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A beginner....

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Joined: 04/02/2018 - 17:56
A beginner....

Hello all,

I have recently been inspired to do more with my astronomy than attempting to take pretty pictures, and having seen Gary Poyner deliver an excellent talk to the StargazersLounge community (in fact, I co-hosted it) I am looking to take up measurements with my CCD camera.

My equipment currently stands as:

  • Sky-watcher EQ6-GT goto mount
  • Skywatcher Esprit 100mm f/5 refractor
  • StellaLyra 6 inch f/12 Cassegrain
  • AtikOne 6.0 CCDD camera - Clear, RGB, Ha, Oiii, Sii and 650nm IR Pass filters (I swap the RGB and Broadband filters according to the object, leaving the Clear (focussing) and IR Pass (Luminance) in situ)
  • ZWO 120Mini CMOS camera

Hopefully this will give me a wide spread of possibilities!

My thinking is that I will mix taking measurements of objects in with my normal sequences of capturing the pretty picture data, and in this, I have a couple of questions:

  1. Would any of the existing filters be suitable for this work? If not, I had thought of obtaining a CCD V filter, and using that for double duty as a focus filter
  2. I assume that all images will be corrected with bias, dark and flat-field frames. Is there anything else I should do to ensure my data is properly calibrated?

Hope everyone is well is the current climate, and looking forward to getting started in variables!

Kind regards

Darren

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CCD photometry

Hi Darren,

It is truly pleasing to learn of someone who wishes to do science as well as art!

I would strongly recommend getting a Johnson-V filter. It is by far the most used photometric filter and although it is possible to emulate its results by taking images in several other filters at the same time, that process is really not recommended unless you have no choice.  It requires more exposure time with complicated calibration and data reduction processes to give inferior results.

Until your filter arrives you can spend your time productively working through the data reduction process on whatever images you have to hand. The results will not be compatible with measurements made in standard pass bands but you will learn much about what needs to be done. There are all sorts of niggles that need to be dealt with, as I have learned the hard way.

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Hello Darren,

Hello Darren,

You have some excellent equipment for CCD observation of variables. For many years I too used a 100 mm refractor. For many branches of VS work you don’t need a filter, so that might be the place to start to see how you get on before buying a V filter.

This includes photometry of cataclysmic variables, such as looking for outbursts of dwarf nova. There is a thread on here about IX Dra at the moment; I use unfiltered CCD for that. Also have a look at the VSS CCD target list which was developed to provide people who are new to the field of CCD photometry of variable stars with some interesting targets to which they could turn their CCDs, whilst developing their techniques. The G filter could also be deployed if you wish. It’s close to V. The only processing you will need for photometry is the calibration you referred to.

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Good point.

True.  I had overlooked the possibility of actions such as detection of outbursts and period searching in time series of observations.  Crazy, really, because I generally go unfiltered when observing exoplanet transits.

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No filters required

I'd just like to emphasise Jeremy's comments. Ultimately it would be good to aim to get some filters but proper photometric filters are very expensive and a lot of work can be done without them or even using the much cheaper filters designed for colour imaging. 

If you are interested in trying something slightly different you might want to look at our project to monitor the comet 29P/Schwassmann-Wachmann. Have a look here for details. This comet is well placed at the moment and normally sits around mag 16-17 but it can flare up and catching the eruptions early is very important. Richard Miles runs this project on behalf of the comet section. It might be a little faint for your kit but try some images of the field and see what you get.

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Doing science via photometry - Bias images

Glad you are looking to go one step further ...

Can I just point out about the need (or not as the case might be) to take Bias frames. Professionals do this because lots of people use the same instrument and they all probably want to use different exposure times. For the amateur, I would recommend NOT taking Bias frames. After some investigation, I decided it was a pointless exercise. Better to take a series of Dark frames using the SAME EXPOSURE TIME as your Light frames. A good compromise for a standard exposure time is 60 seconds but that will depend on your mount's tracking performance.

More important is the need to average a good number of Dark frames (and Flats) so that you achieve a satisfactory SNR. How much SNR you need in the Flatfield depends on what you are trying to target for observation. Say 20 frames of each are a good compromise. Also - Don't forget to take the Flat-Darks with the same (short) exposure time as your Flat frames. HTH

P.S/ Nick James mentions Comet 29P - you might find you will have to sit on that target for say 30 minutes with your 6" Cass. to build up enough SNR. Feel free to try.

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A biased opinion.

We are going to have to agree to disagree on bias images.  As an amateur photometrist I often take Light exposures which vary between a second or few to a minute or few --- roughly two orders of magnitude --- for each sub. Bright stars saturate in less than five seconds; faint ones are barely measurable after stacking an hour's worth of subs.

Taking bias frames is a matter of a few minutes work in my experience. Each image has essentially zero exposure time (almost by definition) and at most a few seconds download time. Also in my experience, processing bias frames is essentially cost-free in most (all?) software. After co-adding the bias images to create a master bias, flush their subs to recover the storage.

Very strongly agree with the advice about taking a good number of dark, flat (and bias!) frames to average out the noise.  I generally use somewhere between 10 and 100 depending on my patience and the brightness in each filter of the fluorescent panel used for the flats.

You may have gathered that there are almost as many opinions as photometrists...

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Well I have never used a bias, ever!

Paul is right that there will be lots of opinions. Here is mine based on many years of imaging work.

My calibration steps involve having a library of dark frames of different exposures so that I never have to scale darks. Since I don't scale my darks I don't need bias frames. I generate the darks when it is cloudy and generally average 30 - 100 raw dark frames at each exposure to get my library dark (I do 5, 10, 15, 30, 60, 120, 300s darks). My camera has good temperature regulation and so I can use dark frames from 6 months ago and they are fine. If you have a CMOS camera you probably will find that scaling darks is not always successful so, again, it is a good reason to keep a library of darks at different exposures rather than trying to scale them. 

I have always used sky flats, again taking 30 - 100 raw flats, subtracting the flat dark and then normalizing and averaging them to get the master flat. Since the right conditions for making twilight flats don't come along very often I tend to re-use flats for a month or so. I'm lucky, I have a permanent observatory and the camera never comes off the telescope but new dust spots appear with monotonous regularity. You can see an example of that in the lower right of this image.

When you do the averaging is best if you can use floating point format output FITS files rather than 16-bit integers. This applies both for the calibration frames (flats, darks) and for your final stacked images. Certainly don't using summing with integer files since you will end up saturating brighter stars as their summed pixels hit 2^16.

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Bias and CMOS

If you have a CMOS camera be aware it has two possible problems with bias frames.  First, amp glow is not linear with time. Two CMOS can have different readout modes for very short exposures such that extrapolating "long" dark frames of different lengths back to zero time does not give the same value as a "short" bias frame.

Regards Andrew

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Joined: 04/02/2018 - 17:56
Thank you all for your

Thank you all for your replies!

Noted on the use of the G filter as a starting point while I add the photometric filter to the 'I really need...' list - it's amazing how long that list gets!

My Atik camera is a cooled CCD and does a good job at holding the set point. My normal calibration routine consists of:

  • Camera normally running at -10 degC (absolute, not 10 below ambient)
  • 101 bias frames, averaged to a Master
  • 21 Dark frames of varying exposures, all averaged to a Master (refreshed on a 6 month cycle)
  • 11 Flat frames per filter, all averages to a Master, taken at each session
  • Use Pixinsight to create the Master Bias/Darks
  • Use Pixinsight to calibrate the flat frames using the Master Bias [However, it would not be an issue to take Flat Darks and use these instead]

The guiding (off-axis) with my refractor on the EQ6 is usually very good - typically I would take 15m or 20m exposures, but have done 30m with no problems. The Cassegrain is a recent purchase and not yet managed to get time to train the guiding for the longer focal length.

I'm looking forward to getting started, and offer my apologies in advance for the raft of questions that will no doubt follow!

Kind regards

Darren