Jupiter Tonight?

[Fay]

Oh,I was thinking, as everyone seems to want a video camera that does 60 fps, the more the better,it seems

[Fay]

Just read a post on a forum, which said 5fps for a mono camera, minimum compression, which confirms what you say Mac. So mus spply to colour as well as Mark has colour SPC. So why do 60 fps on say, DMK camera?

[Mac]

so why do 60 fps on say, DMK camera?

well for one it gives you a lot more frames to choose from,
dont forget even though the air might be still to our eyes,
when your imaging anything the air will still be turbulent (within reason)

If your imaging at 60 fps, then at some point during those 60 frames, you will have a few perfect frames to choose from.
where at 5fps you only have 5 to choose from.

so you have 12 times as many frames to choose from per sec at the faster frame rate.

Mac.

[mickw]

I would have thought a dedicated astro camera should give better results than a webcam as it's designed ground up to do the job - a webcam is a compromise .

A bloody good compromise in the right hands.

[Fay]

I have been outside tonight, having a practise with the SPX, DMK ATIK, for comparison, Moon & Jupiter. The Atik does a good job of the Moon, not Jupiter, with my 6" anyway. There seemed to be a lot more air movement with the SPC @ 5fps than the DMK @ 60 FPS.

[RobertM]

Fay,

Before using the DMK at 60fps you might want to read this article which was posted on Cloudy Nights...

http://www.astro-imaging.de/astro/dmk_artefacts_20091211.html

Robert

[Fay]

That was interesting Robert. I must admit I usually operate the DMK @ 30 ps & will now make sure I always do in the future

[MarkS]

As usual, I make this stuff up as I go along, with the help of a bit of maths.  But here's my analysis:

USB2 easily supports 15fps or more with the SPC900 without any compression problems.  But I use 5fps because it is the only way of getting the slowest shutter speed of 1/25sec.  A webcam is fairly noisy and the slow shutter speed maximises the signal-to-noise ratio.  With twice the shutter speed, 4x the number of frames is required to get the same SNR.  Hence if you double the shutter speed it means that you need to double the total exposure time to achieve the same SNR.  Trouble is, Jupiter rotates, so you need to keep the total exposure down.  So the whole thing is a compromise.

I'm sure the DMK is a much less noisy imager so the best compromise just might be a higher frame rate - but I'm not sure.  People always talk about using a high frame rate to catch the fleeting instants of good seeing.  However if, for instance, there is a fleeting half second of good seeing then. from a SNR point of view, it is far better to capture it with 2 frames at 1/4sec shutter speed than with 30 frames at 1/60sec.

Though people boast about how many thousands of frames they used, the total number of frames used is largely irrelevant - it's the quality of each frame that counts. I took this principle to it's logical extreme when I captured this detailed image of the Copernicus crater with just 20 frames:
http://www.markshelley.co.uk/Astronomy/2009/copernicus2009.html

Mark

[Fay]

With the EOS as well, Mark. All interesting Mark. I have taken a few good one's with the Atik just 10 frames, but seeing had to be good.

Sadly, I cannot fix a 2" fitting on the 6" Mak, it originating aperture is only 1.25

[RobertM]

Yes Fay, the more the better ... but the frame quality has to be good otherwise there will be poor s/n.  The camera has to be very sensitive and the subject bright.  The brighter the object the shorter the exposure can be and the more (hopefully) usable frames you can capture.

Robert

[Tony G]

Sadly, I cannot fix a 2" fitting on the 6" Mak, it originating aperture is only 1.25

Fay,

You can with this! 

Tony G

[MarkS]

You don't need 2" fittings to use the Canon for planetary imaging - just need a 1.25" T adapter - it fits into the eyepiece socket or, as in my case, into the 1.25" Barlow.

[Fay]

But being that the hole in the back of the Mak is only 1.25, would I just get get a central image on a Canon chip? I have enquired witha couple of suppliers before, but they said that it cannot be done, were they speaking with forked tongue?

[MarkS]

It wouldn't work well for deepsky imaging where you need to fill the whole CCD becaue you would get very serious vignetting.  But for planets it would work fine because you are only using the central region of the CCD.

The only thing you need to check is if you will have sufficient back focus.

Mark

[Fay]

Robert lent me an adaptor once, I thought it would be the answer, but as you say, only the centre of the chip is usable, imaging the Moon would not be an option, but planets woud, Can you only see the centte of your chip when using the EOS, Mark?