Xilman

7 minutes ago, Astro Waves said:

Side topic, I did post this is imaging discussion but its also very appropriate here seen as how no one answered it yet. Do the imaging settings need to be the same when imaging on multiple nights? I've managed to get two nights on the Whale galaxy but with different exposure times. They are also slightly framed different ...

Nope, none of exposure times, exposure dates and field centres need be the same.  To see the latter to be true, consider the number of mosaics that are out there: overlapping images which have been stitched together. All you need is co-adding software which can correctly position the subs. Personally I plate-solve first and then use SWarp to stack the subs. SWarp reads the co-ordinate system from the individually solved subs.

SWarp also handles the multiple exposure time issues by weighting each sub by its exposure before co-adding.

Do you get the idea that I really like SWarp?

12 hours ago, JRWASTRO said:

Greetings Steve,

Implement the SG filter and presenting the results will need some software like MATLAB (very expensive) or OCTAVE (free open source software).

Create a list of your raw data (CSV) that can be read by OCTAVE and it will be straight forward to implement a SG filter and or a Moving Average (MA) filter.

Octave is a good program but I dispute the necessary part. Anyone who followed my advice and started investigations with the Wikipedia article would have seen that coefficients up to degree 25 were published as long ago as 1965.

A link in the wikipedia leads to https://sites.google.com/site/chandraacads/resources/sg-filter/db

where you will find tables of coefficients for 1 through 4 dimensional data and for a wide range of polynomial degrees and window sizes.

Once you have the coefficients it is relatively easy to implement the filter in the programming language of your choice, Excel-compatible spreadsheets if you wish though, to be honest in high dimensions it will run rather slowly unless you use FFT convolution.

That said, if the tabulated coefficients are not suitable for your needs, you will need to compute them. Free software exists to do this for you. Another link from the Wikipedia page leads to C source https://zenodo.org/record/1288901

Have fun!

1 hour ago, vlaiv said:

Too much work as you can get the same result in easier way.

By doing that you are effectively interpolating between original data points. There is no more data to be gained - these intermediate stacking steps will have high correlation with other samples - so you might as well just interpolate existing 10 points with some interpolation algorithm to get nice curve.

check out this for example (hit example on the page and play around with data)>

https://tools.timodenk.com/cubic-spline-interpolation

Also look into Savitzky-Golay filtering. That method uses a least-squares polynomial fit to runs of data points. It works very well in practice in reducing noise without greatly broadening genuine features.

A simple variation can be used to find maxima and minima in the data, perhaps from an eclipsing binary.

Descriptions and code for S-G filtering are all over the net. Finding them is left as an exercise. Hint: Wikipedia is a good starting point.

6 minutes ago, AndrewRrrrrr said:

we use MJD at work - the integer part of the MJD is a much smaller number than the JD and the "day" starts at midnight. The JD "day" starts at noon. 

The JD is probably more useful to an astronomer because the integer number does not roll-over at midnight, in the middle of a viewing session.

So it depends what you're doing really. Maybe something to do with being at sea in the old days - the only time you could determine exactly was midday when the sun is at its highest point in the sky. 

Just a few guesses really!!!!

In these days when 64-bit cpus are widespread the extra size of the full JD (of which there are many variants BTW) is not such a big deal as when 32-bit floating point was commonplace. There is a big difference between 6-7 digits precision and 15-16 bit precision!

Your "more useful" comment belies your location. 😉  Someone in Oz or NZ may politely disagree.

44 minutes ago, robin_astro said:

 Ideally it is better to analyse the subs separately, dealing with any outliers at this stage eg with a sigma clip (or if present in all of them with a bad pixel map)

Seconded!

10 minutes ago, han59 said:

A practical test would be interesting.  Just see what the photometric standard deviation is for a reference/check star over a longer period.  Any long image series will do.  I have no problem to measure the standard deviation,  but I don't have a program which can stack median.

I use SWarp, which has a number of different stacking modes.

2 minutes ago, han59 said:

In general taking the median value brings more noise to the signal compared with average (mean).  For photometry average (mean) will be better.

