Thu, 10/22/2020 - 22:23
Hello my senior AAVSO colleagues,
I am fairly new to AAVSO and recently transitioned from AstroPhotography to DSLR Photometry.
I want to use my Canon DSLR camera only and not the telescope. I have a wide variety of high focal length lens available to me (18 mm-70mm, 35mm, 75-300mm, 650-1300mm)
As I understand that especially for earth-like exoplanets, transit depths are quite small and it requires high precision measurements. Can I capture the exoplanet transit by the method proposed above?
I asked this question to Dr. Dennis Conti and he suggested to ask this question to the Exoplanet group through this forum.
I will appreciate your candid response.
There is a whole buch of things to figure out on this path. My best advice is to just dig in and try it. Start with the 1300mm focal length and pick an exoplanet that eclipses without crossing your meridian during the eclipse and has a depth of at least 20 mmags or 20 ppt on the Swarthmore site. Plan on taking around 200 to 300 identical pictures for the series, keeping the star on the same pixel. Look at he various exoplanet databases to see what the transit should look like and compare to your data. Build confidence by doing.
Ray
For exoplanets, you need precision. A classical way to test precision is to repeatedly (continuously) measure one standard (known non-variable) star whose magnitude and color are similar to that of your target exoplanet. It's probably good to cover a range of sky altitudes. If your precision is good relative to the exoplanet's transit depth, then you can approach the exoplanet with some confidence.
The best part is: you can do this anytime. You need not wait for an exoplanet event to test various lenses, exposure times, processing, etc.
NB: Your 18-70 mm lens and 35 mm lens are not high focal-length lenses, and probably won't resolve stars well enough for photometry.
Also, if your optical train is not tracking the sky, the length of time you can measure without resetting your camera will be very limited. There do exist inexpensive devices to cause a DSLR to track the sky for an hour or two.
Thanks Eric for your suggestion.
I will give it a try with a higher focal length lens.
I do have a tracking I-Optron mount also but need accessory to mount my camera on it.
I like your suggestion of trying it out!
I may need further guidance on my first target.
Can you propose me an easier target (exoplanet and its parent star and its RA/DEC position) for these days in high elevation in the sky? I will try 300mm, 650mm and 1300mm. I know that my FOV will narrow down considerable so finding the target would be a challenge. Can I use Astrometry.net to identify my target?
Thanks, again.
This is very handy for finding a transit that can be seen from your loacation:
https://astro.swarthmore.edu/transits/transits.cgi
Eric's suggestion of doing time-series photometry on all the stars in any of your fields of view is a good one. You will learn a lot in a few good nights of staring at a star-studded view of the the galactic plane. Others have suggested tracking difficulties, but you can really only learn that by doing it and perfecting with better technique and/or better mounts. Brighter transited stars, as suggested by others here, is good. I suppose, because they might tend to be closer, their hot Jupiters easier to see. Higher elevation, lower airmass is good for reducing signal to noise, as is stacking.
You now have way more information than we did when we started. Your best teacher now is you actually doing it. Good hunting.
Ray
Hi
Below are targets ranging in magnitude between 7.6 and 9:
HD 149026b, HD 17156b, HD 189733b, HD 209458b, HD 80606b, HD97658b
François
Let me explore to find them first. If I am unable to find them, I will ask for more help.
Typically, I start out with Stellarium and AAVSO's Variable Star Plotter finder charts.
Is there a good Exoplanet website where I can read more about these targets?
Using a longer focal length lens would be best particularly if it has a fast f/ ratio. When I started doing DSLR photometry I was using a Canon L-series 70-200mm zoom lens, and quickly found that the focal length drifted over a series of exposures, making registration impossible. So if you are really interested in doing this, buy yourself a decent 200 - 400 mm non-zoom lens, or maybe a small refractor. I used a William Optics Z71 with a focal reducer, and found it was well suited, primarily because of its very excellent focuser. (Camera zoom lenses, even high quality ones like the Canon L-series, tend to undergo minute focus changes over time when pointed upward or downward. Not good.)
Another thing: Exoplanet observing is a wonderful application for ensemble photometry. Using every non-variable star in the field within 2 V band magnitudes of your target makes high accuracy differential photometry possible, because the variance of a single measurement is proportional to the reciprocal of the number of comparison stars, all other things being equal. So even if you don't know what the actual magnitudes of your comp stars are, it doesn't matter because what you are trying to do is sense a change in the relationship between the target and the ensemble average. Pros typically use ensembles of ca 40 comparison stars to enable realization of millimagnitude detectability.
Also, plan on defocusing and making Green filter (approximately V-band) measurements only. This because you want to make sure you have greatest sensitivity and constant pixels on target.
I agree that I need a better prime lens in 200-400mm range.
Can you explain: "Pros typically use ensembles of ca 40 comparison stars to enable realization of millimagnitude detectability" further?
I understand that for exoplanets we need millimagnitude detectability and especially for earth-like exoplanets the transit depth is extremely small due to small masses involved.
