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Dave Smith

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About Dave Smith

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    White Dwarf

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    http://astrosnaps.co.uk

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  • Gender
    Male
  • Location
    Maldon, Essex
  1. Hi Billy. I have never had any concerns about atmospheric correction because my comparison stars have always been in the same field of view. If there are no "official" guide stars in the FOV I choose others using Guide software with the UCAC4 star database added. Mostly, however, I use the comparison stars given on an AAVSO chart. My FOV is around 30'. When I used my 90mm refractor the FOV was approx 1.5 degrees but had no problem with atmospheric correction. https://www.projectpluto.com/ Hope that helps Dave
  2. Lovely weather last week including clear nights. Here are my variable star results from 4 nights. All are HADS stars. Stars that pulsate in size and brightness. Dave
  3. Presumably if you didn't plot the comparison stars then it will have a better scalefor the main star. Dave
  4. I like the shape of that curve. The cloud certaily is showing its effect on the last readings. Patrick will likely find this useful. He uses a curve fitting analysis which doesn't require a whole period, although he does prefer a period or more. I also have not been able to get AIJ to plot absolute magnitudes, although I must admit that I haven't really tried. My technique is to generate a BAA report and import that into Excel where there is much flexibility to how it is plotted, labels etc You seem to have well and truly been hooked onto variable stars Dave
  5. I was fascinated by Graeme's graph for this star so just had to have a go. Fortunately it was clear most of the time last night. Dave
  6. Why wait, He's already expecting data from you. I usually send the BAA data file. He prefers that you do not submit the data to BAA or AAVSO straight away so that he can do his stuff first. I was advised 3 months but I have only waited a month as otherwise I would forget altogether. Dave
  7. Wow - that is brilliant and an unusual shape. I wonder if it is multiperiodic although it is not marked as such on the HADS spreadsheet. With variable stars there is no need to worry about focusing, in fact some defocus slightly to avoid saturation, as long as neighbouring stars don't start to merge. Congratulations on your first HADS Dave
  8. I can't get a direct link but the page I had in mind can be found by following the Beginners link on the BAA website. http://www.britastro.org/vss/ There is also much detail on visual observing on Gary Poyner's website http://variablestars.co.uk Gary has impressivly submitted more than a quarter of a million visual observations from light polluted birmingham. The Society of Popular Astronomy also have a guide to visual observing which is linked to on this page https://www.popastro.com/main_spa1/variablestar/ Hope they are useful. Dave
  9. Some valuable variable star results are produced by visual observers. There is a guide to visual observation of variable ststs on the BAA web site. I will post a link in the morning. Dave
  10. Observing Variable Stars Why observe variable stars? It is fascinating to directly observe the variation in brightness of a star and ponder as to why it is varying. Some vary because the star is intrinsically varying e.g. due to it regularly expanding and contracting and some vary because there are actually two stars eclipsing one another. Some variations are highly predictable while others are not. Professional astronomers who study stars are highly dependent on the results of amateurs which mean any observations are very useful. What equipment is needed? Stars can be observed with just the naked eye but to be of scientific use then the minimum requirement would be just a pair of binoculars. Many variable star observers make valuable contributions to the databases using binoculars or telescope by visually comparing the brightness of the star with that of nearby standard stars over a period of time. More precise estimates of brightness can be obtained with a telescope and camera (DLSR or CCD camera). Software (Free or paid for) is available to enable the measurements of variable stars compared with standard stars. Anyone who can take an astro-image and apply darks, bias and flats would be able to take useful variable star measurements. When starting out it is not necessary to purchase any filters. Once you are capable of taking a reliable set of readings the first filter to consider is a Johnson V filter. Even then a filter is not vital to make a valuable contribution. Which software? I started off using MaximDL simply because I already had it. I would not recommend buying it (it is very expensive) for this purpose as better free software is available. There are two free astronomy applications available and I have used both. Firstly Muniwin. It does the analysis on images and produces a spreadsheet ready for submitting to the BAA VSS database and the AAVSO database. I have found, however, that it sometimes rejects some of the data for no apparent (to me) reason. More recently AstroImageJ has become available and seems far more consistent. Both of these two packages come with help files. Collecting data My experience has been entirely using a CCD camera for variable star observations and so the descriptions that follow are biased towards that. There are links at the end which will give guidance on visual observing or using a DSLR camera. I am restricting the software description to using AstroImageJ Choosing a target star Fortunately there is much help