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Advancing into Photometry

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#1 garyhawkins

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Posted 14 January 2021 - 01:16 PM

In recent weeks I started a course on Photometry run by Boyce Research Initiatives and Education Foundation (http://boyce-astro.org/). In particular, I'm currently taking their B.R.I.E.F Introduction to Photometry and Exoplanets.  I started with the IntroSTARS™ online course for the astrophysics fundamentals needed to enroll in all Boyce-Astro seminars and programs.  The aim of my current course is to train folks for exoplanet transit monitoring.

 

It soon became clear the equipment/software I had for EAA suited photometry well.  All I needed to do was add a processing tool - AIJ appeared very comprehensive. Several nights ago, I imaged my first variable star BH Aur - the period is approx. 0.45 days.  I took 6sec x 86 images at one frame every three minutes, covering a three hour period with the hope of catching some of the 1.5 mag change reported for this variable.  I processed the data in AIJ, selecting five comp stars varying in magnitude from 10.6 to 12.6.

The attached image shows a partial light curve for BH Aur, compared to the three closest magnitude comp stars.  All five comp stars exhibited flat light curves, while that for BH Aur clearly demonstrates the slow (relative) fading of the star during this part of its period.

 

I'm looking forward to participating in this branch of the hobbie.  Any pointers would be greatly appreciated.

 

CS Gary

Attached Thumbnails

  • BH Aur Light Curve.jpg

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#2 555aaa

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Posted 14 January 2021 - 02:28 PM

AIJ = AstroImageJ?



#3 GaryShaw

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Posted 14 January 2021 - 02:35 PM

Gary

I started learning photometry about 2 months ago. I joined the AAVSO and received a volunteer Mentor who has been outstanding. I’d encourage you to do the same. Also, the AAVSO has a great online tool, called VPHOT, for analyzing the star image or sequence of images and it’s all quite automated to save time and allow you to immediately see your data in context with lots of other observations. 
 

Before uploading into VPHOT for analysis, your images must be callibrated and plate solved. You can plate solve in VPHOT but there’s often a que which can take a long time. Another key point is to take 5 images for each observation and stack them after they’ve been calibrated and before you plate solve. If you’re using a OSC, like ASI294MC, you’ll need to deal with the color issue which I won’t get into in the interest of time. If you’re working with a mono camera and filters, it a whole other process. Proper filters are also not presently available right now either, so a OSC is a good beginning approach. 
 

gotta get back to work...,

cheers

Gary

 

ps: I was advised to work with Maximdl. So far, it’s been easier to do the image captures, darks and flats with Sharpcap. Then I switch to Maxim for the rest of the image calibration, stacking, color plane creation and plate solving. 


Edited by GaryShaw, 14 January 2021 - 08:28 PM.

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#4 garyhawkins

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Posted 14 January 2021 - 09:35 PM

Yes.

 

AIJ = AstroImageJ?



#5 Ed Wiley

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Posted 14 January 2021 - 10:03 PM

Congratulations on the move Gary! The Boyce family runs a true quality foundation and I am sure you are getting some quality guidance. Like you, I use AIJ, but only for calibration. I mostly due eclipsing binaries, and since I do ensemble photometry with BVIc filters and transform my data I have switched from VPhot to LesvePhotometry for most data processing post-calibration. Its also free and powerful, but you need to by PinPoint to use it (not cheap!). I used to use MPO Canopus a lot, but cannot get it to run on my particular Win10  computer (others have had success).

 

As you get into it, please consider joining AAVSO. You will find lots of friendly folks and many interesting avenues for photometric work to explore. We have a number of courses of interest (I teach the first CCD course) and you might find some of them interesting. 

 

Ad astra,

Ed


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#6 StarmanDan

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Posted 15 January 2021 - 10:51 PM

AIJ is an excellent tool.  We use it exclusively for the exoplanet observations my club makes.  If you want to get into TESS observations, you will have to know AIJ.  Exoplanets are some of my favorite things to observe.  With AIJ it has the ability to calibrate and plot images on the fly so you can see a light curve generated in real time as images come off the camera.  It's cool to watch a star's light get dimmed suddenly as a planet passes in front then return to normal as it exits, all happening in real time.  Here is a plot of WASP-80b I did when I first got started doing exoplanet observations.

