On December 16, 2024 I purchased (on Kijiji) a used Canon lens for Astrophotography when travelling, the Canon EF 100mm f/2.8 Macro USM. I paid $300, which seems good. The camera is not to be confused with the Image Stabilization (IS) version that costs close to $1250.00. This camera costs around 800.00 CAD new.
The camera was previously used as a macro camera for food photography and now it will be asked to do the opposite. Astrophotographers have rated this lens highly.
It is difficult to put a 50 pound telescope, a 50 pound mount and 50 pounds of other gear in a backpack when hiking. There’s hardly any room for a computer. The solution is to use small lenses, a small but solid tripod, and do widefield imaging. Camera lenses are excellent for this.
I have an electronic controller (from AstroMechanics) that fits between the lens and my ZWO Astronomy cameras and allows electronic control of the focus and aperture from the computer via a tiny USB cable (connected to the camera and thereby to the computer). The main limitation of this device is that it is not suitable for full-frame cameras. I can connect full-frame cameras with other adapters but I need to focus manually. Mostly I’ve decided to use the ZWO ASI 294MC camera which is perfect for the AstroMechanics adapter (but the field of view is smaller than a full-frame sensor).
My travel setup includes the ZWO ASI-294MC camera and a tripod (and batteries, laptop computer, etc.) with the Canon EF-S 24mm F/2.8 pancake lens. This lens has a very big field of view and is “fast”. 24mm is excellent for wide-field milky way shots and has been extensively used for this. The lens is fast because the aperture is relatively big compared to the focal length. The F/2.8 tells us this.
I have also used the “TPO Ultrawide 180” travel telescope, with a 180mm focal length at f/4.5. This little telescope can also be used at 220mm and F/5.5. F/4.5 is described as “fast” but it isn’t really very fast for me. I am used to the Celestron RASA telescopes at around F/2. The time to capture an image goes with the square of the F number. The TPO telescope would take (45*45)/(28*28) or 2.6 times longer than the Canon pancake lens to form an image. Given the terrible Ottawa weather and the trees surrounding me I want to image as fast as possible.
There is a huge difference between the TPO focal length (180mm) and the Canon pancake (24mm). The TPO is slower because the focal length is a bit too long (relative to the aperture). In other words the rectangle on the sky that is admitted and focussed on the sensor is just too small for quick imaging. The TPO also requires different adapters/connectors from the Canon. I should also note that I am pointing the telescope at the targets manually. The smaller the target (longer the focal length) the more difficult this is. Overall I have found that the TPO is not good for travelling, which is precisely why I bought it.
Enter the Canon 100mm lens: This lens will be interchangeable with the 24mm pancake, is just as fast at F/2.8 and is renown for sharp stars. It isn’t a very long focal length, so it is for wider fields of view than the TPO. Hopefully it hits the sweet spot.
I also own the Canon 50mm F/1.8 portrait lens. This lens is even “faster”, although I’m told the image quality is not adequate until the aperture is slightly reduced. Even then it is faster than the TPO and the 24mm or 100mm lenses. I haven’t used this lens much but will be trying it out soon. This lens is pictured in figure 1. The 24mm pancake lens is smaller than the 50mm lens.
Figure 1: Canon 50mm portrait lens beside the new Canon 100mm lens (spoon for scale)
Image 2: Different fields of view over an image of the Orion Nebula.
In image 2 green is the 24mm pancake lens, in blue is the 50mm portrait lens, in yellow is the new 100mm lens and in purple is the TPO 180. To capture the various interesting parts of the Orion constellation (the target shown) the 50mm lens seems best.
The options:
TPO 180 with 294MC camera (Purple in Figure 2)
Field of View = 6 by 4 degrees
Pixel scale = 5.3 arcseconds/pixel
Speed = F/4.5
Canon 100mm with 294MC camera (Yellow):
Field of View = 11 by 7.5 degrees
Pixel scale = 9.5 arcseconds/pixel
Speed = F/2.8
Canon 50mm with 294MC camera (Blue):
Field of view = 22 by 15 degrees
Pixel scale = 19 arcseconds/pixel
Speed = F/1.8 (in practice a bit slower).
Canon 24mm with 294MC camera (Green):
Field of view = 44 by 30 degrees
Pixel scale = 40 arcseconds/pixel
Speed =F/2.8
Equipment Weight
24mm lens = 125g
50mm lens = 160g
100mm lens = 600g
TPO 180 = 400g for the main tube plus various pieces... Maybe 800g?
ZWO ASI 294 camera = 410g
Astromechanics focus/aperture controller = (uncertain, perhaps 200g)
Laptop computer = ~1450g
Tripod, mount, batteries,,,
TOTAL is probably 5kg.
Subject to some further testing, I expect that the TPO will stay home on my next trip and one or more Canon lenses will come along. The Canon lenses are interchangeable so adding one or two additional lenses will not be significant.
Comentarios