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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:
My diy Onstep GoTo controller is basically an Arduino Mega 2560 with a RAMPS 1.5 shield, rated for 12V normal, 20V max & uses about 2A max.
It is powered from a 12V car battery. The lead has crocodile clips at the battery & connects to 5A screw terminals on the RAMPS.
When I "power-up" by connecting the clips there is a spark at the terminal. This is expected, but does the sparking reduce the life of my electronics?
If so is there a cheap/simple way to reduce or prevent this?
I know I could put a switch in the lead but I assume the sparking would then happen inside the switch, making no difference.
This will be a thread detailing some of the changes and additions I will be doing to my ASC/Weather Station project. This is version 2.0 as I'll be making some very big changes from the initial project and I think continuing on in the existing thread would not have made much sense.
So, I still want to use an APS size sensor as after seeing the quality and light capturing capabilities of the now defunct Opticstar DS-616C XL camera and Meike lens I simply cannot go back to using a smaller lens/sensor combination. One thing is certain, I won't be paying £400 or potentially more for another APS astro sized camera so with that in mind I plan on heavily modifying a Nikon D50 DLSR and use the same lens. I chose the D50 primarily due to it having a CCD sensor (ICX453AQ) very close in specs to the one in the Opticstar (ICX413AQ) and the fact that I got a hold of a fully working body for £25.
Now there's a few issues with going down the DSLR route which I plan on addressing as follows:
The oversized camera body can be stripped down to bare essentials and fitted in the existing case with some moving of parts around Uncooled, the sensor is quite noisy so to cool it I plan on using the existing Opticstar enclosure with the TEC and hopefully get it purged with Argon to avoid dew formation. Also, since the box will need to be completely sealed to achieve this, there's simply not enough room inside for the main board to which the sensor connects to. The only way around this is using an 39pin 150mm long FPC extension which I managed to find and will be arriving shortly. This means I can have the sensor completely sealed with enough slack in the connection to place the mainboard anywhere I want. The D50 uses the NEF file extension as a "RAW" file format but it's not truly RAW and a heavy median filter is applied to all long exposure images to smooth out the noise. It works great for day to day shots, but in an application such as mine it'll most probably eliminate or severely affect my stars as most of them at the FL I'll be using the camera at will be a few pixels across and the Nikon median filter is very aggressive with such small features. The way around this is what's commonly known as Mode 3 on Nikons. Nikons have a additional Noise Reduction mode which takes the long exposure light first then straight after an equal length dark with the shutter closed, then applies the dark on the light and you get a further noise reduced image which again works very well, but not so much for AP. With mode 3 you essentially have the NR feature on and take an exposure but then immediately shut down the camera after the light has finished exposing. What this does is it causes the camera to dump a REAL RAW image onto the SD card without applying the median filter OR the Noise Reduction process. This obviously results in a much noisier image as expected, but all the stars will still be there and the image in this way can then be dark-subtracted and processed to my liking. I'll post some test shots I've taken to illustrate this. The D50 uses a hybrid shutter, both the CCD electronic shutter and mechanical shutter are used depending I think on the exposure length. If a high enough exposure is used, from what I understand, one can use exclusively the electronic shutter, but for longer exposures the shutters work in conjunction. Now I know the ICX413AQ in the Opticstar is more than capable of taking long exposures solely with its electronic shutter despite the fact that in its datasheet they recommend a mechanical shutter for proper use. So, my thinking is since the D50's sensor is similar to the ICX413AQ the only thing preventing the camera from being able to take any exposure using exclusively the electronic shutter is that its mechanical shutter is in the way and I don't think that the camera would prevent the CCD electronic global shutter itself to still open and close when required. However, this is all a theory at the moment and the only way to confirm it is to test the camera with the sensor outside when the FPC cable arrives. More on this later... In terms of capture software available, the D50 is actually very poor and I could only get digiCamControl to see and control the camera via USB. But I won't be using this as when the camera is hooked up to the PC its SD card is identified as a storage drive and the camera can be used as it would normally with the images appearing on the drive after being written to the SD! Since I'm using my VB app to process the images I would just point the app to that folder and should work. That's all I can think of for now but if and when new ones come up I'll add them here.
Next I'll be describing some of the other changes planned.
So my birthday just past so money to splash on astro stuff , i will have my 1000D modded by juan at cheapastrophotography and also have ordered an autofocuser from deepsky dad https://deepskydad.com/autofocuser i know they can be done DIY but this is a neat package and costs about the same as a SW autofocuser and a hitechastro focusmaster and i`m no electronic wizard and pavel seems to have a good product and works with ascom and confirmed it works with APT i will update in a few weeks time when hopefully i will have received and tried out .
I would like to share my design of a smart barn door tracker. It is a simple to build isosceles barn door tracker with tangent error correction through a Arduino micro-controller. I have shared all the details about the tracker including the mechanical design, electrical circuit and the software source code here:
If you are interested in the math and other details about a barn door tracker, more details are there in the blog. Also find an online calculator which helps in calculating various parameters while designing a barn door tracker. The blog is here:
M46 and M47 shot with this tracker.
135mm lens, ISO800, 15sec X 200 subs (50 minutes) exposure. Cannon 500D.
Comments and suggestions most welcome!