What am I doing wrong now?

[Kevdan]

Hi All,

A mate o fmine gave me a pentax DSLR body so I got the adapter to fit the scope, Only problem is even with the 2x barlow the image I got of saturn was so small it was hardly recognisable. The image must have been less than 1/2mm dia so unless I used a 50x barlow (I dont suppose they make one) How do I get a decent sized image, My philips 880 gets better pics than this and I thought DSLR's were supposed to be better. Or once again what am I doing wrong?

Camera is a pentax IstDL if that helps.

Kev.

[GazOC]

Hi Kev, despite the huge difference in cost a half decent webcam is a better tool for imaging planets than a DSLR. Its all about how many frames you can get and how quickly you can get them.

[Cornelius Varley]

The reason webcams and similar types or camera are better at planetary imaging is because they can take hundreds of images in short space of time as an AVI file. The size of the sensor in a webcam also means that more of the sensor is covered by light from the planet compared to the bigger sensor in a DSLR. In reality the number of pixels being covered is about the same for both types of camera. Focal length also has an effect on the image size. Normally, for planetary and lunar imaging, it is usual to shoot at between F20 and F30 so, for your telescope, you would need aleast a 4x barlow to get into the right sort of focal length for planetary imaging. This article Sampling and pixel size gives a good explaination of matching focal lenght to pixel size.

Peter

[Kevdan]

Hi,

Thanks for the replies but it was just that if I am only getting a very small Saturn and lets face it Saturn is a big planet, then how do I get good images of nebulas, there's no way Im going to be able to veiw through the camera eyepiece for example the trapezium inside the orion nebula so how can I possibly focus on it and capture it. The camera is only 6mp so the image should be using quite a bit of the sensor inside it.

great article by the way Cornelius.

Kev.

[Ad Astra]

Hi Kevdan,

It isn't that Saturn is big or small - it is all about "angular diameter", in other words - how much of the sky does it cover? The Moon covers about 1/2 degree of sky, the planets typically between 5 and 50 arc seconds (1 arcsec = 1/3600th of a degree... same fraction as 1 second to one hour of time.) Because planetary images are so small, you need lots of mag to see them at all.

Web cams take VERY short exposures - the air turbulence doesn't figure in that way - and then stack the relatively dim images to make better photos and bring out detail. Single exposure images and high magnification are a very difficult combo!

Nebulae and clusters by comparison are often several degrees wide - literally covering thousands of times more areas in the sky - and on your camera sensor. This is where your DSLR will shine, and lower magnification and wider field will take away much of your problems with 'seeing' (air turbulence) as well.

I hope that helps,

Dan

[Sparrow]

If you download the free programs CCDCalc, and put in the details of your scope/camera, it will show you what sort of scale image you can expect to capture.

The New CCD Astronomy Home Page

Regards

John

[ronin]

Saturn subtends an angle of 0.0115 deg.

Your scope is 650mm focal length.

This would give an image at the focal plane of 0.065mm.

The 2x barlow would double this to 0.13mm

That is what should appear on the sensor unless there are others optics in the path.

As you say small but at 1,300,000,000 Km away what do you expect?

[ronin]

Oops, while making coffee I realised that the figures are wrong, off to recalculate again.

Size of Saturn is 0.0057 degrees, not 0.0115, forgot the 2Pi factor. This ~20 arc seconds. (0.0057*60*60)

Still makes the image about 0.13mm in size (with the barlow) however as I only had the radian to degree conversion wrong and only used radians in determining the image size.