A Guide to Determining ISO and Exposure with the Celestron Origin

Authored by Stephen Dow and James Paulson

We have been experimenting together for a couple of months now making all kinds of mistakes to learn this new device called Origin and its needs. We have been working together in tandem through any challenges we run into. We have come up with some general guidelines and observations on how best to expose various targets on the Celestron Origin using both the 178C and 678C camera.

Here in a nutshell is our very basic starter guide. Please note that we are exposing in Bortle 5 to Bortle 7 skies – therefore somewhat light polluted and typical. At some point down the road, we will attend a star party in Bortle 2 skies and at that time we will evaluate our needs for that region. I assume all that will mean is being able to increase the length of subs because there will be additional well-depth in the capture, along with more signal and less noise.

We note that the Origin works as advertised.  It is light/portable, quick to set up (even in EQ mode), auto align, auto focus, has an auto dew heater and is easy to break down at the end of a session.  It really does allow you to maximize photon capture time and enjoyment. It is a great scope and we both thoroughly enjoy using it as an imaging unit.

We haven’t noticed much difference in ISO/exposure settings after changing from the 178 to the 678 cameras.  The 678 does have a smaller pixel size (2.0 um vs 2.4 um), and there is a change in aspect ratio and total pixel count, all of which seem to make 678 images a bit sharper overall.

It is important to note though that with the wedge (EQ mode) exposure times will be different than in the Alt/Az configuration.  The wedge eliminates field rotation at the zenith so you can gain some additional exposure duration if needed. Alt-Az limits you to shorter sub-exposures.

In general, the exposure settings combination depends on several parameters such as object magnitude, the type of object, overall size, surface brightness, phase of the moon during capture, proximity to the moon in the sky, the evening’s seeing conditions, atmospheric transparency, wind, dew point and relative humidity, and of course if the target is at the horizon or zenith and where in between. Higher is better because you are looking through less atmosphere.

We also find that there is a value in having a level tripod, along with having good quality and properly created darks/flats/bias frames, couple with a good tight and close polar alignment, tight focus (always refocus with filter or with a temperature change) and it also helps to have the scope cooled to ambient temperature. Small things matter if you want great results.

You really should have the external power supply connected if you are running for more than three hours or at lower temperatures.

One interesting thing worth noting is that with experience, you can usually tell if you have appropriate ISO/exposure settings after the first four frames arrive. That right there is a great reason to own and use the Origin.   The fourth frame will denoise.  The final image will look even better after the Origin default processing.

Image quality in the end seems to be most sensitive to frame exposure time.  We generally find that a typical exposure range of 10-30s (default 15s) and an ISO range of 100-500 (default 200) works best with either camera. We would reserve 1000+ ISO for very dim objects, and even then, only with a stable atmosphere with good transparency and seeing, and the moon being out of the way. We also find that a total exposure time range of between 10 min (600s) to 2 hours (7200s) is where everything happens. Keeping ISO low also curtails walking noise to some extent. Until we get dithering in our software suite, walking noise will be present.  It can be removed with PixInsight or Siril using drizzle.

It is interesting to note that not a lot of additional detail is gained with total exposures greater than two hours (7200s).

Another important note is that you can reframe your target if the object is not centered or if a bright star needs to be moved outside of the frame.  Reframing seems to work better than targeting a nearby star.  Origin knows what it’s looking at and will adjust parameters.

Satellite and aircraft streaks are a fact and reality of modern times, and their tracks will fade from the image with longer total exposure times. Just work with it is our advice. If it is early in the session, restart the exposure. If it is late, it may be a write-off, or you might be able to crop it out at the end.

Our post processing with unique Origin calibration frames is an evolving work in progress. We have been using both PixInsight and Siril at this point with much to learn. Results direct from Origin using in-app processing are good as well, but post processing can extract a bit more it seems. Part of the thing we are running up against is the header information in the Origin light frames tends to make stacking them a challenge. That is a whole new area we have left to investigate.  You can stack individual frames or work with the final stacked TIFF image straight from the Origin.

Weighted Batch Pre Processing in PixInsight is what makes using it interesting. Carl Hostetter passed this information along on handling this. WBPP doesn't like having only the 1 bias, dark, and flat FITS file (each) that the Origin adds to the session raw files. To get around this, you can either 1) open each file in PI, rename it so that it has "master" in the name (e.g. "bias003" -> "master_bias003") and then save it as an XISF file; or 2) make 3 copies of each calibration file (for a total of 4 bias, 4 dark, and 4 flat FITS files). WBPP should then work if you just add the whole raw directory to it and let it sort the files out.

Up to this point, neither one of us has purchased the Star Sense Auto Guider but that will be next on the equipment list. This is a pricy piece but will be very beneficial for exposures over 30 seconds and curtailing some of the cropping issues we see on the native Origin output images.

Finally, when it comes to filters, other filters besides those mentioned below have been tested. The Antares UHC filter works OK with brighter objects, but the Origin clear filter works better.  The ND13 moon filter works well for moon shots. The trick is to find filters friendly to use in around f/2 and that can be challenging.

Our Target-Specific Recommendations for ISO/Exposure

Open Star Clusters – open star clusters will image up quickly so no need to either go deep with long exposures, nor deep with ISO and total integration time. We think that something like ISO 200 and about 15 minutes total using 15 second subs to get there should be more than sufficient. We use the clear filter for these.

Globular Clusters – Globular clusters are a special case. Too much ISO and too much exposure will blow out the core quickly. Think ISO 100 to ISO 200, 10 to 15 second exposures and clear filter. You will capture a lot of detail in 10 minutes

Nebula – Nebula can be tricky but can also be very interesting. A lot of how you image a nebula depends on what else is in the image. If you have a big bright star in there get ready for some compromise. It’s hard to have faint nebula and bright stars without blowing out the stars. It will also depend on the type of nebula, and how faint it is. Emission and reflection nebula will have different exposure requirements than dark nebula, which tend to like deep and long exposures. Again, this is very much target specific, so use these as general guidelines only.

We like to shoot our nebula with at least 30 second subs, and about one hour total exposure to go deep. We think perhaps ISO 500 or even ISO 1000 (depending on sky brightness), and we also like to use a nebula filter. We have been using the Antlia Triband RGB Ultra II with good results. We have both used the Celestron Origin nebula filter with good results as well.

Galaxies – the faintest of them all. Galaxies are for those times with exceptional viewing conditions. We like the idea of going deep and long for these elusive deep-sky objects. We would use ISO 1000, 30 second subs, a minimum of one hour of total exposure time, and the clear filter. We’ve been using the Antlia Triband RGB Ultra II as well for galaxies with good results. Still want to try just a basic UV/IR cut filter at some point as well. And even work to ease back on the ISO setting to see just how much it is impacted because lower is often better,

An interesting note is that some galaxies (M31 / M33) are big and bright and need to be treated much like globular clusters. Experience will guide you here.

This summary of our results is shared to help others looking to get more out of the Celestron Origin.