Lunar Images by Philipp Salzgeber

 

 

Philipp
Salzgeber

Wolfurt / Austria

 QHY5 vs. DMK21 for lunar imaging

I bought the QHY5 for autoguiding my astrophotos, but since it is also capable of capturing images of brighter astronomical objects, I gave it a try as a camera for lunar imaging. A few weeks ago our association (VAA) bought a Imaging Source DMK21 camera especially for lunar and planetary imaging.

As I currently have both cameras at my home, I thought it might be interesting to do a comparison between the two.


The Astro-Professional 80ED on my Super-Polaris mount. I used a Vixen 2x Barlow lens and two extension tubes to increase the focal length of the telescope.

  DMK21 QHY5
Active Pixels 640*480

1280*1024

Pixel size 5,6µm 5.2µm
Exposure time 1/10000 to 30 s 1/1000s to 5s (using QGVideo)
Region of Interest yes yes
Interface USB USB
Telescope mount C/CS, 1¼" M48 (T2), 1¼"
ST-4 guideport no yes
Size 50,6 x 50,6 x 50mm 64mm Ø 26.5-32mm length

My experiences with the QHY5 are a little bit mixed, it works great as a guidecam, and even though I am using a 60mm achromatic refractor as a guidescope, I never had to search for a guidestar, but was always able to find one with 1 or 2 seconds of exposure.

For lunar imaging, I ran into the issue that on exposures shorter than 85ms the lower part of the image appears brighter than the rest on some of the images. Because it is more difficult to sort out individual frames from an avi file, than just brows through a folder with thumbnails to delete / deselect the brightened images, I always capture individual bmp images instead of recording an avi. (see: http://qhyccd.com/ccdbbs/index.php?topic=1317.msg7617#msg7617)


A few percent of the images taken with the QHY5 show such a defect in the form of a bright part of the image. This happens only with exposure times below 85ms.

 

A single bmp from the qhy5 is more than 3MB big, so you eat up a lot of harddisk space in an imaging session. Since Registax crashed when I tried to process 500 images in one batch, I limit the files per image to 300. With the DMK21 1000 frames result in an avi file that is about 300MB. But of course the field of view of the DMK is pretty limited (see images at the bottom). If you want to cover a bigger area, you have to create a mosaic.

I have seen both Registax and Avistack crash when processing images from the QHY5.

I use QGVideo which came with the camera (I bought a used one), since the HDCAP software apparently uses a driver which is not compatible with the autoguiding programs (I use PHDGuide). I spent an evening trying out the different software drivers and had to manually edit the registry to get rid of them...

The images below have all been taken with an Astro-Professional 80mm ED Doublet refractor.
A Vixen 2x Barlow lens and two extension tubes were used to increase the focal length to get a useful image scale.

This is 200% enlarged crop, move the mouse over the image to compare the cameras.
The image from the DMK camera was rotated and enlarged to show the same area as the crop from the QHY5 camera.

Move the mouse cursor over the image to compare the cameras:

The image taken with the QHY5 shows some patterned noise, which is absent from the DMK21 image.

One Big advantage of the QHY5 is the bigger chip area, compare the two shots below to get a feeling of how much bigger it is:

QHY5: DMK21:


 

 




All images © Philipp Salzgeber

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