Tangra 1.4


Resolving Common Plate Solving Issues in Tangra

This page covers a couple of issues that may cause the plate solve to fail as well as gives some suggestions on how to improve the plate solving - make it faster and recognizing more reference stars.

Do you have enough stars?

A common reason for not getting a fit is an insufficient number of stars particularly with non integrating video cameras. By default Tangra requires at least 7 reference stars for a successful plate solve. In fact that many stars are recomended for a good position measument. However in some cases you may have only 6 or 5 reference stars. In this case you may still be able to get a good position but you have to configure Tangra to accept 5 or 6 reference stars only. This can be done from Configuration -> Astrometry -> Astrometric Fit -> Min Number of Ref Stars.

If you have less than 5 stars you will need to increase the exposure (integration time). If you have more stars but they seems to be very faint you may try some of the other suggestions below.

Is your limitting magnitude configured correctly?

The "Limitting Magnitude" setting on the calibrated configuration has the meaning of "What is the maximum magnitude that you could get to using this equipment and your longest useful exposure". It is used to determine how many stars will be loaded from the star catalog for the plate solve. If this value is too low then it is likely that you will not get a plate solve. Typically with 8" or larger telescope and integrating video camera that supports a couple of seconds exposure often a magnitude of 15-16 can be reached. It is recommended that values smaller than 14 are not used unless the field of few is significantly large (a degree or more).

If you are using a non integrating video camera for bright NEAs then try to increase the limitting magnitude to say 13 even if your camera can only do 11.5 for example.

Do you have too many hot pixels?

How many is too many depends not only on the number of the stars in the video but also on the brightness of the stars and the hot pixels. The best way to deal with hot pixels is to use dark fields. Tangra can generate dark fields from a video and then this dark field can be loaded and subtracted. Another way to handle hot pixels is to use the "Filter our non stellar objects" setting on the Settings -> Astrometry tab. Because the hot pixels will have either too small FWHM or will be too elongated (when they are hot blocks of pixels) using the "Filter our non stellar objects" will exclude them. However you have to be careful when using this option if your stars are too large or when they are slightly elongated (e.g. as a result of optical alignment problems).

Are your stars too dim?

If your background is too bright because of long exposure or the stars in the FOV are just too faint then Tangra may not be able to recognize them as stars. There are a few things you can do.

(1) Firstly make use of the Brightness/Contrast adjustments offered when starting the astrometric measurement and try to make the stars easily visible.
(2) Secondly on the "Identify Star Field" form when entering the UTC time of the observation at the bottom there are settings for detecting the faintest star in the video. Instead of using the option to "detect automaticallly" try to specify the faintest magnitude yourself (after you have determined it comparing the FOV to a star catalog)
(3) And lastly on the "Stellar Object Requirements" group on the Settings -> Astrometry tab there is a value for "Min Detection Certainty". This value is not used only when excluding non stellar objects but is also used when the faintest magnitude in the FOV is set to "detect automatically" and Tangra needs to determine it.

If you generally work with faint or hard to detect stars you may want to decrease the value for "Min Detection Certainty" to 0.5 or 0.3. The best way to find what value works for you is after you do a plate solve to select a one of the fainter stars that you would like Tangra to be able to recognize and check its "Detection" value (see below). Then enter this or slightly smaller value for "Min Detection Certainty"


Double check your input and configuration values

If what fails is the calibration of your system then it is possible that you have misstypes one of the parameters. Having chosen correctly the camera is particularly important. Some cameras have a PAL and NTSC model and they have different CCD size. Also different models of the same camera may have different CCD dimentions as well. For example Tangra lists all WAT-902H PAL cameras under one item. However it is not impossible a future version of the camera (say WAT-902H4) to have different CCD dimentions. If you are unsure it is best to check the physical dimentions listed by Tangra for the camera model you have selected against the documentation of your own camera. If the values don't match or if your camera is not listed by Tangra then you will need to add it as a new camera. To do this simply press the "New" button on the "Select Video Camera and Recording Configuration" dialog and enter all parameters from your camera technical documentation.

Checking CCD Dimentions

Adding a New Camera

The next thing to check is the orientation of your field of view. Depending on the number of reflections in your optical system (including diagonals) you may have a flipped image. If this is the case then Tangra will not be able to calibrate the configuration if you haven't selected the correct flip settings. It is important to note that there is no essential difference for the plate solving between "Flip Horizontally" and "Flip Vertically". If your field of view is flipped then you need to choose only one of those options. Checking both flip settintgs is equivalent to choosing "Rotate 180 Deg". Rotation doesn't affect the plate solving but could be used for user convinience for example to display North - Up if you need to compare the field with a planetarium program. You may find that even it is not required for the plate solving to work, it will be more convinient for you if your FOV matches your planetarium program which usually means to have North - Up. Sometimes in the southern hemisphere people may prefer to use South - Up.

Checking Orientation

Checking Location

It is also important to note that Tangra can plate solve even videos taken with Alt-Azimuth mounts where North or South could be anywhere depending on the date and time you have taken the video and will rotate with time. In such a case you will not be able to configure North to be Up and should only look at the Flip settings.

Finally it is also a good idea to double check the coordinates of your field center, the date when the video is taken (so star proper motion can be applied correctly), the UT time and the object designation (in a case you are identifying the field by an object in the FOV), the coordinates of your location (in the "Setings" -> "Star Catalogue & Location" tab).

Is your configuration slightly different?

You may have changed one component in your system, for example the focal reducer with a slightly different one or you may have changed the way you record the video. Any change in the optical or recording part of your equipment will likely change things sufficiently for plates to become more difficult to solve or it may not be possible to solve them. The focal length is also important. With temperature the focal length of you system will vary slightly from summer to winter and Tangra allows for a huge 5% variation. However if you change your focal reducer from F/3.3 to F/3.1 this may exceed the 5%. Other changes that would make the system different that you may not think about are for example:

(1) Changing the video timer. As a result the OSD area may change. You will need to recalibrate making sure to redefine the OSD area location for your new timing device
(2) Chaning your video grabber. It may change the aspect ratio or the dimentions
(3) Chaning your video grabbing or recording software. Again may change the aspect ratio or the dimentions

Are your stars too elongated or too large?

If you are using the "Filter our non stellar objects" from the Settings -> Astrometry tab, and your stars are too elongated then they may be excluded because of that. Try turning this setting off. If you need to use it to deal with hot pixels then increasing the "Max PSF Elongation" value could help.

If your stars are too big then they may be also excluded if the FWHM it outside the range specified in the "Stellar Object Requirements" group. In this case again try turning the "Filter our non stellar objects" setting off or increase the Maximum FWHM.

Do you have a white area or lines at the video border?

This is something that could happen (particularly if you are using a video tape recorder) and will cause troubles. Sometimes there may be a white line or other white artefact at the image border (e.g. see below). In this case Tangra may have troubles recognizing the stellar objects in the video. To cut the white edges from processing you will have to use the "Clip Border Pixels" value for your calibrated configuration. Use a value with at least 5-10 pixels larger than your white edges.