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Tutorial: Observing Variable Stars


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Thanks Dave,

The biggest problem I have at the start is in getting my telescope to point at the right place - but I'm working on that.

Steve

 

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20 minutes ago, woodblock said:

Thanks Dave,

The biggest problem I have at the start is in getting my telescope to point at the right place - but I'm working on that.

Steve

 

I find that the plate solving facility in SGPro brilliant for getting the star right in the middle of the FOV.

Dave

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I'm using APT with EQMOD and Cartes du Ciel. APT has a plate solving facility which I'm playing around with at the moment. I'm not quite sure how to use it to get the scope in the right position. When I run it it tells me the RA and Dec of the centre of the image. So my target star is in the field but it's not at the centre. I know the coordinates of the centre but I'm not sure how to automatically move the scope to position the target star in the middle.

Steve

 

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Hi Dave,

I've loaded an image into AIJ and plate solved it which came out ok. Then I've tried a multi aperture photometry test, marked up the test star and the comparison stars and that came out fine except that I wanted the output in apparent magnitude. So I ticked the box on the last but one line to get the output as a magnitude. I define the test star and that's ok but when I click on a comparison star it goes to a website called SIMBAD which tells me that there is no astronomical object found even though the RA and DEC for the point are correct. I assume that the SIMBAD site should be recognizing the star at the coordinates and tell me it's magnitude. 

Steve

 

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Hi Steve,

I don't  use that method at all. I get the comparison stars from aavso.  If that doesn't  work I  get comparison stars from a star chart program "Guide" which has the UCAC3 star database. Usually the charts from aavso are sufficient. Personally I  don't  put the magnitudes in at the AstroImageJ stage but later when putting the data into the BAA spreadsheet

Hope that helps.

Dave

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  • 1 month later...

Hi Dave, another quick question.

On another thread I asked about a problem I had with the coordinate system and I wondered how it might affect me here.

The problem I had was that the star coordinates from the AAVSO website were slightly different from the coordinates of the same star in Carte du ciel. It turned out to be because the coordinates from AAVSO were based on J2000 epoch and while in Carte du ciel were based on the current epoch. Although the coordinates were very close the difference was significant.  So when you plate solve which coordinate does it come up with? In your description you have to enter the correct RA and DEC of the star taken from the AAVSO chart. That is normally the J2000 coordinate. Does that mean that the coordinates worked out by plate solving also have to be j2000 coordinates or doesn't it matter?

Cheers

Steve

 

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13 minutes ago, woodblock said:

Thanks Andrew,

So to make Carte du Ciel point at the right place I'd have to make sure that it was set up for J2000 too rather than Jnow?

I don't  know Carte du Ciel but yes that would be my starting point.

Regards Andrew 

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The thing that slightly confuses me is this. The star catalogues have the coordinates J2000. If I point my telescope at the sky and take a picture and then plate solve it will give me the coordinates as J2000. But the actual stars are Jnow and there may be small differences between their actual position now and the J2000 position. So presumably the plate solving software can allow for that.

Carte du Ciel is like stellarium. I use both but I find CDC easier to use with eqmod.

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1 hour ago, woodblock said:

The thing that slightly confuses me is this. The star catalogues have the coordinates J2000. If I point my telescope at the sky and take a picture and then plate solve it will give me the coordinates as J2000. But the actual stars are Jnow and there may be small differences between their actual position now and the J2000 position. So presumably the plate solving software can allow for that.

Carte du Ciel is like stellarium. I use both but I find CDC easier to use with eqmod.

That's right. When I use The Sky X I give it J2000 it converts it to Jnow and points there. If it plate solves it does it with J2000 catalogues and reports the actual position in J2000 coordinates. Similarly if it needs to offset the interface works in J2000 but the control system converts to Jnow under the bonnet. 

Regards Andrew 

Edited by andrew s
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@Dave Smith, many thanks for taking the time to write your tutorial! Nice to see others using AstroImageJ, its still my goto tool after many years.

A quick question, is there any benefit to stacking subs or not for variable work - what is commonly done? I must confess I've always calibrated, then stacked using average to create a new file. And then compared sequences of those new average files over time. Maybe I've been adding extra steps unnecessarily, as it is quite a bit of extra work.

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22 minutes ago, 7170 said:

@Dave Smith, many thanks for taking the time to write your tutorial! Nice to see others using AstroImageJ, its still my goto tool after many years.

A quick question, is there any benefit to stacking subs or not for variable work - what is commonly done? I must confess I've always calibrated, then stacked using average to create a new file. And then compared sequences of those new average files over time. Maybe I've been adding extra steps unnecessarily, as it is quite a bit of extra work.

