This week, I returned to my roots and enjoyed some deep-sky astrophotography using a DSLR camera and small telescope. Don’t get me wrong, dedicated astronomy cameras and heavy-duty mounts are great, but my latest imaging session in the backyard was a breath of fresh air.
In this post, I’ll share my early results with a new compact refractor telescope, the William Optics Zenithstar 73 APO. I’ve managed to collect some exposure time on a number of deep-sky objects using a crop-sensor (modified) DSLR.
My experiences with the Z73 have reminded me how much I enjoy deep-sky imaging through a wide field refractor with a DSLR. This is where my astrophotography journey began, and it has not lost its appeal over time. Not one bit.
Returning to my roots…
The cooler nights of fall have allowed me to begin using my DSLR camera again for astrophotography. The overnight low has dropped to about 8-10 degrees C, a welcome relief from the scorching, humid nights of summer.
The temperature of the sensor in my Canon EOS Rebel T3i has been hovering around 25 degrees C during my imaging sessions, which is still warm enough to produce quite a bit of noise. A 5-minute exposure at ISO 1600 is a lot to ask of a camera designed for daytime photography.
But enough about my old DSLR for now, let’s get to the fun part. (My new telescope). To stay up to date with my latest endeavors in deep-sky astrophotography, please subscribe to my email newsletter.
The William Optics Zenithstar 73 APO
The William Optics Zenithstar 73 is a compact doublet APO refractor designed specifically for astrophotography. Owners of full-frame DSLR cameras will appreciate its 45mm diameter illumination circle for edge-to-edge images.
After picking up the Z73 from the William Optics booth at NEAF, I am finally using this premium refractor for astrophotography at home in the backyard. A series of rained out camping trips and even a clouded-out star party put a lid on my summer plans to use this portable APO under dark skies.
This compact and lightweight apochromatic doublet refractor have a lot going for it, including an ultra-wide field of view and high-end Ohara FPL-53 objective lens construction. I was fortunate enough to receive a complete package that includes the dedicated Flat73 field flattener, 50mm Guide Scope and more.
When asked which color I prefer, I had to keep the tradition of white and gold alive to match the Z61 APO and FLT 132 refractors. This “big brother” to the Z61 uses a new mounting ring and guide ring design, that match the gold Vixen-style dovetail bar.
After taking the Zenithstar 73 out of the neatly packaged soft carry case, the first thing I did was separate the guide scope rings a notch to provide a more balanced hold of the 50mm guide scope. I removed the Rotolock (which is an added accessory from the standard package) to thread the dedicated field flattener in for astrophotography.
A great place to thread a 48mm filter (such as the Baader Moon and Skyglow filter pictured below), is on the Flat73. Then, you can attach the field flattener to the telescope with the filter inside.
I must say, I was spoiled with a totally complete package that included all accessories. These are additional items to consider when calculating the overall price of the package. The accessories for the Zenithstar 73 include:
- Soft carry case
- Flat73 1:1 Full frame flattener
- 2” Rotolock with M63 threads
- 50mm F/4 Rotolock Guiding scope
- 48mm T mount for Nikon or Canon
What’s nice is, William Optics outlines everything you’ll need for a deep sky astrophotography system – and you can order it all together. You don’t need to go searching for field flatteners or guide scope rings that will fit your telescope. Astrophotography is the number one priority behind everything William Optics makes.
William Optics Zenithstar 73 APO Specs
|Focuser:||2.5" Rack and Pinion|
Deep Sky Images from a City Backyard
The timing of the full moon and the ever-present glow of my urban sky meant narrowband filters were the obvious choice. Even with a color DSLR camera, astrophotography can be enjoyed a great deal more with a simple clip-in ha filter.
The primary DSLR I use for deep sky astrophotography is an old Canon EOS Rebel T3i that has had the full spectrum modification performed. To compare it with a dedicated astronomy camera or CCD, you could consider it to be an un-cooled one-shot-color camera.
An Astronomik 12nm ha filter was snapped into the body of my APS-C sensor Canon T3i for the following images. With the Flat73 field flattener in place, it should come as no surprise that the stars in my image were recorded as pinpoints top the very edges of the image.
Sadr Region in Cygnus (Butterfly Nebula)
The Butterfly Nebula (IC 1318) is a rich emission nebula region in the constellation Cygnus. It is part of a much larger complex of gas and dust residing in the Sadr region. The photo below shows off the wide field of view and crisp stars you can expect when using an entry-level DSLR with the Z73.
