Astronomy Product Reviews by Michelle Stone
The Starlight Express HX516 CCD Camera
Purchasing
I called Adirondack on Tuesday and had the unit on Friday. No hassles. I have a very good feeling about recommending Adirondack as a vendor.
The Camera
The HX516 is a pretty compact device. It's just a few inches long and handily slides into a 2" focus tube or diagonal. It has excellent blue response and has no evident blooming problems even on 20 minute exposures (Jupiter and Saturn are exceptions). The pixel array is composed of 659 x 494 pixels. Pixels are 7.4 microns square. A binning mode effectively combines 2x2 pixels to form 14.8 square pixels. This is the mode that I used exclusively for galaxy shots. Exposure times are greatly reduced using this mode. For the long focal length of my C11 this is a needed feature.
Problems
The software is obviously a first release. It crashes on my machine if you do multiple exposures with the autosave function. It only outputs files in fits format and 8 bit tiff. I am told by Terry Platt that a public domain fits plugin with Adboe Photoshop can be used for file editing. The unsigned integer format can be changed in the 'Show/Edit current FITS details' menu so that Photoshop can load the file. This is done in the Starlight Express software prior to file save. It's find and focus mode takes a picture of only the central part of the camera's array. The need for find and focus is to get the image to the center. I find it hard to get it there by not seeing "around the edges".
The camera's focal plane is extended out from where you think it will be so that you can't use any of the standard accessories as is. You must have an extension tube attached in front of the camera. In addition, it uses Pentax threads (not T threads), so you need some sort of adaptor there. I had to jury rig every thing together to get it to work the first nights out. I put the flip mirror on the SCT, then the filter wheel, then the camera slightly threaded on to a T thread connection with a wire looped around it in case it fell off. The offset provided by the thickness of the filter wheel allowed me to bring the eyepiece to focus. This is not a real problem... it's just not one that I expected to have in the field. I have since purchassed a set of Pentax thread extension tubes which should solve this problem.
Discovering these problems on-site was fairly frustrating to say the least. Although a reference is provided in the manual about extension tubes, I believe that having a set as a purchase option, would be a valuable service to Startlight Express customers. After working through these minor hassles, I got so that I could reliably shoot what I wanted.
What Works Great
The software was extremely easy to figure out. I love a good software GUI that is simple and straight forward. Hx5 fills the bill. For "in the dark all thumbs" people like me, a simple CTRL Z (two keys right next together for me) sets up the dialog for a repeat exposure. And that's one thing that I end up doing a lot of when I am focusing and finding an object.
The software does have some very nice and simple to use contrast stretching algorithms. I found these extremely useful as compared to those in Photoshop. The camera is small and doesn't weigh a ton like other units. Other units need lots of cooling so that dark current can be minimized so they have lots of additional metal heat fins (which adds to the weight).
The focal plane problem that I identified in the "problems" section actually has a benefit in that the camera can be mounted directly into a 2" focusing mechanism.
I found that the dark current was barely a problem and didn't even bother with dark frames on short 3 minute shots. On the 10 and 20 minute shots, I did shoot a dark and flat frame. Looking back after sorting through the data, I don't believe that I will worry with flat frames.
When asking about blooming on the CCD mailing list, Terry Platt responded:
The anti-blooming of these cameras has no effect on the linearity
at all. The old linearity 'chestnut' is a hangover from the Texas
chips with 'clocked' antiblooming (e.g. TC211, TC241 etc). The
Sony chips have a precisely defined potential barrier under the
pixel wells and overflow over this barrier is tightly controlled.
Many users have conducted accurate photometry with these CCDs
(including myself) and there is no sign of non-linear effects
until overload is reached.
To me, I don't see the old "bleeding star problem" that I have seen on other CCD cameras.
Conclusions
I am still having problems with keeping the mirror to hold steady in the C11 and working through the problems of getting my guide scope to set on a guide star properly (the good stars are never "right" where you need them. Move to the guide star and the object moves out of the field of view). But I am very pleased with the camera. After working through the painful lessons of centering the object and focusing (which has to be done with any imaging device), the exposures were dead on. The camera itself works flawlessly while the software lacks somewhat in features and quality. So far, the hardware problems have been simple to address and only involved the purchase of extension tubes. I haven't had to send anything back. I am very impressed with Terry Platt's online help to both me personally via email and on the CCD mailing list. It is very helpful and makes me feel more at ease. I look forward to spending many wonderful evenings with this device.
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