Photographing the new Comet

Yesterday, I drove from Salt Lake City, Utah to Arches National Park with a plan to photograph the new comet, Comet C/2025 A6 (Lemmon). My plan was to align the comet with Balanced Rock, from the backside, since the comet is appearing in the northwestern night sky.

Where is the Comet? At the moment, the comet is in the constellation of Canes Venatici, just to the left of the Big Dipper, in the northwestern sky. According to all the astronomical information I've been reading, the comet is expected to peak in brightness between October 21 (tonight) and November 1, 2025, with its closest approach to Earth on October 21. It's supposed to become visible to the naked eye during this period under dark skies, especially during the latter part of October and early November. It's closest approach to the Sun will be on November 8.

I started looking for the comet about one hour after sunset, using a pair of binoculars, and found it at a compass reading of 291º, at about 15 degrees above the horizon (Arches has a latitude of about 39º N). It will appear a little further to the West each night, and a little higher in the sky, giving you a few more minutes each night to photograph it before setting.

Even with binoculars and a dark sky, you can expect the comet to be very faint.

How to Photograph the Comet. Up until recently, the comet has only been a deep sky object, photographed with telescopes. In the last few day, it has become larger and bright enough to be a "nightscape" object.

For best results do the following:

• Photograph in a dark sky area, at least a Bortle Class 4 (Balanced Rock is a Bortle 3 area).

• Wait until the Astronomical Dusk (last night, in Arches, that was at 8:01pm, or about 90 minutes after sunset); however, you should start doing alignment and test shoots about 15 minutes prior to the A.D.

• Even if you don't have a star tracker, try stacking with short exposures (to stop most of the star trailing), using a fast (f/1.4) 50mm or 85mm lens. Shoot at least 12 stacks (20 is better, as this will not only reduce noise, but eliminate the air traffic (satellites and airplanes) that is more prevalent nearer to the horizon. Try an exposure time of 5-seconds with a 50mm and 3-seconds with an 85mm lens (shoot wide open at f/1.4, with an ISO of 6400). Coma corrected lenses, like those made by Samyang and Rokinon have very little coma, even at f/1.4.

• If you have a star tracker, try using a longer focal length, such as 135mm (like a Samyang f/2.0) or even 200mm (like a 70-200mm f/2.8). With these lenses, a tracked exposure of 30 seconds, at about f/2.8, with an of ISO 2500 to 3200 seems to work quite well. Stacking 10 to 20 exposures will not only reduce noise and build detail in the comet tail, but the shorter exposure time will allow you to use laser alignment, rather than the more accurate (but harder to set up) optical alignment. This combination of many tracked and stack exposures works better for me than doing one longer tracked exposure (at a lower ISO and smaller aperture), for this type of nightscape.

• If you do track with a longer lens, be sure to do a foreground exposure with the tracker off, and refocus on your foreground object. You will then have to blend or composite the sky and foreground exposures.

One of my 20 raw, stacked exposures, using an 85mm lens (f/1.4, 3 sec, ISO 6400). Note the 16 different satellites or airplanes!

20 stacked images (in Starry Landscape Stacker) with "S" Curves post processing to add contrast, but to protect highlight detail in comet.

A few feet to the right of my real-time alignment with my 85mm lens, I had this tracking setup, using a 200mm lens.

The above tracking setup did NOT include alignment with Balanced Rock, as you would get with a true "deepscape" where everything is done using one tripod location. Instead, I chose to shoot only the enlarged comet and replace the sky in the 85mm shot with a larger view of the comet. This is often termed as a "focal length blend" (using an 85mm foreground and a 200mm sky). To get the same effect in real time, the 200mm lens would have to have been placed about 150 feet further back from the Rock. Here's my final focal length blend:

A "focal length blend" of the comet, using a tracked 200mm sky (30" • f/2.8 • ISO 2500 — 8 stacked exposures), blended with the 85mm Rock.

I would have gotten better results if I had done about twenty 30" exposures and stacked them together (rather than just eight stacks); however, I started too late last night and the comet was already starting to sink closer to the horizon, and into thicker atmosphere.

Luckily, the comet will be higher in the sky, if you go to photograph it, and you'll have a little more time to do it right.

One caution: Moon interference. Tonight, and for the next three days, you'll have no moon interference; however, starting on 10/25, you'll have to wait about 49 minutes after the start of the Astronomical Dusk for the moon to set, shortening your available shooting time. Luckily, the comet will be that much higher in the sky, so you'll have more time until the comet sets.

Just keep in mind that that interference from the moon gets a little longer each day.

Postscript: Today is Oct 23rd. I had planned to photograph the comet through Double Arch on the 21st, but I had to go to the hospital for a kidney infection (I'm home now and doing much better). Ironically, Kenneth LeRose had the same idea and pulled it off with this real-time video!

There's still time for you to capture the comet. Adler Planetarium reminds us that Comet Lemmon should be visible to the naked eye through November 1st and brightest on October 29th. Here's a map from them showing the location of the comet in the sky: