Raw Beginner Questions

[NedS]

I have read Conti's Guide.
Guide mentions use of standard filters.  When to use what filter?
How many images to take?  How many data points are recommended for a three hour transit?

[Dennis Conti]

Hi NedS,

Thanks for your questions. See my responses below:

1. For "casual" exoplanet observing, a filter that produces the highest SNR for the particular host star would be appropriate.  If you don't have a full set of standard filters, a V filter is a good default. However, for observations as part of a pro-am collaboration, example TESS, the preferred filter is often stated. The selection of a particular filter in this case is to allow observations in different bandpasses in order to determine any chromaticity differences. If there are such differences, then it is likely that the "event" is actually due to an eclipsing binary vs. an exoplanet transit.

2. As far as how many images to take and how many data point for a three hour transit, I would suggest the following. Prior to the actual observation, first determine what exposure time results in an ADU count for the target star that is, say, 50%-75% of the way to where your CCD camera becomes non-linear. For example, if your CCD camera becomes non-linear at 50K ADUs, then select an exposure time that produces 25-30K ADUs. This can be determined ahead of time by imaging the target star, or alternatively, a star of equal magnitude. However, you must also take into account that the ADU counts will increase as the star approaches zenith, so you want to allow for some "cushion" to account for this. Then the exposure time will determine the number of images that can be taken within the observation time frame.

Hope this helps,

Dennis

[Dennis Conti]

Actually, to be more accurate, I should have said "meridan" instead of "zenith."

Dennis

[Dennis Conti]

Sorry, meridian not meridan.

[BPVietje]

I Hope its OK to slide a question into this thread?

I'm wondering why the aim is 50%-75% of the maximum linearity limit?

Since there are accurate timing questions of interest, would a minimum SNR threshold be even more useful?  Say for example, the data can only be used if the SNR is above 100 for the entire transit.  I would think that the minimal exposure that reaches an SNR of 200 would be a reasonable target, allowing a lot of latitude for a deep transit SNR to never fall below 100.

Perhaps I'm way off the mark on the threshold, but would an SNR exposure guideline be useful, or have I failed to understand something fundamental here?

The CCD camera I use is linear to roughly 60,000 ADU, so 50%-70% would be 30-45K, which is easy enough with the brighter TESS targets, but could result in longer exposures that might miss temporal details on dimmer targets.

Woudl love to learn additional details on this.

Clear skies,

Brad Vietje
Newbury, VT
www.nkaf.org

[karenacollins]

I think any recommendation of targeting 50-75% of the linearity limit would be for the case where you want to maximize SNR. Of course if you only need a certain SNR, then design your observation accordingly. For transit photometry, a cadence of 2 minutes is usually fine to sample the transit shape (5 minutes is often acceptable), but that of course depends in the event duration and shape of the expected transit.

Karen