HOW TO TAKE PART

Go to the HOYS SERVER and click on Login, then Register.

Follow the instructions.

This is a telescope / camera combination. If you plan to submit images from several telescopes/cameras, please set up a device for each of them (you can do this at any later stage as well).

When you set up your device, please make sure that in the description box you also put your name and affiliation details, as well as all necessary details of your telescope and camera. We will use these should any of the data be published.

After logging in, click on ‘Upload’ and either drag files into the box or click select files to open a browser window. You can select multiple files (by either using Ctrl+click or Shift+click and ‘ok’ or using the ‘Select files’ button several times) before pressing ‘Upload’ on the form. They will all be uploaded in one go.

It’s useful if your file-names at the very least contain information of the target (either name or number – as on the target list), the filter that has been used and (if you have more than one) the Device the image has been taken with. During the other processing steps you will be asked to identify the Target region and Device used manually and if all your files are called “200.fits” you wont be able to tell which region the file is from. Furthermore, the software will attempt to read the filter information from the FITS header but if this fails you will have to provide the filter information manually – and if that info is in the filename this will be useful.

This is the first real processing step. After you upload the files click on ‘Process Files’ and you will see all your uploaded files with 4 processing buttons/steps next to it. The one highlighted in darker blue is the next one to be done for each file and initially all of them should be set to ‘Observation target’. The four steps have to be done in order for each file, but it does not matter in which order you do them for your files. You can process each file from the beginning to the end, or do the first step for all of them – your choice. Note that the current manual has a ‘Photometry’ button which we have now taken out – it is part of the last button called ‘Photometric Calibration’.

When you click on ‘Observation Target’ you will be asked to select from a drop-down menu which target region the file is for and which Device the picture was taken with. We could, in principle, identify the target region using the coordinates, but as some targets are very close to each other and there is overlap (depending on the field of view), this will cause errors – hence the decision to ask the user. The question for the Device is only to: i) speed up the astrometric solution (since you had to provide the pixel scale) and to ii) store this information for later. For example one might find that data from certain devices have systematic errors and we then can account for this in the analysis if we know the Device the image was taken with.

After you click ‘Enter details into database’ you will get back to the Processing Files screen and can do the next step – “Meta-data”. If you click on this the software will try to extract the following information from the FITS header of your file: Time and date of the observation; Filter that has been used; Exposure time (in seconds) of the image. The date/time is vital to plot lightcurves, the filter needs to be correct to calibrate the data, and the exposure time (together with the Device information about main mirror/lens diameter) will be used for statistics.

This meta-data extraction is usually automatic, but the first time you process a file for a particular device you need to teach the software which information is stored in which header card. Hence, this slightly more tricky process needs to be done only once (and is also described in the manual).

Basically you will be asked in which format the date/time is stored. This can either be as date in one card, time in another card, or both in the same card as e.g. 2017-09-11T18:06:23, or as Julian date. If you have selected the format you then need to select the FITS header card in your file which contains the information for that field, e.g. the Julian date. Similarly you need to identify the card that contains the filter used and the total exposure time. Note if you have stacked images we’d like to total exposure time and not the single exposure time for the individual images.

If some of this information, e.g. the total exposure time, is not in your header you can leave this field blank, but you will then have to put this information in manually for every image you process. Note: We currently are not able to cope with Devices that have two filter wheels where the filter used is e.g. either in FILT1 or FILT2 in the way that one has: FILT1=Blank, FILT2=Visual or FILT1=Blue, FILT2=Blank In this cases we’d ask you to leave the filter card blank and manually add it every time you process an image.

Once the time/date format and header cards are correctly identified, click on “Add header cards to device” and you are done. If you now use “Meta-data” for another image from the same device the website will display what it has extracted from the header and you can check it – the full header is also displayed at the bottom of the page. Behind the extracted filter information the page also displays what it thinks the filter name means i.e. a letter such as B, V, R, I, etc. So if your card entry is ‘Visual’ or ‘visual’ or ‘vis’ or ‘TG’ or tg’ it will interpret it as ‘V’ and calibrate it into the V-Band.

If any of the information is wrong you can manually edit it using the blue: “This is incorrect, modify values” button. You will have to do this every time if you have left one of the header cards blank. But once you set up everything the first time, this step is very quick. Note that after this is set up, you cannot change the header cards. If you change the card names in your FITS files you will need to set up a new Device for these images!!!

After the above steps, this one requires basically no input, only checking. After you click on “Astrometry” the software will use the astrometry.net software to identify the stars on the image and to determine the plate solution for the picture. This is speeded up by knowing the approximate plate scale from the Device setup. But the process still can take a while, in particular if the field of view is small or many users are working in parallel. Please do not close the browser during this process, just wait patiently.

When the process has finished it will display your image with stars over-plotted as red circles the software has identified in it’s database. You can click on the image to get a higher resolution version. If the red circles are on the stars, then it has worked and you can click the ‘It’s correct’ button. Note that not all stars will typically have a red circle around them. If the process times out or does not work for an image, but you can solve it on the astrometry.net website, please let us know.

