% transmission is at 495nm. The main reasons I use it over something like a Johnson R filter were the improved transit SNR (R cuts out a lot of 'useful' green and NIR light) and inexpensive cost of £10. Interesting to note that this filter is also used by KELT for the same purpose. Wratten #12, #15 and #21 are available if you are after more blocking of shorter wavelengths. The Baader 495nm also looks very promising, peak transmission is ~98% which I presume is due to AR coating vs the Wratten coloured glass filters which peak at ~90%.