Attached Thumbnails

  • wasp80b.png

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#7 dhkaiser

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Posted 16 January 2021 - 09:16 AM

AIJ is an excellent tool.  We use it exclusively for the exoplanet observations my club makes.  If you want to get into TESS observations, you will have to know AIJ.  Exoplanets are some of my favorite things to observe.  With AIJ it has the ability to calibrate and plot images on the fly so you can see a light curve generated in real time as images come off the camera.  It's cool to watch a star's light get dimmed suddenly as a planet passes in front then return to normal as it exits, all happening in real time.  Here is a plot of WASP-80b I did when I first got started doing exoplanet observations.

Very nice precision.  What scope and camera?



#8 StarmanDan

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Posted 17 January 2021 - 09:51 AM

Very nice precision.  What scope and camera?

This was done with a .6m f9 RC and a Princeton Instruments Pyxis 2048 camera.

Attached Thumbnails

  • sFNHlAc.jpg


#9 Older Padawan

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Posted 17 January 2021 - 09:58 AM

In recent weeks I started a course on Photometry run by Boyce Research Initiatives and Education Foundation (http://boyce-astro.org/). In particular, I'm currently taking their B.R.I.E.F Introduction to Photometry and Exoplanets.  I started with the IntroSTARS™ online course for the astrophysics fundamentals needed to enroll in all Boyce-Astro seminars and programs.  The aim of my current course is to train folks for exoplanet transit monitoring.

 

It soon became clear the equipment/software I had for EAA suited photometry well.  All I needed to do was add a processing tool - AIJ appeared very comprehensive. Several nights ago, I imaged my first variable star BH Aur - the period is approx. 0.45 days.  I took 6sec x 86 images at one frame every three minutes, covering a three hour period with the hope of catching some of the 1.5 mag change reported for this variable.  I processed the data in AIJ, selecting five comp stars varying in magnitude from 10.6 to 12.6.

The attached image shows a partial light curve for BH Aur, compared to the three closest magnitude comp stars.  All five comp stars exhibited flat light curves, while that for BH Aur clearly demonstrates the slow (relative) fading of the star during this part of its period.

 

I'm looking forward to participating in this branch of the hobbie.  Any pointers would be greatly appreciated.

 

CS Gary

Fantastic Gary I just hope that you and your daughter keep those EAA videos coming while you work on this.


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#10 garyhawkins

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Posted 17 January 2021 - 10:25 AM

Yes, we're planning to do more videos.  In fact, look out for an announcement in the next couple of days.

 

Fantastic Gary I just hope that you and your daughter keep those EAA videos coming while you work on this.


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#11 levyjoelh

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Posted 23 January 2021 - 05:31 PM

This is great.   Thank you for sharing.   My setup should be finally operational next month and this is what I plan to focus in the most. 

 

Any particular reason why C3 had that extra volatility in the mid point of the curve?

 

Best,

Joel

 

 

 

 

 

 

In recent weeks I started a course on Photometry run by Boyce Research Initiatives and Education Foundation (http://boyce-astro.org/). In particular, I'm currently taking their B.R.I.E.F Introduction to Photometry and Exoplanets.  I started with the IntroSTARS™ online course for the astrophysics fundamentals needed to enroll in all Boyce-Astro seminars and programs.  The aim of my current course is to train folks for exoplanet transit monitoring.

 

It soon became clear the equipment/software I had for EAA suited photometry well.  All I needed to do was add a processing tool - AIJ appeared very comprehensive. Several nights ago, I imaged my first variable star BH Aur - the period is approx. 0.45 days.  I took 6sec x 86 images at one frame every three minutes, covering a three hour period with the hope of catching some of the 1.5 mag change reported for this variable.  I processed the data in AIJ, selecting five comp stars varying in magnitude from 10.6 to 12.6.

