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12-21-2020, 02:56 PM   #1
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Suggested settings for the 'Christmas Star'

Hello all.

If I get a clear sky tonight I think I'm going to see if I can get Saturn/Jupiter with my Q7 and/or K3 + 600mm f5.6 A+1.4 TC. With the Q7 that's approaching a 4000mm lens. I've done this before in the past, but never seem to nail it. I can see Jupiter's moon fine, but not much else. I know it is going to require some fine tuning in the field, but can anyone suggest a ballpark exposure? Or is a better approach to take a lot of underexposures and pool them together?

Thanks!

12-21-2020, 03:10 PM - 1 Like   #2
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For the planetary discs, I recently used ISO 800, 1/80", effective f/8 aperture. You're already at f/8 wide open, so you could use the same settings. For the moons you'll need several stops brighter, but I forget how much. That'll be easy enough to see by reviewing your images as you shoot. Good luck.
12-21-2020, 03:54 PM - 1 Like   #3
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First image below is a cropped SooC from two nights ago, taken with my K-3, DA* 300, and the Pentax 1.7 TC (the one that can focus for you). The effective aperture is ~f/6.7 at ISO 800 and 1/4 second. You can see (with a bit of imagination - the focus isn't all that good and even at 1/4 second, there is earth rotation smearing) Saturn's rings. The faint objects stretched out at about 10:30 wrt to Jupiter (the really bright object) are the Jupiter moons. So, this exposure will get you the moons.

The big challenge will be tracking with long focal lengths for any exposures longer than ~0.1 second. I will be setting up my tracking mount for this evening.

The second image is a screen grab from the Stellarium program, showing the planet pairing as well as three of the Jupiter moons (Ganymede is in front of (or behind - not sure which) Jupiter at this time!!) for about 6:15 MST this evening. Note that the field of view is about half a degree. The elevation is around 13 degrees, and the conjunction is setting. Get out there just after sunset, or you are likely to miss it.

If you are too lazy to set up for this, I suggest watching the live stream from (our local) Lowell Observatory (Lowell.edu ; they seem to be down at the moment - wonder if they crashed due to folks wanting to watch!?!?), starting at 5 PM MST (midnight GMT, December 21/22).

---------- Post added 12-21-20 at 04:18 PM ----------

Or, go straight to the Lowell live stream on Youtube:
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12-21-2020, 04:23 PM - 1 Like   #4
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In theory, the equivalent of the "sunny f/16" rule for Jupiter would be a "Jovial f/3.1" based on the fall-off of light from the sun (Jupiter gets about 4% of the sunshine compared to Earth). (The one for Saturn is f/1.7.)

That means that if you use f/3.1, then the right shutter time is 1/ISO or the right ISO is 1/shutter time. Apertures other than f/3.1 shift either the ISO or the shutter time (or both) as required to compensate for the added/subtracted number of stops.

Thus, at a lens aperture of f/6.7 (about 2 and a quarter stops dimmer that f/3.1), the shutter speed for ISO 800 would be about 1/160 second. That's a starting point -- the optimal exposure might be a bit slower than that due to T-stop issues with the lens and atmospheric attenuation because Jupiter is a bit low in the sky right now.

12-21-2020, 06:25 PM   #5
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I was just guessing and shot many different settings. Unfortunately, my tripod is really not heavy enough. Had a little motion blur in a lot of shots. Used a K70 w/DA* 300mm + 1.4x teleconverter with 12 sec. delay. Shutter speed was from 1/15 to 6 sec. at f:5.6 and iso from 100-6400.
12-21-2020, 07:41 PM   #6
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Thanks Y'all. While the Q7 worked okay, the best image was with the K3. It is a composite of one image exposed for Saturn and one for Jupiter
12-22-2020, 08:16 AM   #7
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Maybe some multiexposures and stitching into one could make sense?
12-23-2020, 01:02 PM - 1 Like   #8
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QuoteOriginally posted by photoptimist Quote
In theory, the equivalent of the "sunny f/16" rule for Jupiter would be a "Jovial f/3.1" based on the fall-off of light from the sun (Jupiter gets about 4% of the sunshine compared to Earth). (The one for Saturn is f/1.7.)

That means that if you use f/3.1, then the right shutter time is 1/ISO or the right ISO is 1/shutter time. Apertures other than f/3.1 shift either the ISO or the shutter time (or both) as required to compensate for the added/subtracted number of stops.

Thus, at a lens aperture of f/6.7 (about 2 and a quarter stops dimmer that f/3.1), the shutter speed for ISO 800 would be about 1/160 second. That's a starting point -- the optimal exposure might be a bit slower than that due to T-stop issues with the lens and atmospheric attenuation because Jupiter is a bit low in the sky right now.
And I already found the 'loony 11'-rule challenging. ;-)

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