Can someone more competent in maths then I am, help me? I sometimes need to determine Position angle of double stars from plate-solved images. Therefore I got position angle of the image, RA and Dec of the centre of the image and RA and Dec of star I’m interested in. How do I calculate position angle for that star?
I know that best method would be to do drift imagining of that star to find out position angle. But I just take as many images as possible during the night and then during the processing of images and checking them against Skymap I may find some objects that interest me and I try do Photometry or measurement of separation and position angle.
Position angle is defined relative to South-North direction, anticlock-wise.
So, PA is:
PA (stars pair) = PA(stars pair, measured on plate) + PA (frame).
If PA (frame is 0, PA = PA (measured)
If for example, PA frame is 5°, and PA (stars pair measured) =15°. that means PA (star par) is 20°.
If result is greater than 360°. then you should subtract 360° to get the proper result.
I agree that the best method to more accurately determine the PA of the frame.would be to take one frame without tracking.
Yes Bojan that is right. But I need to know position angle of star that can be anywhere in the image, With plate solve I only know the PA fom centre of the image. Once I know PA of the star of interest (and pixel/ arc sec ratio) I input it in Reduc software and it will calculate PA and the separation for that double star system.
I did not express it correctly I think. I need to calculate frame orientation (PA) relative to the star of interest from parameters available from plate solve.
Bojan, here is image of gamma Leporis, double star system. Plate solve gives me frame PA +01 55’. But gamma Lep is not in the centre of the frame. So I can not use frame PA to calculate PA of gamma Lep double. Question is how to calculate PA of frame with gamma Lep in its centre.
By the way if you are interested in free software that calculates double stars PA and separation have a look on http://astrosurf.com/hfosaf/
Ah, now I see.
If the frame is small, then you can use frame PA as given by this software, there will be some error depending on frame size and DEC..
However, if you need more accurate measurement or the frame is wide-field, then give me some time and will come back with the formula, it is relatively simple spherical trigonometry.
One of the issues is lens and FOV distortion, result of projection from celestial sphere to flat surface of the sensor.
If we assume ideal lens (pinhole), then the problem can be solved easier.
However, I need some more time to think about it, unfortunately I forgot a lot of this stuff since I used it last time ....
I will be back with this.. no way I will give up now . especially because I am interested in this kind of work.
Thanks Bojan, but don’t worry about it too much. I knew that it would not be easy to solve. Trigonometry was never my strong point; it is not used much in electronics. And what I learn at collage some forty years ago – it’s mostly forgot by now.
The frame size is about 27 X 35 arc minutes. In this image the error would not be so big as the star is close to the centre. But sometimes I find interesting doubles close to the edge of the frame.
I found this sort of measurements interesting because according to WDS catalogue, some double stars last measurement was done 100 years ago.
Trigonometry ... it is not used much in electronics. And what I learn at collage some forty years ago – it’s mostly forgot by now.
Exactly my case
Still, I want to know what measurement error we can expect.
I know that error will be zero at equator, and very big near celestial pole.. but how much, that is the question
Last night I was pondering how to calculate PA for any star in the image. As I said before my math abilities are very rusty by now so I thought there must some other way how to do it. Usually there is. So I come up with this.
Use centroid to find pixel coordinates of the star.
Convert FITS image to Bitmap.
Open it in some Paint program. I use Corel Paint
Calculate pixel coordinates of four corners of rectangle with the star in the centre of it.
Select and copy.
Paste as new image.
Back in Maxim convert Bitmap to FITS
Solve Plate.
And we got PA for the star of interest. The only possible error is the accuracy of pixel coordinates determined by centroid. But that error would come up anyhow when calculating PA of binary.
I have used newly created image in Reduc program to calculate PA and the separation. It comes up with believable result.
It is bit of mucking to do simple thing, but allows me to use almost any old image to measure PA and separation of the binary stars in that image. It will not work if the binary is too close to the edge of the image as there may not be enough stars in reduced image for plate solve to work.
Trigonometric solution would be easier to use as one could write small basic program to do calculations and would not have to do all that converting of the image to get same result.
Fairly simple to calculate, but it depends on the specific usage.
The difficulty is to convert Right Ascension into arcsec/arcmin, so as the position are in terms of the Declination (already in arcsec/arcmin).
Basically, the aim is to find the positions in x, y co-ordinates.
I've attached the method and examples to do this in a pdf
Image Calibration using ALADIN (and finding PA's...)
Following up on the previous post, I was surprised that no one commented on using the Java software ALADIN. Most seem to think this software is very complex, but is open-ended enough to allow very useful physical data on deep-sky objects or positions of double stars. This 778k to 1.7Mb software can calibrate known positions of stars from your jpg, png or gif images, and can even correct for field image orientation and projections. You can also create and use many basic and advanced filters or write your own Plugins.
Coupled with the SIMBAD Database, there is little one can do that someone only dreamed at a few years ago! Ain't technology just grand!
I have created and attached a slightly useful pdf file here to do basic calibrations, so you can measure doubles etc. - as well as answering the interesting position angle problem expressed here. (Compared to the mathematical pdf in my previous post - well it is not so mind-numbing.) I have particularly dumbed down the text, mainly not to put people off - but also to highlight the real power of ALADIN for those who are not aware of its existence. Interested amateurs can explore this software in far more detail to cater more for their personal interests. Best of all it is FREE and is not operating system dependent!
Last edited by Enchilada; 31-12-2008 at 09:19 PM.
Reason: Second improvement of the attached pdf
Enchilada, thanks for those links. I will have look on Aladin. But I’m quite happy with Maxim DL + PinPoint and Reduc software I’m using at present. It calculates centroid of the star automatically and it also calculates statistical errors when using multiple images. My problem with PinPoint is that it can calculate PA for centre of the image only.
But I’m quite happy with Maxim DL + PinPoint and Reduc software I’m using at present. It calculates centroid of the star automatically and it also calculates statistical errors when using multiple images. My problem with PinPoint is that it can calculate PA for centre of the image only.
Karl,
ALADIN also has similar features that can do this too. I do understand your reluctance in wanting to work with your experience with your pre-existing software. But after reading you post on 20-12-2008, 01:46 PM, I thought you were doing conversion procedures that were taking much time and effort just to simply get the PA.
Without prying too much, and I'm pretty curious what are you actually doing with these applications? I know you are doing double star work, especially if you are using Reduc software - but to what end? (Never used PinPoint, though) What kind of field-of-view size are you covering with these images?
Note: At least you now have the trigonometric calculations specifically originating for double star observer usage.
I'm pretty curious what are you actually doing with these applications? I know you are doing double star work, especially if you are using Reduc software - but to what end? (Never used PinPoint, though) What kind of field-of-view size are you covering with these images?
Note: At least you now have the trigonometric calculations specifically originating for double star observer usage.
What I’m doing and why, good question. I have very obstructed and polluted view of the sky from my back yard. I take images of minor stars in constellations that are accessible to my telescope. Later when I process the images I compare them with planetarium program and if I see something of interest I may try to do photometry (I don’t have a photometric filters) or find out PA and separation of double stars. My equipment and my methods are not good enough to produce results of any value to astronomy. I’m doing it mostly to satisfy my own curiosity about stars in the images I took. I’m using 120mm f5 Achromat and GStar CCD camera. That gives me 2.85 arc seconds per pixel resolution. Not good for calculation of close binaries. I will try 2X Barlow when I get extension tube.
I have installed Aladin and it looks interesting. Now I will have to make effort to learn how to use it. As I’m getting older I find it more difficult to learn new things. You know what they say about old dogs.
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Calculating Manually PA from images
Image Calibration using ALADIN (and finding PA's...)