The attached image shows a partial light curve for BH Aur, compared to the three closest magnitude comp stars.  All five comp stars exhibited flat light curves, while that for BH Aur clearly demonstrates the slow (relative) fading of the star during this part of its period.

 

I'm looking forward to participating in this branch of the hobbie.  Any pointers would be greatly appreciated.

 

CS Gary



#12 garyhawkins

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Posted 23 January 2021 - 07:47 PM

Hi Joel,

 

This was my first test run of the equipment and I only did this data run.  Thus, I've not really put much effort into analyzing the data except to confirm the light variation for the target star BH Aur. I'm guessing, from what I know now, that this was a poor choice of reference star (I picked five at random).  Some stars are much more stable than others - so this could be some variability of the star itself.  Cloud variations tend to show across all stars as my FOV is relatively small - about 0.4 x 0.4 degrees.

 

CS Gary 

 

 

 

 

This is great.   Thank you for sharing.   My setup should be finally operational next month and this is what I plan to focus in the most. 

 

Any particular reason why C3 had that extra volatility in the mid point of the curve?

 

Best,

Joel


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#13 GaryShaw

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Posted 24 January 2021 - 10:11 AM

Hi

In case you’re not aware of it, AAVSO provides detailed information on all the approved Check and Comparative stars in their database. That includes information on the statistical error identified for each of the stars and presented based on filters being used. 
 

That information works together with the AAVSO’s VPHOT analysis application to allow you to select the right combination of comparative stars and a check Star for each magnitude analysis. The system will also handle fast ‘batch’ processing of many sequential observations of a given star ( such as a short period cataclysmic variable) based on the same set of comparative stars and check Star....a very slick system. 
 

cheers,

Gary 

 

ps : apologies if you folks already know all this.


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#14 garyhawkins

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Posted 26 January 2021 - 10:26 PM

It sounds like you're familiar with VPHOT, Gary.

 

I'm sure my equipment is pretty capable of doing some decent photometric measurements - see attached plot.  These more recent results are from six hours of imaging on the eclipsing binary V474 CAM.  One minute subs, processed, but not plotted in AIJ.  I used a guided C8 SCT with no filters

 

image.png

 

So my questions to Gary and others are:

 

1) Does AAVSO require images shot with appropriate filters (Sloan, etc.), or can you submit unfiltered images?

2) Does VPHOT offer considerably better functionality than say, AIJ?  Is it worth becoming a member of AAVSO to get VPHOT functionality?

3) If you don't have filters, can you still select comparison stars in VPHOT?

 

 

 

Hi

In case you’re not aware of it, AAVSO provides detailed information on all the approved Check and Comparative stars in their database. That includes information on the statistical error identified for each of the stars and presented based on filters being used. 
 

That information works together with the AAVSO’s VPHOT analysis application to allow you to select the right combination of comparative stars and a check Star for each magnitude analysis. The system will also handle fast ‘batch’ processing of many sequential observations of a given star ( such as a short period cataclysmic variable) based on the same set of comparative stars and check Star....a very slick system. 
 

cheers,

Gary 

 

ps : apologies if you folks already know all this.


Edited by garyhawkins, 26 January 2021 - 10:47 PM.


#15 Ed Wiley

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Posted 27 January 2021 - 10:08 AM

Gary H. (contra Gary S):

 

You can submit data taken with a clear/no filter time series for eclipsing binaries, it is just a matter of stating what you used to take the shots. Details on how to do this are i the VPhot documentation. Also I highly recommend that you join AAVSO, take the VPhot course, and use VPhot. It simplifies your data flow and Ken Menzies is a master at teaching the VPhot course.

 

You can also do more: one of the major aspects of studying eclipsing binaries is taking times of minima (ToM) and following changes in ToM over the years. You can easily determine ToM from you light curve and even tell if it is a primary or secondary eclipse. If the EB is part of the "Legacy" EBs you can add them to the pile compiled by Gerry Samolyk and published periodically (your contribution acknowledged). Or you can compile a bunch and publish them yourself.

