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DSLR Astrophotometry Beginner - Typical Exposure Times?

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Phil Edmonds
Phil Edmonds's picture
DSLR Astrophotometry Beginner - Typical Exposure Times?

Hi All,

I am just getting started in astrophotometry with my Nikon D5300 on a Skywatcher 4" ED and an AVX mount.

I'm aware of the stuff about ADU saturation etc., just a simple question, what sort of ISO/exposure length combination do you chaps typically use when doing photometry with this kind of setup? Just ballpark numbers to get me started? Say for stars < mag 6?

I'm just looking for something like ISO100/ 1s   or ISO400/100s etc., just to get me starting in the right zone, none of the stuff I've read actually gives me any starting numbers!

Thanks, Phil :-)

SFS
DSLR photometry beginner

For a given telescope and camera, the answer depends critically on the target.  You need to experiment.

But to get you started,here is an example. I use a 71 mm f/6 William Optics Zenithstar with a 0.8 focal reducer and a Canon T3i (APS-C sensor, 18.1 Mpx).  Most of my current targets are in the 7 - 9+ V magnitude range, so I use ISO 200 with a 15 second exposure, because ISO 200 provides the maximum linear dynamic range for my particular camera.  I sometimes use ISO 400, for example with dimmer RR Lyrae stars or DCEPs having short periods.  That way it takes me less time to accumulate my chosen number of exposures.

I recommend you take the DSLR photometry CHOICE course; I did and it helped me avoid a lot of the problems one can have if one is just starting.

Phil Edmonds
Phil Edmonds's picture
Many thanks for this, just

Many thanks for this, just what I was looking for!

Roger Pieri
DSLR manual

Hi Phil,

this subject is reviewed into details in the DSLR manual , chapter 2, equipment overview, in particular in pages 24-26. There are examples in tables 2.3 and 2.4. Please, note that the ISO settings are missing in columns 7 and 8 of the table 2.3 , it should read "ISO 400" in column 7 and "ISO 100" in column 8. Table 2.4 is ok. 

https://www.aavso.org/dslr-observing-manual

I would recommande to check there is no saturation at start of each session, a margin should be applied depending the air-mass evolution during time series. The focus change with temperature could also determine a saturation. Then the level of saturation - counted in ADUs - could be significantly different in function of ISO in certain camera models (recent EOS: 16383 ~ 13000 ADU depending ISO and 2048 ADU offset).

The sum of exposures to stack shall be at least one minute, preferably two, that to integrate the scintillation. ISO 100 is usefull to avoid saturation or too short exposures in case of bright stars, it provides a better SNR even if the true dynamics is not improved from ISO 250 (counted in electrons). 

Clear Skies !

Roger

Bikeman
Bikeman's picture
Exposure times

Here is a simple rule of thumb that you can use:

A magnitude 0 star will (very very roughly) give you 1 million photons per second per square cm of aperture in the green channel.

A 4 inch (diameter) optic has a ~ 5cm^2 * pi ~ 80 square cm aperture , so you get ca 80 million photons per second from a magnitude 0 star.

For a mag 6 star, you get only ca 1/[2.51^6] as many photons per second, so here

80,000,000 / (2.51^6) ~= 80,000,000 /250 = 320,000 photons per second

Each pixel of your sensor can only collect so many photons, and to stay in the linear response regime, you want to use this capacity only to about max 50%. Typically this would be no more than a few 10,000 photons at most per pixel. Let's assume (say) 8,000 photons as a max to be conservative.

You usually defocus quite a bit to spread the signal among several pixels, also only half the sensor area is covered with green pixels and you lose some photons because the optics and sensor are not perfectly efficient in transmitting and capturing photons, so let's say that the brightest pixel belonging to the 6 mag star image  gets at most 3% of the total green photons, that means that  in total you don't want to collect more than ca 270,000 photons per exposure to stay on the safe side to avoid overexposure. For our example, this means that even one second of exposure could already be too much (and note this doesn't depend on ISO level!) for your equipment and a mag 6 star. Note that for SFS's example, the aperture was smaller (half the diameter) and the stars dimmer (mag 8 is dimmer by a factor of 2.51^2 = 6.3 ). So his exposure time of ca 15 seconds are consistent with this back-of-the-envelope-calculation. 

For the NIKON D5300, the unitity gain ISO level seems to be between 200 and 300 (means that 1 ADU ~ 1 electron = 1 captured photon ) , so ISO 200 should be fine. http://www.photonstophotos.net/Charts/Sensor_Characteristics.htm

This little example calculation should demonstrate that defocusing is really important. If by accident your focus is too sharp, a lot more than the 3% of the photons from the example will fall into the brightest pixel, and overexposing a star will happen fairly quickly.

So I'd say for your equipment:

brighter than mag 6: use a lens with smaller aperture

mag 6: start with 1 sec  and heavy defocusing, or use optics with a smaller aperture

mag 7: 2 sec

mag 8 : 5 sec

mag 9 : 15 sec

mag 10 : 30 sec

etc.

At some point. longer exposure times per frame get inconvenient or impractical and you will  just combine more frames into a single measurement.

HB

 

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