Citizen Science: Photometry with the Celestron Origin and Siril

By Stephen Dow and James Paulson

With special thanks to Richard Berry for his guidance and insight on this endeavour.

FORWARD

This project came about because of a single slide in a presentation delivered by Eric of Celestron on his appearance on the ZOOM ASTRONOMY TALK meeting that we run once a month. When we asked Eric about the light curve on the slide, Eric reached out to a fellow by the name of Richard Berry of the AAVSO to see if we could learn more direct from him since he had generated it. I recognized that this was the Richard Berry that was the chief editor at Astronomy Magazine from 1976 to 1992. Richard is also an astronomy writer and software writer.

PREAMBLE

(As written directly by Richard Berry - and we quote)

“The Origin is well suited for photometry. The aperture is large enough that scintillation is not a big issue, and the star images are clean and free of coma and chromatic aberration. The big gotcha is that the sensor is a Bayer array device, so the star image falls on an ever-changing array of R, G, and B-filtered pixels. What I've done with the Origin is what I call "cowboy photometry," i.e., rude, crude, and rough. What I've done is to pretend that the Bayer array is not there, and combine the array pixels. That said, the combined signal from the Bayer array pixels is a fairly good approximation of classic Visual or V-band photometry."

"I read the photometry section in Siril's documentation. It looks like Siril's photometry is simple aperture photometry, which is fine for bright objects with large ranges of variation. (The Bayer array messes up fancier methods of photometry, like point-spread fitting.) Use it for your work since you have it. I used AIP4Win, a program that I wrote and was included in the Handbook of Astronomical Image Processing, for my basic aperture photometry." 

"For photometry, you want the longest dynamic range the sensor can deliver, which means that you should take photometric images at the lowest gain setting. With the Origin, in ten seconds or so, the central pixels in stars brighter than about magnitude 10.5 or so will saturate, which means photons are effectively lost. Before a photometry run, check that the highest pixel values in star images are somewhat lower than saturation, say, less than about 40,000 ADUs (saturation is normally 65,535, but my Origin never reaches that value). Be sure this is true for your variable, comp, and check stars."

"Individual data points will be noisy because the exposure time is short, the full well of the Origin sensor is shallow, and the signal is grabbed as 12-bit data (it's scaled to 16 bits, but it's really 12-bit data). But smoothing the curve, you can get good looking light curves.”

SPECIAL THANKS TO RICHARD BERRY FOR PRESENTING THIS AMAZING CHALLENGE AND USEFUL INFORMATION TO GET US STARTED

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PRESENTATION IMPACT and NET RESULTS

Stephen Dow has gone 100% in on this project. Much of the heavy lifting on this one has come from Stephen. We together wrote to Richard with inquiry, and also got some information and much more from his web pages. BL Cam was the chosen star that Richard had used when he created that graph with the Origin. Stephen thought this was a good place to start.

PROJECT SCOPE (With additional information from AI)

Stephen has used his Origin Mark II at his home observatory to collect much data in order to graph the variable and determine its period. BL Camelopardalis (BL Cam) is 

a fascinating, extreme SX Phoenicis variable star in the Camelopardalis constellation, known for its rapid, multi-frequency pulsations (shrinking/expanding) and low metal content, making it a crucial object for studying stellar evolution, especially the transition to white dwarfs, and possibly being part of a binary system, revealing complex behaviors like period changes.

 Key Characteristics of BL Cam:

• Type: SX Phoenicis variable (a type of "dwarf Cepheid").

• Pulsations: Rapid radial pulsations (0.03–0.08 days) with significant light changes (0.3-0.7 magnitudes).

• Metallicity: Extremely low metal content (Population II star).

• Location: A field star in the constellation Camelopardalis (The Giraffe). 

Why It's Important:

• BL Cam shows a complex, multi-frequency pulsation pattern, unusual for its class, challenging existing models.

• Its properties help astronomers understand stars evolving towards the white dwarf stage.

• Evidence suggests it might be in a binary system with a long orbital period (around 10.5 years), affecting its observed pulsations. 

DATA ANALYSIS AND CONSTRUCTION

Doing Photometric plots in Siril created with data from the Celestron Origin

Process created and refined by Stephen Dow.

Collect all individual fits lights files of the star field. You need to turn this feature to save all lights on in the Origin app. Pay attention to exposure duration and put your focus on the star you will be targeting. That is your opportunity to impact data quality. Details now show up in results later, so do it right.

After you have your data collected, it is time to process it (next day)

1.     Open Siril

2.     Go to HOME (little house) and set to point to the directory with the fits light files in it

3.     Go to SIRIL->PREFERENCES->FITS tab, uncheck the option “Bayer information from the header file if available.” Change the Bayer mosaic pattern to BGGR from RGGB, Click apply to save changes.

4.     Go to SEQUENCE – search for sequences, going to do the light sequence. Ensure that you are using the correct extension whether it is to be .fit or .fits

5.     Go to REGISTRATION and select GO REGISTER

6.     Linear Autostretch and choose the GREEN data because it makes it dark

7.     TOOLS->ASTROMETRY->IMAGE PLATE SOLVER and search for your star by name.

8.     TOOLS->ASTROMETRY->ANNOTATE and choose via AAVSO standard

9.     Click on the bullseye and then right click to place target (aperture rings) on variable star and comparison star (AKA comp star) and check star (AKA check star). There is work to do in learning how best to use the aperture rings, such as in adjusting it tight to the variable, and working hard to ensure the background measurement is as clean as it can be at the same time.

10.  TOOLS->PHOTOMETRY->CREATE OUTPUT-> LIGHT CURVE and save as a name of something – just use a sample name for now.

11.   Now it plots your light curve. This is your data in visual form. That result is very satisfying. When you see that curve, and realize it was built using data you created, that is cool.

IMPROVING YOUR RESULTS 

The quality of the results you collect depends on the quality of your data. The quality of your data depends on the seeing conditions of your evening. When considering the acquisition of your data, use your imaging skills to produce quality consistent and usable data because the results depend on your care and attention. We need to also recognize that the Origin is an amateur level instrument and that professional grade results come from systems designed to perform this process at a high level. With that in mind, you will be amazed with what you can do as an amateur.

Look at the following two light curves created by Stephen showing the difference in data collection with an over-saturated data run versus a correctly saturated collection. There is no “one size fits all” solution to this and that is where your ability comes into play. You don’t want to clip your data but you do want to properly scale your data. You can see the difference in the plots.

OVERLY SATURATED 

CORRECTLY SATURATED

Stephen has now taken on a new project inside of this quest, and he is going on to do a more in-depth analysis inside of this area and if you wish to learn more about it, please reach out direct to him. He is doing things like determining and plotting the absolute magnitude as part of the plots so that the data and results have additional scientific usefulness.

Plot of Relative Magnitude shown above

K-C Diagram shown above

I can provide Stephen's email address to you if you submit a specific request via the form on the bottom of the Hyperstar Gallery page on the Wildcat Astronomy website at www.wildcat-astronomy.com

 Your consistent efforts with this process will result in the best data. Most of all, we want you to have fun with your Celestron Origin. This is what astronomy is all about.