A Review of the Canon Digital Rebel XT 350D

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by Hap Griffin

This article was originally published in the December 2005 issue of AstroPhoto Insight astrophotography newsletter.

I have to admit that I was once a staunch film astrophotographer. Several of my buddies in my observatory group (http://www.machunter.org) had various SBIG CCD cameras…an ST-5, an ST-237 and an ST-7, but I was never very impressed with the black and white images. Also the resolution wasn’t near that of my scanned slide film photographs at the time. I answered their accusation of being an astrophotography luddite by saying that when an affordable, high resolution, one-shot color camera suitable for astro work became available, I’d jump on it. In late 2003 after seeing numerous high quality photographs in Astronomy and Sky and Telescope magazines taken with the new Canon Digital Single Lens Reflex cameras (DSLR’s), I decided that time had come.

My first DSLR was a Canon 10D. I chose the 10D over the less expensive Digital Rebel (300D) because it had mirror lockup (required for short planetary and lunar shots where vibration might ruin the exposure) and I wanted the extra focusing options and other features for serious terrestrial daylight work. I’d always had Nikon cameras and have several nice Nikon lenses that I wanted to be able to use for astrophotography, so I made sure that an aftermarket Nikon-to-Canon lens adapter was available before making the purchase.

NGC 7635 - The Bubble Nebulaby Hap Griffin w/Canon 350DCopyright Hap Griffinhttp://www.machunter.org/hap_bubble_d.html
Enlarge
NGC 7635 - The Bubble Nebula
by Hap Griffin w/Canon 350D
Copyright Hap Griffin
http://www.machunter.org/hap_bubble_d.html

The first night at my observatory in December 2004 with the new 10D was a turning point in my astrophotography hobby. I had some reservations of how the digital camera would work against the tried and true film platform I had been used to. After mounting the camera to my 10? LX-200 telescope and getting things balanced before darkness fell, I realized that a good target, the galaxy NGC253, was still above my observatory walls and decided to grab a few three minute, ISO 800 frames of it. I used DSLRFocus to focus and sequence the captures and ImagesPlus to quickly stretch and evaluate the frames as they were acquired. The very first image blew me away! In three minutes, I had captured what would have taken at least 30 minutes with my film camera. After a total of twelve frames and subsequent stacking and processing, I had a presentable image of one of my favorite galaxies…all without having to wait on the film to be processed the next day at the camera store! The instant feedback for focusing and frame composition sold me. The next day, I took all of the film stored in my refrigerator and gave it to one of my other film photography buddies and haven’t looked back since.

The 10D served me well as an astro camera until I became aware that it was sorely lacking in sensitivity to hydrogen-alpha light which is so important in photographs of emission nebulae. I wanted to keep the 10D stock for daylight work and decided to purchase a Canon Digital Rebel (300D) body and modify it by removing its internal IR filter. This internal filter is the bottleneck to HA sensitivity in all DSLR’s. Once modified, the 300D was used with my LX-200 and Orion ED80 refractor to take the best astrophotos I had taken to date. A comparison between the old film photographs and the Canon digital projects can be seen at http://www.machunter.org/hapspics.html.

NGC 891 - Edge on Spiral Galaxy in Andromedaby Hap Griffin w/Canon 350DCopyright Hap Griffinhttp://www.machunter.org/hap_ngc891_d.html
Enlarge
NGC 891 - Edge on Spiral Galaxy in Andromeda
by Hap Griffin w/Canon 350D
Copyright Hap Griffin
http://www.machunter.org/hap_ngc891_d.html

When the Digital Rebel XT (350D) came out, I immediately knew that I wanted one. Some of the features which make it an even better astro camera than the 300D are its higher resolution, lower noise, mirror lockup, and the very fast USB 2.0 image downloads. I purchased a 350D body and once satisfied that all features worked out of the box since the modification voids the warranty, proceeded to modify it by removing it’s internal IR filter. The 350D body is considerably smaller than the 300D, thus making the modification somewhat more difficult and tedious. Externally, the smaller body which is standard black in color is much better looking in my opinion than the larger standard silver body of the 300D.

The new CMOS imaging chip in the 300D is very similar to that in the more expensive Canon 20D. It boasts a matrix of 3456 x 2304 6.4 micron pixels for a total resolution of 8 megapixels. The image processing system, known as DIGIC II, has also been improved for faster processing, lower power consumption and less noise than the 300D. While the higher resolution doesn’t necessarily make for better pictures than the 6.3 megapixel 300D, it certainly doesn’t hurt and is helpful to have more pixels to crop if need be. I find that the “amplifier glow?, the reddish glow along the right side of long exposure DSLR frames, is much smaller and less intense than in the 300D. The amp glow is related to camera temperature, but even at 70 degrees ambient, the 350D has very manageable amp glow in five-minute frames at ISO 800. Amp glow can easily be removed with proper master dark frame subtraction while calibrating the image frames before stacking and processing.

The imaging system in the 350D seems to have very noticeably less noise than the 300D. My first night out with the 350D was in July 2005 with an ambient temperature of around 70 degrees (cool for a July in the South, but warm for an astro camera). The lower noise was evident in the first frame. Stacking multiple frames reduces the noise even further with the final noise reduction coming in processing. I have noticed that I don’t have to be as aggressive with the noise reduction in ImagesPlus nor NeatImage as I was before with images from the 300D.

Mirror lockup is another feature that is useful for astrophotographers. It allows one to lock up the mirror and delay opening the shutter to reduce vibration from the relatively heavy mirror slap. While this is not generally necessary on long exposures of dim subjects, it can add significant blurring to planetary and lunar shots where the total exposure time may be under a second. The brightness of the subject allows significant data to be gathered while the camera, telescope and mount might still be oscillating from the mirror slap. Mirror lockup was not available in the stock 300D, but could be added by updating the camera’s firmware using a “hack? commonly available on the internet which added a few of the 10D’s feature’s to the less expensive 300D. Mirror lockup is standard on the 350D.

However, the one feature that in my opinion is worth alone the price of upgrading to the 350D is the ultra fast USB 2.0 camera to PC communications. The standard in astrophotography is to shoot in the RAW mode…that is, the highest resolution possible and with the picture saved or downloaded to the PC in an uncompressed (or at least non-destructively compressed) mode. The RAW frames from a 300D were approximately 6 megabytes in size which took nearly a full minute to download over the USB 1.0 connection. Thus if I were taking an hour of total exposure in five minute frames, the total time for the session would be 72 minutes…60 minutes of exposure plus 12 minutes, one minute for each frame to download. With the 350D and its nearly 7 megabyte RAW files, the download time is only about one second! Thus 60 minutes of image capture only takes 60 minutes. I can be more productive in the limited time I have available at my observatory. This productivity is increased even further when focusing and composing the frame where each of the trial downloads takes only a second.

In this review, I have only touched on the features of the Canon 350D that are of prime importance for astrophotographers. There are many other favorable reviews on the net of this camera for daylight and terrestrial use. In short, the 350D is a very fine instrument for astronomical photography, especially when modified for increased hydrogen-alpha sensitivity. I plan to soon replace my 10D for daylight use with one as well.


Hap Griffin is Vice President of Engineering at South Carolina Educational Television. His many hobbies include astronomical photography, ham radio, collecting music from the 60s and 70s, playing guitar, building and flying high power rockets, and riding off-road motorcycles. Hap holds a B.S. in electrical engineering from the University of South Carolina and is a graduate of the South Carolina Executive Institute. He is a Registered Professional Engineer in South Carolina.

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