Webcam & stacking advice

[DeepThought]

Hi

I am looking to mod a webcam to get some basic shots of planets and need a spot of advice.

Fitstly, am I right in assuming that you need to remove the lens from the webcam and then focus the light through the EP directly onto the sensor? If so then how do you focus the image? Is it just a matter of putting the it the right distance from the eyepiece?

Secondly, will Registax be able to process a moving image (as in a planet drifting across the field of view) or is this something that's not available to me as a dob owner?

I don't have any plans to take up AP in the near future and I made my choice of scope based upon this, but the odd basic image within my limitations would be nice to have.

Cheers

[x6gas]

Hi DeepThought.

Yes you do remove the lens from the webcam, but you do not use an Eyepiece. The focal plane becomes the sensor of the camera and you simply adjust focus to achieve focus at the sensor.

And yes, Registax will stack planets moving across the field of view. You can actually use some software to centre the planet in advance which actually cuts down stacking time in Registax but I don't bother and can't recall what the software in question is called .

[nephilim]

http://www.astrokraai.nl/castrator.php this free software will allow u to clip ur avi so u can stack the most central image, from what i've learnt the webcam acts as a 5-6mm ep & i'd recommend using a barlow aswell (the much increased magnification wont affect the image as it would if u were using an ep.)

Steve

[StuW]

Planetary Image Pre Processor, or you could use Castrator

Sent from my HTC One X using Tapatalk 2

[x6gas]

Castrator - that's it! Thank you - was killing me trying to remember!

[DeepThought]

Thanks

The speed of response on these forums never ceases to amaze me.

is there a way to calculate the magnification I would get based on a 150mm aperture and 1200mm focal length? If it's equivalent to a 5-6mm eyepiece then we're looking at 200x to 240x.

Sorry for the silly questions, I haven't studied optics since my first year of secondary school and I'm a bit rusty.

Mark

[DeepThought]

Actually, thinking about it, I'm guessing the size of the sensor on the webcam has something to do with it.

Might just make the thing and see what happens.

[x6gas]

Actually, thinking about it, I'm guessing the size of the sensor on the webcam has something to do with it.

Might just make the thing and see what happens.

Yes, you're right that the size of the sensor does have an impact.

I find this field of view calculator really useful. You can pick your scope and sensor from a list or add the information manually. In my experience it gives a pretty reasonable view of what you can expect, although the angular size of the planets varies quite a bit, of course...

[JamesF]

Magnification doesn't really have much meaning when you're imaging. You can calculate the image scale though. In arcseconds (probably the most useful measure for planetary imaging) then the number of arcseconds across the sky that correspond to 1mm across the image sensor is given by 206265 / focal length. If you want it in degrees substitute 57.3 for 206265 or for arcminutes use 3438.

So, if you had an SPC900 with pixels 5.6um across in a telescope with a focal length of 1200mm, the image scale is 206265 / 1200 = 172 arcseconds per mm. Jupiter is about 48 arcseconds across at the moment, so it would appear 48 / 172 = 0.28mm across on the sensor. That sounds tiny, but each pixel is only 0.0056mm across, so the planet's image will be 0.28 / 0.0056 = 50 pixels across. Put a 2x barlow in and that doubles the focal length so you end up with an image 100 pixels across and so on.

By comparison, the moon is about 1800 arcseconds in diameter. Using the same maths that works out as 10.5mm wide. The SPC900 sensor is only 2.7mm high though (480 pixels * 0.0056mm), so clearly it's never going to fit in one go.

(I've rounded the numbers in some of the calculations slightly here to make it easier to see what's going on.)

Obviously if you use a sensor with smaller pixels then the same image will end up more pixels wide in the final image. And if you use a bigger sensor you can fit more in. Typically however it's just not that simple because you also need to get enough light onto the individual pixels for them to work properly and there's not much point having far more pixels than are required to meet the limit of resolution for the telescope.

James

[Guy Wells]

Castrator...sounds..er ..useful.