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I have just joined and have been looking around, and putting in various searches to find the answer to my question(s).
I have already found some valuable information, but i can't find a specific answer to a question i have relating to exposure times.
I have shot the milky way several times before, from a tripod and a wide angle lens. I am aware of and understand the "500 rule" and that worked fine for me at first when i was shooting with my Canon 6D Mark II. When i moved over to the Sony A7III i noticed significant trailing using the same rule and that led me to the NPF rule (Via the photopills app incase people dot know).
I am heading back to Tenerife once again in about 6 weeks time and want to buy a star tracker so i can get some really detailed images.
I have done a fair bit of research and in principle, the whole thing doesn't seem to be too daunting or difficult.
I have purchased the Polar Scope Align Pro app so i can align Polaris as accurately as possible, i will practise putting the unit together and familiarising myself with the different parts etc, but it is the exposure times that i do not understand.
My best glass is the Carl Ziess 50mm F/1.4 Planar, the 18mm F/2.8 Batis, the Sigma 35mm F/1.4 Art & the IRIX 15mm F/2.4 Blackstone.
I currently do not own, nor have i ever used a tracker, and I cannot find any information relating to which aperture, ISO and Shutter length any of these focal lengths should or could be shot at.
Is there anything similar to the 500 rule or NPF rule that relates to using a tracker with varied focal lengths? or is it just a case of stepping the lens down for sharpness and then trial and error?
Thanks in advance,
By Captain Magenta
Last year I was given a Unihedron SQM-L, the narrow field of view version of their gadget for measuring night-sky brightness. Since then, I’ve nipped outside to take zenith readings whenever I’ve been able, often a few times per night. As a result I now have 85 data-points, all from my back garden in Sunbury on Thames which rates a 19.04 on www.lightpollutionmap.info . As it turns out, this agrees well with the data I’ve collected.
The darkest I’ve measured at this location has been 19.13, with 4 records better than 19.05 and 10 better than 19.00.
Plotted against Moon altitude, it looks like:
One thing I noticed very early on was that the reading generally gets darker and darker as the night goes on. The chart below suggests the data agrees, but how strongly I’m not adept enough yet with my statistics to work out. If anyone fancies doing this for me, I’d be grateful, I’ve attached the data .csv file I think to the end of this post.
The data itself: each record contains date, time[GMT], SQM value, Moon phase, Moon altitude . For the purposes of my analysis, I’ve converted the time value into hoursafter6pm, which allows the intercept of the regression solution to be loosely considered as the “6pm starting point” for the darkness estimation, which is OK for this dataset as my data is all from this latest Autumn/Winter.
I’ve done an “ordinary least-squares” regression with multiple input variables. At first glance it seems to me that the SQ vs altitude chart above should not behave well with that: there’s a clear kink, intuitively obvious I guess, at the point the Moon altitude goes negative.
To cope with that, I divided my data into two and did three separate regressions: “Moon up” data, “Moon down”, and “All data” but treating phase and altitude as zero if the Moon is below -5 degrees (I chose -5 degrees arbitrarily).
With Moon up, I decided the SQM value will depend on Time of Night, Moon Altitude and Phase. With Moon down, it only needs to depend on time of night.
Thus my regression model is:
SkyQual = a + b.timeafter6pm + c.phase + d.altitude + residual
residual = a + b.timeafter6pm + c.phase + d.altitude – SkyQual
The analysis involves minimizing the sum of (the squares of the) residuals, by hunting around for the appropriate values of a, b, c & d which yields this minimum. I used MS Excel’s built-in Solver to do the “hunting around”.
The following table summarizes the results:
In words, using “Moon Up” as my subject, my Sky Quality, in magnitudes per arc-second, can be estimated as
plus 0.0314 /hour
minus 0.864 /full-phase (or 0.216 /quarter)
minus 0.0186 /degree above horizon (or 0.186 /10 degrees).
This is a pretty simple analysis. I’m sure there’s theory and formulae available relating Moon-altitude and -phase to extra sky brightness, but I haven’t used any of that here. And the “error model” I’ve used implicitly assumes that the relationships between SQM-reading and the variables are linear.
If anyone is curious and wishes to do their own analysis, my raw-ish data is available as a .csv file attachment at the end of this post.
A note about the data collection: each reading is an average of a few readings at a given time, with outliers rejected. For instance, often the first press yields an outlier, and over the following few seconds subsequent ones tend to settle down. So the series of readings 19.05 (me getting excited), 18.85, 18.86, 18.86 , which is a quite typical pattern, would cause me to record 18.86. My highest recorded reading, 19.13, was indeed where it settled down.
Other “one-on-one” charts:
I gave a demonstration/workshop at my local Astro Group* about a simple way of removing light pollution from an Astro Photo.
The description I gave was deliberately for beginners, using a wide angle tripod shot photo and using one of the easiest packages to get to grips with (Paint.net).
The attached pdf covers the basic technique.
I'd appreciate any feedback on it.
* The Mid Cheshire Astronomical Group - all welcome, we meet on the last Friday of the Month.
Congratulations to Welsh photographer Alyn Wallace for getting National Geographic Photo Of The Day for these pictures of the Elan Valley night sky.
Second attempt at astrophotography with my canon 1300D untracked (first was orion ). Shot under dark skies of himachal pradesh (India).
Stacking done in DSS and processing in Gimp.
Any suggestions would be appreciated. especially regarding the trees at the bottom.
Total exposure time - 20*20 seconds
Shot with - Canon EOS 1300D (untracked) (unmodded)
Flats and Biased frames included