 

Check out the AASVO Eclipsing Binary pages for details. https://www.aavso.or...inaries-section

 

Ed


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#16 GaryShaw

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Posted 27 January 2021 - 10:26 AM

Hi Gary,

 

1. Since you have a OSC camera, the best approach, according to my AAVSO mentor, is to use a high quality UV/IR Cut filter for image acquisition. The following is the process I’ve been taught works best with OSC cameras :

- acquire 5-10 sequential images ( about 20-30 seconds of data total)

- process with Dsrk and Flat - no bias

- convert these 5-10 calibrated images to color

- align on the middle image and stack using the ‘average’ setting

- save the stacked image and toss the components

- separate the stacked image into 3 planes using ‘tri-color’ setting and then tossing the original, the red and the blue plane - saving the G plane ( it’s closest to the V) 

- bin 2x2 this G plane image 

- plate solve this image 

- check and edit the Fits Header to be sure you have FILTER  listed as “V”, Your name as OBSERVER, include the AIRMASS at the midpoint of the exposures. I assume AiJ preps a nice Fits Header for you. I use Maximdl for all this- can be very automated once you learn the app.

- the image resulting from the above can be uploaded to VPHOT. Once the submission is made to AAVSO, go in and ‘edit’ the brief report line to change ‘V’ to ‘TG’. This clarifies that ‘tricolor green’ was used in the magnitude determination. There is a drop down menu to select this.

 

2. Joining AAVSO makes huge sense to me since I want my data to be available for research and educational purposes. Joining also entitles you to be assigned a Mentor who will work with you to learn the ropes and help with all sorts of education and support as you learn. AAVSO has their act together - barring a few issues with the recently updated website, and streamlines everything about variable star observing. If you want to respond to ‘calls’ for data, you can pitch in on active research projects and make a real contribution.. 

 

3. VPHOT and all its features are available to members and the process I outlined above is fine with VPHOT. It’s quick and easy to upload and analyze your images whether they be one at a time or in a time series of images as you’d do for short period variables or cataclysmics, etc. 

 

So, I tried to answer the questions but the process from acquisition to processing to analysis and, finally, submission, is fairly detailed and precise. There are lots of nuances I couldn’t get into relative to image quality, SNR, FWHM and selection of ‘comp’ and ‘checks’ stars. Having a Mentor as ‘guide’ through all that is, for me anyway, a necessity and a pleasure. When you post your data and display a light curve with yours and others’ data depicted, you really feel great to see your data lining up nicely with others’ and telling the star’s story. 
 

Feel free to PM if You’d like.

Good Luck,

Gary


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#17 garyhawkins

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Posted 27 January 2021 - 08:59 PM

Hi Gary,

 

I went to AAVSO to get a list of comparison stars in my FOV around V474 CAM, but if I was looking in the right place, it only identified one star, and that was brighter, and thus likely saturating, in the image.  Perhaps, if I'd used VPHOT I would have got a wider selection.

 

No apologies necessary, believe me, I'm literally three weeks into this. I assume in your later post where you describe workflow, the tool you were using was VPLOT? If VPHOT capable of dealing with field rotation after a meridian flip - I've been struggling to deal with this in AIJ?

 

CS Gary

 

 

Hi

In case you’re not aware of it, AAVSO provides detailed information on all the approved Check and Comparative stars in their database. That includes information on the statistical error identified for each of the stars and presented based on filters being used. 
 

That information works together with the AAVSO’s VPHOT analysis application to allow you to select the right combination of comparative stars and a check Star for each magnitude analysis. The system will also handle fast ‘batch’ processing of many sequential observations of a given star ( such as a short period cataclysmic variable) based on the same set of comparative stars and check Star....a very slick system. 
 

cheers,

Gary 

 

ps : apologies if you folks already know all this.



