New Camera and Dobsonian Telescope

Testing a New Combination

Although my 12-inch Dobsonian telescope doesn't track stellar motion caused by Earth's rotation, I thought it would be interesting to test short exposure times with my new Nikon Z6 2 camera attached to this large aperture. On February 6th I gave it a try. A special adapter is required to attach the camera to the scope. The camera would not reach focus with first adapter I purchased. Eventually, I bought another adapter that brought the camera's focal plane a couple centimeters forward, enough to achieve focus.

The telescope effectively acted like a 1500 mm telephoto lens. I quickly discovered stars would trail unless exposures were very short. An approximate rule for avoiding star trails in unguided photos is to keep exposure times in seconds less than 500 divided by the lens focal length. For my telescope the rule said 500/1500 = 0.3 seconds would be the longest exposure I could take. Trial and error proved the rule accurate. To compensate for short exposure time I cranked the ISO up to 51200! The next image is a 0.3-second exposure of the Orion Nebula, M42, at ISO 51200. In spite of background noise associated with high ISO the image captured a lot of detail in only 0.3 seconds!

The Pleiades were high in the sky above most light pollution before the Moon rose. Another 0.3-second exposure at ISO 51200 captured only part of this cluster as seen in the next image. I should have moved the field of view around, taken a series of images, and then combined them to capture the entire cluster. Maybe I'll try this another time. The diffraction spikes on bright stars are kind of pretty!

I was particularly interested to see how Jupiter would look in a short exposure. The following 0.05-second exposure at ISO 500 overexposes the planet but allows all four major Jovian moons to be visible.

Based on the previous picture it looks like this particular telescope/camera combination might be great for capturing certain kinds of planetary conjunctions.

On this night, February 6th, comet C/2022 E3 ZTF was passing near the star, Capella, in the constellation, Auriga. I searched the neighborhood of Capella while looking at the camera's viewfinder screen, but couldn't find the comet. Instead, I took a picture of bright Capella itself with a 0.3-second exposure at ISO 51200. Yellow overexposed Capella shines nicely with accompanying diffraction spikes in the next image.

In my light polluted sky I misidentified the star Almach in Andromeda and photographed Mirach instead. I was surprised to see a faint, fuzzy object to the lower right of bright Mirach in the following image. The faint object is actually the 10th magnitude elliptical galaxy NGC 404! It's amazing how this faint distant object actually showed up in just a 0.3-second exposure at ISO 51200!

Finally, I tried a 0.2-second exposure at ISO 51200 to capture most of the star cluster, M41, in Canis Major. 

Soon after capturing M41 the nearly full Moon rose making the sky even worse for imaging. It was time to quit.

These results with very short exposure times make me wonder how nice it would be to have a 12-inch telescope on a tracking equatorial mount. Longer exposures with no star trailing would really show amazing stuff!