on this on the BAA variable star section database. http://www.britastro.org/vss/ Follow the link to Observational Programmes – CCD Eclipsing Eclipse predictions. I would recommend as a first choice an eclipsing binary star as the variability is continuous which makes detection of some variability highly likely on first attempt. Finder charts Some are available on the BAA site but I find the aavso site more useful. https://www.aavso.org/ The charts that can be printed from there include comparison stars with their precise magnitudes. E.g. EG CEP Locating the star. Personally I think SGPro is superb for getting the star in the middle of the FOV especially if plate solving is used. There is no need to worry about orientation, just get the star centred in the FOV. Depending on your scope and mount guiding may be necessary to keep the star in position. The task is then to obtain images over a period of time such that the target star and comparison stars are not at or near saturation. Using AstroImageJ Download it from https://www.astro.louisville.edu/software/astroimagej/ On first use it is important to upgrade to the latest version by clicking on “help” “update” and then going up to “latest build”. To check that star image is not close to saturation Open the file to be checked and then place the cursor over star and ALT-CLICK. A profile of the star will appear. As long as the highest point on the graph is well below saturation point (around 65000 for a 16 bit camera) and the sky background level is reasonably below the signal level all is ok. It does not matter if there is a star in the field of view that is saturated as long as it isn’t being used as a comparison star. Data collection, calibration and photometry Once the data has been collected AstroImageJ can be used to do the calibration. Click the DP button and two windows will appear. You need the Data Processor window. The other can be closed. On the first line click the first blue folder symbol and navigate to where the light files are stored and click select. Then click on the second blue folder symbol and click on the first file in the series and click open. Further down do similar selections for your master dark and master flat. If you want to use AstroImageJ to construct the masters then please read the full AIJ instructions (see links) Once that is done click START at the bottom and the files will be calibrated and put into a sub-folder pipelineout. (You can change this name is you wish). Platesolving It is not absolutely necessary to plate solve the images but it can save a lot of trouble. If all the images are well aligned and there has not been a meridian flip then you may get away without the plate solving stage. I have found that plate solving the images leads to smooth running of AIJ in the photometry stage. It is also well worth installing a local version of astometry.net as explained on the AIJ forum (see references). It takes under 20 seconds per image with the local plate solver. If there are many images the computer can be left to itself while solving. To plate solve with AIJ proceed as follows. Import the calibrated files to AIJ and select the astrometry icon. Most of the selections are clear but it is important that you input the correct arcsec/pix for your scope camera and the R.A. and DEC of the target star (from the aavso finder chart). Then click START and if all is ok it will proceed with the plate solving. Running AIJ photometry Import the plate solved files into AIJ and click on the Multi-aperture-Measurements icon The following window will open. Tick boxes as shown and then click on the Place Apertures button. First left-click on the target star (the variable) and then the comparison stars. The variable will be labelled T1 and the comparison stars C2, C3 etc. When finished multiple windows will open including a plot and a measurements spreadsheet. The measurements spreadsheet needs to be saved as Measurements.txt The plot can be configured to plot any aspect from the Measurements file including the star magnitudes (if the box on the penultimate line of the Multi-aperture measurements is checked. Submitting results to the BAA and/or AAVSO databases. You do not need to be a member of the BAA or AAVSO but you do need to apply for a login. See the BAAVSS database or AAVSO website for details. To generate files in a format to upload to either organisation, the BAA Photometry Spreadsheet is required and can be downloaded from the BAA database. There is also a full guide to AIJ and the Photometry Spreadsheet available. I find it convenient to use Excel to plot a graph of magnitude against time as there is more flexibility with presentation. e.g. If you get stuck at any point and cannot find any help in the references then do not hesitate to contact me by pm on SGL. Dave Smith Feb 2019 References BAAVSS website http://www.britastro.org/vss/ There are many links there to VSS database Eclipse predictions Charts CCD (Target lists) and many more Beginners AAVSO website https://www.aavso.org/ AstroImageJ https://www.astro.louisville.edu/software/astroimagej/ AIJ user guide https://www.astro.louisville.edu/software/astroimagej/guide/AstroImageJ_User_Guide.pdf AstroImageJ Forum http://astroimagej.1065399.n5.nabble.com/ Astrodennis guide to exoplanet imaging using AstroImageJ http://astrodennis.com/ There is also much detail on visual observing on Gary Poyner's website http://variablestars.co.uk Gary has impressivly submitted more than a quarter of a million visual observations from light polluted birmingham. The Society of Popular Astronomy also have a guide to visual observing which is linked to on this page https://www.popastro.com/main_spa1/variablestar/
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