#18 GaryShaw

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Posted 28 January 2021 - 11:19 AM

Gary 

VPHOT is strictly for uploading the final calibrated and plate-solved image and then determining the magnitude. It also provides the target Star location on the image along with nearby comparative stars - assuming you have previously plate-solved the image.

 

VPHOT is not an application for acquiring the original images. I just do that as I normally observe using Sharpcap. I’m on an AZ mount so no meridian flips.

Gary 



#19 garyhawkins

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Posted 29 January 2021 - 12:03 PM

Sorry, my previous post was not clear.  I have an EQ mount, so for long data runs, I'm must do a meridian flip.  I thought initially all that was necessary for the second data set was rotate it 180 degrees to match the first set.  However, it appears this rotation can lead to some misalignment of the stars, particularly towards the edge of the FOV where the effect is magnified.  I believe this is associated with distortion relation to the sensor plane of my camera.

 

This slight shift creates problems for the photometry measurement of stars in the region as the aperture is no longer centered on the star.  Can VPHOT handle this, or do the data sets need to be processed separately? 

 

Gary 

VPHOT is strictly for uploading the final calibrated and plate-solved image and then determining the magnitude. It also provides the target Star location on the image along with nearby comparative stars - assuming you have previously plate-solved the image.

 

VPHOT is not an application for acquiring the original images. I just do that as I normally observe using Sharpcap. I’m on an AZ mount so no meridian flips.

Gary 



#20 GaryShaw

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Posted 29 January 2021 - 07:46 PM

Gary

Sorry, I’m not really clear on the issue or what you mean by the reference to ‘data sets’. I just take a SC capture series of 5, 5-12 second exposures of each variable I’m observing that night, whatever exposure avoids saturating the target and the key comp stars. If I’m observing a very short cycle variable, I might set up Sharpcap to take a set of 5 images every 15 minutes over a long enough period to capture a full light cycle of the star. I’ve never had to do a ‘flip’ but I don’t see why VPHOT would care which side of the flip the image came from, as long as the images have the same exposure, post processing and plate solving. 

 

VPHOT processes individual images to determine a single magnitudes for each.... or, it will automate the process to analyze several hours worth of images spaced out at 15 minutes or whatever interval you used. 

 

You might want to PM Ed who seems experienced with AAVSO or contact Sara Beck at AAVSO. She’s very helpful. sara@aavso.org

 

Gary S



#21 Ed Wiley

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Posted 29 January 2021 - 11:31 PM

GaryShaw,

VPhot does not care if your images are filpped or not due to a meridian flip. This is because it plate solves each image and thus know the orientation. Likewise for images that are slightly shifted, no need to alignment. You can also stack images directly in VPhot to improve SNR. So, for example, you can feed an entire image series into VPhot that includes images before and after the meridian flip and VPhot will spit out the results with no problem.

Ed



#22 garyhawkins

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Posted 30 January 2021 - 11:25 AM

When I referenced data sets, I was referring to the set of images captured before the meridian flip (DS1) and the set of images captured after the meridian flip (DS2).  

 

I was expecting that if I took DS2 images and rotated them by 180 degrees, and imported DS1 and DS2 into my photometry tool, AIJ, the aperture locations I defined for my target and reference stars would work.  The aperture location for the target star worked, as the images were all platesolved and my target star was right in the center.  However, for the reference stars, when AIJ got to the first image in DS2, it was clear the reference star apertures were now not completely centered on the chosen reference stars.  It was not much of a error, but enough to effect the calculated data.

 

I was struggling to understand why this happened.  Then I ran the CCD inspector that is part of ASTAP (which is a great piece of software for photometry pre-processing) and got the following result.

 

Sensor plane.jpg

 

The result should be a square smile.gif.  It not being a square indicates my sensor is not well aligned in my optical chain.  The result is that when I do a meridian flip, the part of the camera sensor that was imaging a particular reference star rotates 180 degrees, and that reference star is now measured by the camera pixels diagonally opposite.  If the sensor plane is distorted and this distortion is not corrected, which currently it is not, a reference star in DS2 will effective shift slightly on resulting images compared to DS1 images.  This shift will get worse the further you are away from the center of the image.  As my aperture locations are fixed based on DS1 locations, then when AIJ looks at the DS2 images the apertures are not quite centered.