Yes but. It depends. If the SNR is high enough, median can allow for better sky estimates. The median stack is undoubtedly noisier, but not by a huge amount.

If the SNR is very low, perhaps only 3 for bare detection (i.e. astrometry, not photometry) then every last bit of SNR is worth having and so summing or averaging is definitely the way to go.

As long as you keep all the subs it is straightforward to experiment with what does best in various circumstances. Binning or other forms of smoothing, with or without background removal, can also be helpful on occasion when making quantitative measurements.

Shame. Oh well. No way I can contribute.

As noted at the Deep Sky Section of the BAA (https://britastro.org/section_front/12) globular clusters are  amongst the finest objects to observe in the night sky.

Over the last six months I have been collating a list of globular clusters and their properties, and have written an article which proposes three challenges for amateur astronomers. The first two challenges are now revealed to public view at http://www.astropalma.com/Wotnot/GCM/GC_Marathon.html and at https://britastro.org/node/25753. The lists are available in both HTML table format and as a spreadsheet, the latter being available at http://www.astropalma.com/Wotnot/GCM/GC_MW.ods

These challenges are to observe as many as possible of the globular clusters associated with the Milky Way galaxy, either all of them (the full challenge) or those which rise above your local horizon (the restricted challenge.)

There are 158 objects in the full challenge. A few of them are naked eye objects. A few are ferociously difficult targets, even for ambitious and well-equipped imagers. The great majority lie somewhere in between. An important aim of the challenge is for you to determine the present limits of your skill and technology and to spur you on to exceed those limits.

A forthcoming article and challenge extends these to extragalactic globular clusters, of which thousands are accessible to amateur imagers and a good number to visual observers --- one was catalogued by Messier!.

On 31/01/2021 at 21:28, MartinB said:

The imaging challenges are back! 

 

Can you clarify the rules please?

In particular, must the image(s) be taken on or after the start date of the challenge?

My interest is that my scope is in La Palma and I am stuck in Cambridge until at least the late summer.

I've a whole bunch of images taken last summer, some of them of satellites many here will have never seen before.

3 minutes ago, andrew s said:

Your next Doh! Maybe forgetting to remove it 😏.

Been there, done that.

All too frequently. 8-(

My Doh! moment related to focusing was when I realized that I could make a Bahtinov mask out of a sheet of corrugated cardboard. Printed out a full-scale design onto 12 sheets of A4 paper (this for a 0.4m scope), glued them to part of the packaging in which a replacement kitchen oven was delivered a couple of weeks earlier, then attacked it (very carefully!) with a Stanley knife and a steel ruler.

Works a treat, after two years or so of focusing by guesswork I now get perfect focus very quickly. Bahtinov masks at this aperture do not seem to be available and extrapolation from what is on sale had led me to suspect that a custom product would be serious dosh.

I confess that I never thought I would rule an optical-wavelength diffraction grating with a craft knife.

How long is a piece of string?

Seriously: stretch until you can see what you need to see within the limitations of your data and then stop.

See https://britastro.org/observations/observation.php?id=20200805_212500_dc7c69e9f34bb551

and

http://www.astropalma.com/Projects/Satellites/caliban.html

for extreme examples of mine.  The former  completely destroyed the appearance everything except the target of interest. AFAIK, this is the only amateur image of Thebe ever taken, and I have yet to see a professional one not taken by a probe in Jovian orbit.

22 hours ago, dave_tucker said:

My day job is as a Software Engineer and I was wondering how I might put those skills to use on the Uk's famous cloudy nights.
I've been playing with KStars/Ekos and INDI and it's awesome, but it's unclear to me (from the website at least) how to contribute my time (Ikarus will gladly take my money though lol).
So I'm wondering if any of the maintainers hang out here and might be kind enough to point me in the right direction... or if there are other popular projects that might need a helping hand.

I and a number of other people would very much like an INDI driver for the K8055N Velleman USB board. In my case it is used to control the dome. The LesveDome product works well under Windows/ASCOM but it would be nice to be able to use it under Linux/INDI.