What is "ca 40 comparison stars"?
"Ca. 40" means "circa 40", that is, "approximately 40".
For asteroids I use every available non-variable star in the ATLAS catalog, so typically 30-150 comp stars in each 36x24 arcminute field of view. Yes, that's an extreme example.
The good news: for your work, you don't need all the available stars--averaging even 4-10 comp stars will help a lot (and will help you to detect and reject any problem comp stars).
Eric, can you provide us with the link to the ATLAS catalog?
Thanks!
François
ATLAS reference catalog version 2 is at: https://archive.stsci.edu/prepds/atlas-refcat2/#intro
Its magnitude entries include Sloan (the future) but not Johnson-Cousins. Its many millions of stars to mag 16 take up about 6 GB on the hard drive, and I had to write my own python routines to read it, though that wasn't too difficult.
My previous reply was really to encourage ensemble comp stars, not so much to encourage the ATLAS catalog itself (excellent though it is).
Thanks, Eric, for the ATLAS reference catalog.
I understand it is better to do more ensemble comp stars photometry.
It might be instructive to also have a look at the KELT survey (and their planet discoveries), which found a bunch of exoplanets via the transit method with extremely little telescopes (that's what E-L-T in the name stands for :-) )
The telescopes had only 80 mm focal length and about 40 mm aperture (!), and a plate scale of 23 arcsec/pixel (!!)
https://keltsurvey.org/telescopes
But look at that beast of a mount!!! I think there is no easy answer to your question, a lot will depend on your mount's ability to track a star with great precision. The longer the focal length (for a given sensor ), the smaller will be the plate scale and the more difficult it will be to do precision photometry if you want to keep fainter stars from drifting in between pixels. Also longer focal lnegths lead to smaller FOV which limits the number of useful , high SNR comparison stars.
I would strongly encourage to consider looking for used, even "vintage" fixed focus length lenses for projects like this. You can get really fast lenses from the chemical-film-era of the 20th century for a fraction of the price of a similar spec'ed modern "digital" lens. Remember that those chemical era mass produced consumer lenses were designed to provide good performance across a field of 35mm film , something which is now called "full frame" and only used for the more expensive DSLRs. Those vintage lenses will lack a lot of modern features like stabilization, motor focus, auto-focus interface, aperture control interface, etc etc, but you need none of that for photometry, really, One needs to make sure that the focus ring is still "sticky" enough to be stable and that there is an adapter ring to use the lens in question with your modern DSLR, e.g. the Canon EOS mount can be used (with adapter) together with all Pentax-K mount lenses.
Even if the exo-planet project turns out to be too difficult, a fast telephoto lens will always be useful for general DSLR photometry, so I don't think it will be a wasted investment in any case.
Cheers
HB
I understand this is a challenging mission but worth exploring!
Per your suggestion I will try to fing a fast higher focal lenth lens that I can use for both Phtometry as well as Astrophotography.
Any suggestion on cost efficient purchase? If I go by brand name as Canon they are very expensive. However there are some good chinese brands available these days which will fit my budget. I have a Yongnuo 35 mm F/2.0 prime lens and works great! I have done my initial 20 photometry readings using that wide FOV lens. Shall I go for 135mm or 200mm prime lens as my next purchase? Will appreciate your candid opinion. Any suggestion where I can purchase used lens?
I think the 135mm ... 200mm range is a very good choice to look for lenses that are good but not super expensive.
I have a series of Pentax SMC lenses that must be from the late 70s or perhaps early 80s, like 50mm 1:4 , 85mm 1:2.0 , 135mm 1.2.5 and a Pentax at 300mm 1:4. I got the 50mm at a "real" shop for some 25 EUR or some such, the adapter ring was more expensive :-). I got all the other lenses on eBay, so far I had not a single bad experience buying there. If a legancy lens is any good you will find reviews for modern-era use for them on the Internet, like this site that discusses Pentax lenses, e.g. the 135mm I have:
https://www.pentaxforums.com/lensreviews/SMC-Pentax-K-135mm-F2.5-Lens.h…
Note that this site will also give you an idea about the average street price for the lens, e.g. here (at the time I'm writing this) 180 US$ for the 1:2.5 135 mm lens.
I think that's not too bad, if you go for a slightly faster 1:2.0 new lens from Canon you will be paying...I think....about 800 US$ ??
If you go for a new, but manual focus 135mm 1:2.0 from Samyang (South Korean brand that has many fans) , you are still paying ca 500 US $ I think
These are just examples but I think you get a good impression what the differences in prices for new , new but manual focus, and vintage lenses are. In between will be used, modern lenses of course.
Also think about the risks of dropping that lens or exposing it to dew when working in an astronomy context ... my 135mm actually has a small dent from when I stumbled over a cable causing a tripod to fall over...it still works but I would not want that to happen to a super-expensive lens :-)
CS
HB
Yes, you are right. I will look out for a used lens on ebay, canon or other photo shop sites.
These lenses are more expensive than the body of the camera!