 

So I think there are at least four potential solutions 1) treat each data set separately and then combine the results from AIJ, 2) correct the sensor distortion, 3) use a processing tool that knows how to process combined data sets with a meridian flip, or 4) never meridian flip when doing data measurements.

 

I'm going through your workflow suggestions now - thanks for the detailed information - it's very helpful as I try and get into this topic.

 

 

Gary

Sorry, I’m not really clear on the issue or what you mean by the reference to ‘data sets’. I just take a SC capture series of 5, 5-12 second exposures of each variable I’m observing that night, whatever exposure avoids saturating the target and the key comp stars. If I’m observing a very short cycle variable, I might set up Sharpcap to take a set of 5 images every 15 minutes over a long enough period to capture a full light cycle of the star. I’ve never had to do a ‘flip’ but I don’t see why VPHOT would care which side of the flip the image came from, as long as the images have the same exposure, post processing and plate solving. 

 

VPHOT processes individual images to determine a single magnitudes for each.... or, it will automate the process to analyze several hours worth of images spaced out at 15 minutes or whatever interval you used. 

 

You might want to PM Ed who seems experienced with AAVSO or contact Sara Beck at AAVSO. She’s very helpful. sara@aavso.org

 

Gary S


Edited by garyhawkins, 30 January 2021 - 11:28 AM.

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#23 Ed Wiley

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Posted 01 February 2021 - 11:48 AM

Gary Hawkins,

I think this is one of the reasons LesvePhotometry (LvP) and VPhot use Pinpoint: Many programs are good at determining the center of an images, not so good at determining the position of all the stars in the image due to distortion. Pinpoint analyzes images for distortion to a degree that allows it to reach a solution for each part of the image this correct placement of the apertures. (That is not a technical description, just my understanding.) I am not that familiar with AIJ except in terms of calibration, so I am not sure what happened. However, in using MPO Canopus I always separated my images into those before and after the meridian flip and totally avoided trying to do things like rotating the images. It was a PITA as it required two analyses, but it worked (and took advantage of other features of MPO Canopus). You might try not rotating images and analyzing pre-flip and post-flip images separately. The alternative is search for another program or programs that fits your needs. I settled on a cost-benefit solution of  LvP/Pinpoint for my times series analyses  and VPhot (Pinpoint "built in") for my other photometric work.

 

Ed


Edited by Ed Wiley, 01 February 2021 - 11:53 AM.

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#24 garyhawkins

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Posted 01 February 2021 - 02:59 PM

Now I'm a member of AAVSO, I'm planning on using VPHOT as soon as I go through the tutorials.  As you said, one of the simple things to do is avoid meridian flips altogether, or process them separately.  I could do with trying to resolve those issues in the optical chain as well.  Thanks for all the feedback - much appreciated.

 

Gary Hawkins,

I think this is one of the reasons LesvePhotometry (LvP) and VPhot use Pinpoint: Many programs are good at determining the center of an images, not so good at determining the position of all the stars in the image due to distortion. Pinpoint analyzes images for distortion to a degree that allows it to reach a solution for each part of the image this correct placement of the apertures. (That is not a technical description, just my understanding.) I am not that familiar with AIJ except in terms of calibration, so I am not sure what happened. However, in using MPO Canopus I always separated my images into those before and after the meridian flip and totally avoided trying to do things like rotating the images. It was a PITA as it required two analyses, but it worked (and took advantage of other features of MPO Canopus). You might try not rotating images and analyzing pre-flip and post-flip images separately. The alternative is search for another program or programs that fits your needs. I settled on a cost-benefit solution of  LvP/Pinpoint for my times series analyses  and VPhot (Pinpoint "built in") for my other photometric work.

 

Ed


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