[Aavso-photometry] Automated star finding

Wed Aug 8 16:09:52 EDT 2007

I'm really sorry if my description of th MExtract module for Mira Pro ended 
up sounding too much like an advertisement, since I was describing a product 
more than a technique. The trouble is that the technique is embodied in the 
way the software works and the product is unique, thus I ended up describing 
a specific product from a user's standpoint. In the future, I'll try harder 
to be more generic rather than just just shooting from the hip like that. I 
apologize in advance if it offended anyone.

Michael Newberry

----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Wednesday, August 08, 2007 10:39 AM
Subject: Re: [Aavso-photometry] Automated star finding

> Jim,
>
> That is a good list to start from. One important omission form your list 
> is
> Mira. Mira Pro with its MExtract Module will adderss the stated problem in 
> a
> number of ways but it is more versatile even that this application (Mira 
> AL
> offers a targeted subset of the MExtract capability). The MExtract module
> has been available for about 1-1/2 years, during which time it has been
> refined and expanded. At the algorithm level, automated object detection 
> is
> far more complex than one might suspect. I hope no one minds me getting a
> little detailed here but I think that, by giving a lot of detail, people 
> can
> appreciate some of the important issues behind automated object detection
> and property extraction. And I do appreciate up front that this will
> probably sound like a promotional ad for the MExtract Module.
>
> To see how MExtract integrates into photometric measurements, let's start
> with Mira's aperture photometry package. The photometry package can work
> with any number of unknowns, any number of standards, and any number of
> images (that is, an image set of 1 to n images). Photometry can be done in
> the following ways:
>
>    1) manually by marking objects (in at least one image, if measuring an
> image set),
>    2) by loading a catalog file containing (RA,Dec) or (X,Y) coordinates,
> and, optionally containing some standard star ID's, mags, and/or weights,
> and
>    3) internally, as part of the MExtract object extraction pipeline.
>
> For any of these methods, the photometry routines produce a table of 23
> columns for each object, about half resulting from measurements and
> calculations Mira has made (other info, such as name filter, date, time,
> etc. are taken from the image header). I will focus on option 3, using
> MExtract. This tool involves processing a pipeline of 5 stages: Procedure,
> Background, Detect, Match or Difference, and Post-Process. All stages are
> "optional" so that a procedure can be configured to the application at 
> hand.
> To use MExtract, you click on the tabs and controls to setup an MExtract
> "profile" (a collection of parameters) describing which stages to use in 
> the
> procedure, which methods to perform in each stage, and whch parameters to
> use for each method. You can run that profile or save it with a profile
> name. Recalling a saved profile allows you to run the same pipeline on 
> other
> images or image sets. For example, you might create a profile that 
> produces
> only an object list for use in option 2 above (processing a catalog file
> through the photometry package). Or you might load the catalog as in 
> option
> 2, and manually add or delete objects (as in option 1) before processing 
> the
> objects.
>
> As I mentioned, Photometry is a method in the Post-Process stage of the
> MExtract procedure. To use it, you check it on the Post-process tab and
> configure its parameters (aperture  sizes, orientation, ellipticity, 
> etc.).
> When using MExtract for photometry, the object coordinates and properties
> result from the Detect, Filter, and Match/Diff stages of the pipeline, as
> these feed the target list into the photometric measurements.
>
> MExtract was designed to be extremely versatile so you can do things like
> find stars and tabulate photometry. But there are many possible variations
> on just that theme. For example, you might configure it to find all the
> objects meeting "criteria" (more below) and then measure them in one 
> image.
> Or you can have it do that for an image set (also called a "stack"). You
> might have it toss out those objects that match, or do not match. at a
> "similar" coordinate in every image ("similar" means the detection
> coordinate is within some radius of a position in the previous image). 
> This
> can feed a list of transient objects (objects that are moving or that
> suddenly appeared) into the photometry routines. You might also just have
> MExtract create a table of the star coordinates for future measurements on
> other images or image sets using photometry option 2. You can load that
> object list into the photometry package and add or delete objects from it,
> save it as your master catalog for monitoriing a field (see end of 
> paragraph
> above).
>
> I mentioned detecting transients: One application of MExtract is to search
> for variables and novae. One of the options for the post-processing stage 
> is
> to compare the photometry of each object detected in an image set with a
> threshold value for the variation. For this, it uses the magnitude 
> measured
> in each image. For example, you might have it find objects in an image set