The ‘Photometric Calibration’ button does two things:

1. It runs source detection and photometry software (SourceExtractor) over the image. This detects all stars, measures their fluxes and converts them to instrumental magnitudes. These magnitudes need to be calibrated into apparent magnitudes, or at least an instrumental magnitude system which is the same for all images.
2. Thus, we need to calibrate the photometry. For this purpose, we have for each target and filter a ‘calibration master frame’. Most of the calibration process is completely automatic, and you, the user, doesn’t need to do much, except checking if the result is ok or not. This is done via two graphs which are displayed when the calibration has finished. Like with the astrometry, this process can take a while, in particular if there are a large number of stars in the image.

The software first matches up all the stars in the image with the stars in the calibration catalogue. Only stars which are judged to be well detected are considered. A plot is then generated which shows for each matched star the difference between the instrumental magnitude in your image and the magnitude the star has in the reference catalogue. This difference is plotted against the instrumental magnitude of your star. This is the top of the two plots.

The software then tries to find a trend line which describes how this difference depends on the magnitude. This is over-plotted as a blue line in the top plot. If the data was correctly calibrated, this blue line should be at a value of zero for all magnitudes. To achieve this the software fits a function to the blue line. This then needs to be subtracted from the magnitude difference. The result is shown in the bottom plot on the result page.

Figure 1

The bottom plot shows as red points the same stars as in the top plot. But this time the difference of their instrumental magnitudes and the magnitude in the reference catalogue is plotted, considering the fitted correction function. Hence, now all these points should scatter around zero along the y-axis. The x-axis now also shows the calibrated magnitude and not the instrumental magnitude anymore.

There are two further green dotted lines which indicate the typical scatter of the red points from the y-value of zero. Typically about 1/3 of the red points should lie above the top green line or below the bottom green line. The smaller black dots in the bottom graph are all other stars in the image which had some issues with the photometry (i.e. they are near the image edge or have a nearby neighbour, or don’t look like a star). They do however, typically behave the same as the ‘perfect’ stars – red dots.

Figure 2

The task of the user is now to either accept, reject/delete or re-calibrate the image.

When to accept an image (Calibration graphs appear correct – green):

Simply put, when everything looks ok – see Figure 1. Basically the red points in the bottom graph scatter around zero. There is more scatter for fainter objects than for bright objects. The scatter for the bright stars (left) is no bigger than 0.1mag. There is not a significant number of black points to the right of the green vertical line – this line indicates the maximum magnitude considered in the calibration. There is no significant gap between the green vertical line and the red points.

Figure 3

When to re-calibrate an image:

Simply put, when the data looks like it should be accepted, but some things could be improved. This can either be if there is a large gap between the vertical green line and the red points – e.g. Figure 2; or there is a significant number of black points to the right of the green vertical line – e.g. Figure 3. In those cases you need to adjust the value for the ‘MAX_USE’ parameter and press “Retry calibration with the above values”. The MAX_USE parameter basically indicates where the vertical green line is. The software tries to determine it’s position but gets it wrong sometimes.

Figure 4

Hence, if there is a gap between the red points and the green line, use a value which corresponds to the magnitude in the top graph where the points ‘thin out’. Similarly, if there are many black points to the right of the green line in the bottom graph, use a MAX_USE value that is a bit larger than the point in the top graph where the green line is drawn. For the example in Figure 3 you could use MAX_USE=19.3 and then you get the output shown in Figure 4. This would be an acceptable fit.

You can repeat the re-calibration as often as you like with different parameters until it looks ok. But usually, after a bit of practice, only one or at most two re-calibrations are needed.

Figure 5

When do you reject the calibration and ‘Reprocess file’:

Simply put, when the data looks very bad and/or has a lot of scatter, such as in Figure 5. There are two possible reasons for such data:

i) Observations were taken under bad conditions, i.e. extremely variable and/or thick clouds. In these cases, when the scatter of the points is more than 0.1mag everywhere, the data is unusable and should not be included in the database – unless:

Figure 6

ii) For some reason the software has mis-identified the filter the data has been taken with. This can happen if for some reason the FITS header entry is wrong, or if you manually entered the wrong filter. We have done this on purpose for Figure 5. We pretended it is an R-Band image but it actually is an I-Band image. If we reprocess the data, with the correct filter, then the result immediately looks like Figure 6. This will, of course need some re-calibration and with using MAX_USE=19.3 it will look like Figure 7 and would then be acceptable.

Figure 7

When to reject the calibration and to delete an image:

Simply put, when all of the above described things don’t work and the data looks very noisy with lots of scatter whatever you do. If you are unsure if an image is acceptable or not, simply do not press any of the four buttons, but click on “Process Files” on the top of the website to get back to your table of images to be processed. Please get in touch stating the image ID number (the one at the front of the filename given to it by our software – first column in the table of images) and we will have a look at it and send you feedback.