15 minutes ago, mikeDnight said:

Absolutely Louis.  We'd use the Matabeelee gumbo beads or even chocolate buttons if they're worth more than the £, - and most currencies are! ☺

Eh?

In my experience, 1 GBP is worth more than 1 unit of almost all other currencies. Currently, 1.16EUR, 1.40USD,  102INR, 1.78 AUD,  30.22 CZK, ...

On of the very few exceptions is the Kuwaiti Dinar where 1GBP is worth 0.427 KWD,  This does not include such as ounces of gold (1 XAU = 1275 GBP)  or palladium (1XPD = 1715 GBP)

Work your way through www.xe.com if you want to find other exceptions. That is from where I took these figures just now.

1 minute ago, vlaiv said:

You are right - but selection of the storage / transport format should not be considered part of stacking process.

Yup. Saving as 32-bit floating point FITS images is a good trade-off between precision and range in my experience.

1 hour ago, woodblock said:

Thanks, I'd be interested to see your report. How would you send it?

 

An outline here and a tarball of the software attached.  Give me time to get my act together.

Median stacking is more robust against outliers such as cosmic ray hits or satellite trails, though you should still reject any which pass within the aperture radius around the target and its comparisons. The benefit comes from getting a better sky estimate.

Average or summing (almost identical) gives a somewhat better SNR than median for an identical set of subs.

I tend to use 30s subs. except for extremely bright stars which saturate the detector at that integration time.  For faint objects there could  be over a hundred subs.

Can give a report about my pipeline going from a set of subs to a report suitable for submission to BAA-VSS if you wish. My code runs under Linux but see no real reason why it should be successful under Windows or MacOS.

43 minutes ago, Spile said:

So are you going to be taking orders?

Seconded.  I could well be interested.

2 hours ago, SamK said:

https://www.robots.ox.ac.uk/~az/lectures/ia/lect3.pdf

 

Yay, there's a blast from the past! 30 years ago I used to work at robots.ox.ac.uk alongside Andrew Zisserman.

7 hours ago, SamK said:

After many hours of fiddling round with Registax wavelet settings to process my own solar system images, I've always been curious as to how it actually works. In doing so I've put together my own image sharpening program which does something similar to Registax wavelets.

 

Really nice. Kudos!

It took some fiddling to get it to work under Linux but I got there in the end. The stock WINE implementation doesn't cut it because the dotNET framework is not included. To assist others, I followed the instructions at https://www.dedoimedo.com/computers/wine-dotnet-mono.html

TL;DR install winetricks and mono-complete, then run "winetricks dotnet45". Wait for ages, accept licensing details and ignore error messages.

10 hours ago, AlexK said:

 And I do squeeze out every darkness minute available from my multi-hundred mile trips. Even when resting in the chair. Why bother driving so far otherwise? I can rest all month before the next New Moon!

 

There is another difference between our approaches. I don't drive anywhere. Neither, I suspect, would a beginner drive for hundreds of miles.

Just now, Xilman said:

I use a dim red flashlight. Table: accepted. If you're in an observatory we can assume you have one.  If not, a small fold-up table is lightweight and cart around a

AARGH! The above got posted somehow before it was complete and I can't find an edit button.

What I meant to post was:

... cart around and set-up.

IMO, we are both right. Do what works for you and I'll do what works for me.

14 hours ago, AlexK said:

All paper charts require you to work at some table and with a bright flashlight.

...

  I'm crunching hard DSOs by hundreds a night and have time to revisit favorite sky-candies as well 😍

 

I use a dim red flashlight. Table: accepted. If you're in an observatory we can assume you have one.  If not, a small fold-up table is lightweight and cart around a

Your second statement is the real difference between your technique and mine. When observing visually I like to look rather glance at something. "Hundreds a night" --- let's be very conservative and  assume only 200 in an 8-hour night which is 2.4 minutes per object, including overheads. In practice I very rarely observe anywhere near 8 hours in a single session.