> that vary by more than 0.2 mag.
>
> The issue of detection "criteria" is an important one. The MExtract 
> pipeline
> has a number of stages---they are Background, Detect, Filter, Match, Diff,
> and Post-Process. You select a Procedure consisting of which stages to 
> use,
> and then specify parameters to use for each stage. All options are saved
> together as a "profile" (see above). In the Filter stage, you select 
> which,
> if any, criteria you want to use to filter the object list coming out of 
> the
> Detect stage. Here are some examples showing why Filtering is both 
> important
> and useful:
>
>    1) You might choose to keep objects that are within some range of
> roundness (i.e., stars vs galaxies, or to discard highly elongated cosmic
> ray hits),
>    2) Keep objects within some range of orientation angle (e.g., for
> objects or PSF's that are elongated vertically to study your tracking or
> optics, or maybe you want your object list not to contain saturation 
> bleeds
> and hot pixels),
>    3) Keep objects within some range of size (filter out the large blobs
> and/or hot pixels), and/or
>    4) Select only objects which peak within some brightness range (avoid
> objects that are too faint or which should be considered "saturated").
>    5) In addition, if you just want a list of widely spaced objects to
> monitor, it can even reject objects that are too close to other objects.
>
> You can choose any or all of these filtering criteria, and they become 
> part
> of the profile you save. Setting up different profiles allows MExtract to 
> do
> all sorts of projects.
>
> Using Mira's the built-in analysis tools after an MExtract run you are 
> ready
> to go research-wise with the photometry table produced by MExtract. For
> example, you might use the scatter plot tool to compare values from 
> columns
> in the photometry table (or any other measurements table). For example, a
> very obvious thing to do is plot a light curve, with magnitude error bars,
> for object 5 which was reported as exceeding your specified magnitude
> variation threshold of 0.25 mag.
>
> As a final note, I would add that our lowest price Mira AL includes a
> simplified version of the MExtract module that is tuned to detect and
> measure stars for photometry. Schools use this for someting as obvious as
> having students make an HR diagram for objects in a field or in a cluster.
> There are only a few adjustable parameters: 1) Detection sigmas above sky
> noise, 2) Maximum number of objects to find, and 3) whether to measure the
> entire image or just a rectangular region. It runs from one of the toolbar
> buttons on the Photometry toolbar. From my detailed description of the
> MExtract module for Mira Pro, you can imagine that, internally, Mira AL 
> runs
> a pre-wired MExtract profile using parameters chosen for detecting "nice"
> stars on a single image.
>
> I apologize for all the text here---I didn't intend it to be this long! 
> But
> I hope you can get an appreciation for all the issues involved in doing 
> this
> kind of work. I think our MExtract module has broken down the procedure 
> into
> as painless a task as possible but one has to understand the details of 
> how
> it works in order to use it optimally.
>
> Michael Newberry
>
> ----- Original Message ----- 
> From: "Jim Roe" <jroe at jamesroe.com>
> To: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
> Sent: Wednesday, August 08, 2007 7:11 AM
> Subject: Re: [Aavso-photometry] Aavso-photometry Digest, Vol 45, Issue 6
>
>
>>
>>
>> bailyhill at aol.com wrote:
>>> Could you mention some of the windows programs that you have used
>>> successfully?? How about Mira, Maxim, Tfocus, Allos, Temma2, Canopus,
>>> AP4WIN?? I think these are some of the ones that AAVSOers would be
>>> using.?
>>>
>>
>> While you (or someone else?) is at it, would you tell me which, if any,
>> program is capable of listing every detectable star in an image with its
>> RA, Dec, instrumental magnitude, (plus all the other goodies in the
>> header such as date, time, filter, exposure, etc).
>>
>> The closest I've come is AIP4Win that will list most of these but only
>> against PixelX and PixelY coordinates - unless you use their primitive,
>> human input required, plate solution.  (CF, Pinpoint or Charon which are
>> automatic and nearly instantaneous.)  I suppose I could use the plate
>> solutions by either Charon or Pinpoint LE (in Maxim) to transform the
>> Pixel coordinates, but that seems clumsy.
>>
>> Charon does a nice job but requires one to pick off the stars in the
>> image individually.  The good news is, I've asked Bill Gray to see if he
>> would fix this and he said he would look at it - keep your fingers
>> crossed.
>>
>> It looks to me that Maxim is only good for time series (in the
>> photometry front, that is - I like it a lot for other functions).  Am I
>> missing something?
>>
>> BTW, I've been able to run WinXP in a virtual machine from a Linux host
>> using VMware Player.  V.2 supports USB 2.0 and the only big problem I've
>> encountered is using the host serial port - but that is easily fixed by
>> using USB-Serial adapters ($10 each).
>>
>> Jim Roe
>> Wentzville, Missouri
>>
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>>
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>>
>>
>
>
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