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Old 07-04-2013, 11:01 AM
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Nikolas (Nik)
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dslr vs planetary camera crop factor

As the topic states, will the image appear larger than a dslr screen?
As you know a smaller sensor size on a dslr has a 1.6x crop factor as opposed to full frame if you use canon as an example so items appear larger for the same focal length.
Is it the same for planetary cameras? Will their smaller sensor size allow for a larger image for the same focal length?
Anyone have any examples?
Cheers.
PLEASE don't get mathematical as it does my head in
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Old 07-04-2013, 11:21 AM
Dennis
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Hello, Nik

I have used both a DMK21AF04 (640x480) and a DMK31AF04 (1024x768) CCD cameras from the Imaging Source, both are mono.
  • The pixel size on the DMK21 is 5.6μm x 5.6μm.
  • The pixel size on the DMK31 is (smaller) 4.65μm x 4.65μm.
When I record an image of say, Jupiter with the DMK31, it appears larger on screen that the DMK21, because the DMK31 put more pixels on the disc of Jupiter due to its smaller pixel size, compared to the larger pixels on the DMK21.

Canon DSLR's have varying pixel sizes e.g. Canon 7D is approx. 4.3μm whereas the Canon 5D Mk III is around 6.2μm so once again, on a small planetary disc, the 7D will put more pixels on the disc and therefore the image will appear larger on a computer display.

Hope that helps.

Cheers

Dennis
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Old 07-04-2013, 11:35 AM
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Nikolas (Nik)
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Cheers
so something like this
"ASI120MC


Sensor: 1/3" CMOS AR0130CS(Color) / MT9M034(mono)
Resolution: 1.2Mega Pixels 1280x960
Pixel Size: 3.75Ám"
will render a much larger image than a 5dmark 2 on the screen?
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Old 07-04-2013, 12:02 PM
pw (Peter)
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Good question and answer, I've been puzzling about these "high res" CCD cameras that are only 1300 x 1200 odd pixels VS my Nikon D800 with 7300 x 4900 odd pixels.
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Old 07-04-2013, 12:18 PM
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pluto (Hugh)
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Quote:
Originally Posted by Nikolas View Post
Cheers
so something like this
"ASI120MC


Sensor: 1/3" CMOS AR0130CS(Color) / MT9M034(mono)
Resolution: 1.2Mega Pixels 1280x960
Pixel Size: 3.75Ám"
will render a much larger image than a 5dmark 2 on the screen?
Yeah this question is all about pixel size.
The 5DmkII has pixels 6.4Ám square. So if Jupiter was say 50 pixels across on an image from the 5DmkII then an image of Jupiter from the ASI120MC, and through the same scope, would be 85 pixels across.
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Old 07-04-2013, 01:31 PM
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Nikolas (Nik)
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ok thanks for all the info, looks like I'll be getting an ASI120mc then
cheers (should be a lot lighter too)
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Old 07-04-2013, 01:38 PM
Dennis
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There are many variables that contribute towards the overall desired result of “good image quality” and image scale (=pixel size) is but one of them.

My DMK21 and DMK31 are excellent to use on the Moon for high resolution images but they are only 8 bit cameras, so theoretically, they can only display 255 shades of Black through Grey through White, which sounds a lot of shades of grey but often we are only playing with light levels of low intensity and all bunched up together.

My Canon DSLR delivers better image quality, the transition between tones being very smooth and continuous, as well as the obvious advantage of recording a larger chunk of the Moon.

With Jupiter, unless you are Tom (von Tom) who fills the frame with the Jovian disc, often a DSLR chip can offer too much area at the native resolution, which is essentially “wasted” in recording plain background space.

The file sizes for 8 Bit mono DMK cameras are also a lot smaller and are therefore less demanding on your computer specification.

Cheers

Dennis
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Old 07-04-2013, 01:53 PM
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Nikolas (Nik)
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not interested in mono
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Old 07-04-2013, 02:07 PM
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pluto (Hugh)
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Quote:
Originally Posted by Nikolas View Post
not interested in mono
The bit depth the camera captures is relevant to both mono and colour cameras. A colour image from these cameras is made of 3 mono images describing values for red, green and blue.
While DSLRs are usually higher than 8 bit per channel, my 5DmkII is 14bit/ch, you would normally capture planetary video by capturing the zoomed liveview feed, through BYE or similar, and this would only capture 8bit/ch.
I have the QHY5L-II colour, I think this is the same chip as the ASI120MC, and it says it can capture video at 12bit/ch but I've never tried it (and I think it writes it to some weird format).

I definitely agree that there's more to image quality than resolution though and so far the best planetary stuff I've got has been using a friends Canon 7D. The combination of small pixels and sharp image seem like a good balance however I haven't had a really good go with the QHY5L-II yet.
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Old 07-04-2013, 02:20 PM
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mithrandir (Andrew)
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Nik, download a copy of CCDCalc and fill in the values for the scopes and cameras you want to compare.

The field of view is the answer you are looking for.

I also have a 127ED.

A 5dMkIII would give you 129.8x86.5 arcmin. The ASI120MC would give 17.3x13 arcmin. That is about 2% of the area in an image from the 5D.

On the other hand, each pixel in the 5D covers 1.35 arcsec and in the ASI 0.81 arcsec so the ASI makes objects about 66% bigger.
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Old 07-04-2013, 02:33 PM
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Also, don't forget that most planetary imagers use Barlows to bump up the image size on the sensor. You can make the image on the screen any size you want with any camera by choosing the appropriate Barlow.
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Old 07-04-2013, 03:40 PM
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Nikolas (Nik)
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Andrew, Brilliant thanks!
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