I captured roughly 2 hours worth of exposure time in Ha using my Canon T3i through the Z73. The images are free of star-trailing and elongated stars thanks to the accurate tracking of my Sky-Watcher HEQ Pro Synscan mount.
This mount is a twin to the Orion Sirius EQ-G GoTo, which I often recommend to beginners as a robust, astrophotography-worthy mount for a setup like the one shown on this post. With a sound polar alignment routine, this equatorial mount can consistently provide sharp images of 5-minutes or much more. (The longest I’ve shot was 10)
The iOptron CEM60 center-balanced mount I used for the past 12 months has been returned to its rightful owner after a generous extended loan from Ontario Telescope. Luckily, the 5.5-pound William Optics Zenithstar 73 is nowhere near the limits of the HEQ5’s payload capacity.
The Heart Nebula in Cassiopeia
Next up is a rather dynamic looking nebula in Cassiopeia known as the Heart Nebula. As you can see, this massive target fits within a single field of view using the Zenithstar 73 with a crop-sensor DSLR camera. The F/5.9 aperture of the Z73 provides a healthy balance between light gathering ability and sharpness.
Both the Butterfly Nebula and Heart nebula images were produced using 5-minute image exposures at ISO 1600. Astro Photography Tool was used to automate the image captures, with PHD2 guiding helping to accurately guide my HEQ5 mount during each sub.
Dithering between each image and stacking multiple light frames helped to improve the overall signal to noise ratio in the images. The individual light frames were very noisy, but using dark frames in the stacking process (DeepSkyStacker) can really help to correct this.
Why I love a DSLR Camera and Telescope Setup Like This
For the type of astrophotography I’m most interested in these days, it’s hard to beat the photography opportunities available at the 400-500mm focal length. At 430mm and F/5.9, the Zenithstar 73 fits the profile of the ultimate wide field APO for deep sky.
I’ve repeatedly mentioned how much I love to use APO refractor telescopes, and I believe that they offer a better user experience and more consistent astrophotography results than any other telescope type.
Telescopes that offer a longer focal length (of 1000mm or more) are great for small DSO’s and galaxies, but deep sky objects that cover a large area of the sky such as the Heart Nebula are impossible to photograph with a DSLR without creating a mosaic.
One of the advantages of having a wide field of view is the ability to capture multiple deep sky objects in a single shot. It allows you to get creative with the framing of your target next to a star cluster or some interesting nearby nebulosity.
The photo opportunities are endless, and you may find a lifetime of ideas before feeling the need for a telescope with an increased focal length.
Image Processing Narrowband Images from a DSLR
Here is a look at the individual light frames using the Canon T3i through the Zenithstar 73. The images have a red cast because of the strong narrowband filter that was used (h-alpha). I have registered and stacked the images just as I would with a color image in DeepSkyStacker.
Reviewing my RAW images in Adobe Bridge
For the Heart Nebula, I’ve got about 5 hours of total integrated exposure time. This is two nights worth of shots that I’ve separated into their own tabs in DSS. I captured matching dark frames of the same temperature, and also bias and flat frames to help produce the highest quality stacked image possible.
Extracting the Red Channel from an RGB image
The trick after that is – to extract the red channel with the strongest signal in Photoshop. Have a look at the difference in the image quality of the red channel alone vs. the full RGB image with weak Green and Blue channels.
Notice the difference in signal from the red channel to blue when using a 12nm h-alpha filter
I copied the red channel out to a new image canvas and processed it using many of the same techniques as a traditional deep sky image such as minimizing stars, a curves stretch and a bit of noise reduction. This greyscale image can then be added to existing color data, or become a part of a complete narrowband project that include SII and OIII.
For now, I’ll just enjoy the black and white image in good old Hydrogen Alpha.
It felt great to use my DSLR camera for some astrophotography again. I began my journey many years ago with a camera and telescope like this. If you are a beginner that’s thinking about taking the plunge into deep sky astrophotography – I think you should go for it – and a setup like this is likely the best way to get started.
Next, I’ll shoot some broadband color images using a full-frame DSLR with the Zenithstar 73 to really take advantage of the fully illuminated image circle. The Sky-Watcher HEQ5 has proven to be a reliable GoTo mount over the years and continues to deliver incredible results for me. I look forward to more sessions like this in the coming weeks as the longer, cooler nights usher in the new deep-sky targets of Fall.