I've never done that as the stars i have been looking at vary quite rapidly. I was advised that for longer period variations to just submit the individual readings. 

Dave

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1 hour ago, 7170 said:

@Dave Smith, many thanks for taking the time to write your tutorial! Nice to see others using AstroImageJ, its still my goto tool after many years.

A quick question, is there any benefit to stacking subs or not for variable work - what is commonly done? I must confess I've always calibrated, then stacked using average to create a new file. And then compared sequences of those new average files over time. Maybe I've been adding extra steps unnecessarily, as it is quite a bit of extra work.

It's perfectly fine to do this as it increases the signal to noise.  For example the Kepler the space telescope (may it rest in peace) had six second exposures that were combined to create 1 minute and 30 minute cadences.  It's mostly used on fainter objects, but as noted above it can be limiting in short variable objects if you only observe one period - if you observe multiple periods this becomes less of an issue as it is unlikely that the exposures will exactly align with the objects period.

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Hi Dave,

I've carried out my first observation session and I've been working my way through AIJ to process the images.  I have a book called Digital SLR Astrophotography by Michael Covington. There's a short section about variable star observation. In it he recommends that when taking the images you slightly defocus in order to spread the star image  but I'm beginning to think that's not such a good idea and I wondered if you do that.

As a test I did several shots of the star S Cas approximately one per minute. This star has a long period and my idea was that I should get pretty much the same magnitude on each shot over a thirty minute period. As it turned out the estimated magnitude varied between 11.8 and 12.1 and I wondered if that was good or bad. I know there are some things I have to do to improve my game but this was my first attempt.

Cheers

Steve

 

 

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Hi Steve

The purpose of defocusing is to help insure that the star image, and the comparison stars are not saturated. I don't go out of my way to defocus but don't worry too much about being in focus. It is important that you check that it is not too close to saturation.

S Cas has a very long period, you may find it more interesting to try a star with a short period so that you can detect change. Ther are suggestions on the BAA VS  web site.

Dave

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Hi Dave, the reason for picking a star with a long period was an attempt to check my method. I figured if I did a series of images and analysed them they should not show much variation in magnitude. If there was a significant variation then I must be doing something wrong.

I found it quite difficult to adjust the amount of defocussing. If you over do it the stars turn into circles.  I did find that some of the bright comparison stars were saturated so I'll have to adjust the exposure. The problem then is that the dimmer comparison stars you can hardly see to select them.

Thanks

STeve

 

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2 hours ago, woodblock said:

Hi Dave, the reason for picking a star with a long period was an attempt to check my method. I figured if I did a series of images and analysed them they should not show much variation in magnitude. If there was a significant variation then I must be doing something wrong.

I found it quite difficult to adjust the amount of defocussing. If you over do it the stars turn into circles.  I did find that some of the bright comparison stars were saturated so I'll have to adjust the exposure. The problem then is that the dimmer comparison stars you can hardly see to select them.

Thanks

STeve

 

Hi Steve

It is better if the comparison stars are of similar magnitude to the target star if possible.  I've just had a look at the aavso chart for S Cas and can see that it could be tricky. One of the brighter comparison stars have a few faint ones very close by.

Dave

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  • 1 year later...

I realize that I am very late to this discussion but, like Dave, I use a CCD camera, a V filter (most of the time, Sloan r' almost all the rest with a smattering of unfiltered where timing is the principal interest), the AAVSO charts and a local installation of astrometry.net.

Where we differ is that I very strongly recommend Russ Laher's "Aperture Photometry Tool" (another APT, not the one mentioned above). APT is described at https://en.wikipedia.org/wiki/Aperture_Photometry_Tool which is not to say Dave's solutions are not also good ones!

APT is available from https://www.aperturephotometry.org/ Amongst its benefits are the choice of several models for the sky background measurements and elliptical apertures (circles are zero-eccentricity ellipses and they are supported of course). The latter are especially useful for measuring poorly tracked images as well as for photometry of galaxies which is their intended usage.

It runs on anything which runs Java (I use Linux, others Windows or MacOS). It is of very high quality, being written by a professional astronomer at Palomar for use by professional and amateur astronomers everywhere. Russ is very responsive to feedback - I have notified him of a few inadequacies and their prospective fixes; he implements them very quickly and gives credit where due.

APT is easily scriptable and I have written both pre- and post-processing scripts which I use to make BAA-VSS submissions. All my code is freely available on request.

 

Edited by Xilman
Fix parenthesis
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