Web Cam Imaging

[Langy]

Hello

I'm a total newbie to Astronomy (always had some interest over the years) and only just had my first telescope Celestron Astromaster 130 EQ delivered on Monday.

I've already had a good couple of sessions with cloudy skies where I could get good views of both the moon and Jupiter with 4 of its moons.

I have an IT and Photography background so I am keen to eventually get images stored of what I am seeing.

To start with all the videos and articles I've been seeing this last week or so I'm interested in web cams.

Now as you can tell with the scope I'm on a tight budget.

I've just read an article where you can literally butcher any webcam to get it back to the chip and just house that in a relevant tube to replace the eye piece and should get some reasonable images of the moon and close planets, like Jupiter and Saturn.

This now raises a few questions on how things work.

I've noticed that when you remove the eye piece from the reflector or have it seriously out of focus that you see the X from inside the telescope. Now I'm guessing here but does this change to the item you are trying to view when the receiving plane of the item in the eyepiece holder hits the required point? i.e. the base of an eyepiece, the chip on a webcam / DSLR?

Now the focal length of my scope is 650mm, if I was to replace the eyepiece with a webcam or DSLR would that just be a straight 650mm lens for the recording device?

I know that a DSLR magnifies the focal length of the old 35mm equivalent lens by 1.6 due to the plane of where the chip compared to film is, making a 300mm lens into a 480mm lens. With a 300mm lens on a DSLR usually giving a magnification of 10x, how is the magnification worked out when inserting a webcam chip or DSLR on the scope?

Finally I think it's straight forward in that if I insert a Barlows 2x and attach the webcam or DSLR then the focal length of my scope is then doubled to 1300mm and thus doubling the magnification answers for the above questions.

Thanks in advance for any replies.

Paul

[JamesF]

I've just read an article where you can literally butcher any webcam to get it back to the chip and just house that in a relevant tube to replace the eye piece and should get some reasonable images of the moon and close planets, like Jupiter and Saturn.

That's a bit of a simplification. It's possible to do with some webcams with varying degrees of difficulty and many are just not sensitive enough for worthwhile planetary imaging.

 

I've noticed that when you remove the eye piece from the reflector or have it seriously out of focus that you see the X from inside the telescope. Now I'm guessing here but does this change to the item you are trying to view when the receiving plane of the item in the eyepiece holder hits the required point? i.e. the base of an eyepiece, the chip on a webcam / DSLR?

You won't see the spider when the image is in focus either with a lens or camera.

 

Now the focal length of my scope is 650mm, if I was to replace the eyepiece with a webcam or DSLR would that just be a straight 650mm lens for the recording device?

Exactly so.

 

I know that a DSLR magnifies the focal length of the old 35mm equivalent lens by 1.6 due to the plane of where the chip compared to film is, making a 300mm lens into a 480mm lens.

I'm far from certain that this is true. I believe what happens is that you get the same field of view with the APS-C sensor as you would with the longer lens and a full-frame sensor. The actual size of what appears on the sensor doesn't change, in terms of what is covered by, say, a single plxel, as far as I'm aware. I'm prepared to be wrong there though.

 

With a 300mm lens on a DSLR usually giving a magnification of 10x, how is the magnification worked out when inserting a webcam chip or DSLR on the scope?

Imagers don't talk about magnification. It's not a useful term. "Image scale" or "plate scale" is more useful. That is, what angular measure of the sky is covered by some area of the sensor. The general formula for this in degrees is 57.3 / focal length. That is, with a 650mm scope you'll get 57.3/650 = 0.09 degrees per mm (or 5.3 arcminutes per mm).

 

Finally I think it's straight forward in that if I insert a Barlows 2x and attach the webcam or DSLR then the focal length of my scope is then doubled to 1300mm and thus doubling the magnification answers for the above questions.

Adding a 2x barlow will indeed halve the image scale, so as above you'd then get 0.045 degrees per mm, making the image twice as big. There are a number of caveats, in particular maintaining the correct distance between the barlow and the camera sensor (moving the two apart gives a larger multiplier), but you have the general idea.

James

[Langy]

Thanks for the quick reply, most helpful.

Imagers don't talk about magnification. It's not a useful term. "Image scale" or "plate scale" is more useful. That is, what angular measure of the sky is covered by some area of the sensor. The general formula for this in degrees is 57.3 / focal length. That is, with a 650mm scope you'll get 57.3/650 = 0.09 degrees per mm (or 5.3 arcminutes per mm).
 

OK I can see the logic in this calculation. So how do I calculate what my 650mm focal length covers with the 10mm (32.5x) or 20mm(65x) eyepiece so I can use it for comparison?

Before I go and butcher a cheap webcam back to the chip how about using it on the eyepiece for afocal images, my 10mm just about fills the viewfinder with the moon and obviously half the size on the 20mm. If I can get clear images that are not the size of a pinhead due to vignette on the eyepiece I can at least have something to work with. Has anyone any experience of this as it alignment and distance from the eyepiece for the best possible image and focus.

Paul

[JamesF]

I'm not sure what you can realistically do to compare a view through an eyepiece and what happens on the camera at prime focus.

However, you can calculate the true field of view of the eyepiece.  You need to know the actual field of view of the eyepiece (different models of eyepiece are different) and the magnification of the eyepiece (which is telescope focal length / eyepiece focal length ).  Note that magnification is valid here because we're not talking about imaging

Let's say for the sake of argument that your 10mm ep has a 60 degree field of view and in your 650mm scope gives a magnification of 65x (I think you got them the wrong way around).  Then the true field of view is 60/65 or just under 1 degree.  If the Moon almost fills the field of view in the 10mm ep then I suspect I may have been over-generous with the 60 degrees guess as the Moon is only about half a degree wide.

I can't help with the afocal imaging I'm afraid.  It's not something I've ever tried.

James

[Langy]

Thanks James

I think the 20mm has a 50 degree fov and the 10mm is 40 degrees.

At least it gives me an idea on how to compare them.

I will wait to see if anyone replies regarding the afocal.

Paul

[JamesF]

You can get mounts designed to hold a camera for afocal imaging.  That might at least allow you to experiment in a repeatable way.

James

[nicks90]

you can get   decent   images using the 130 (my first scope too!) using a £3 asda webcam!

heres my effort at jupiter last year, first ever time i took a movie with it and no idea how to do any processing in registax (still have no idea and havent done any AP since, but thats another story)

all i did was prise the case apart and rip the lens out of the webcam (which included the IR filter, hence why the piccy is a bit red), and then put the case and nose back on. I then made a hole in the base of a 35mm film canister to stick the nose of the webcam through and used some gaffer tape to strap it to the canister! Popped the canister in to the eyepiece holder. Focussed as good as I could and took a 1 minute AVI through "sharpcap" on my laptop and then used "castrator" to centre jupiter and crop all the unwanted emptiness away and fed the resulting avi through "registax".

no idea what curves are, wavelets, filters etc - so just slid a few settings back and forward till i got something i recognised and then gave up. But that was the result.

I am sure with a better webcam and experimentation with sharpcap and a focussing mask, you can get better results. Also if you know what you are doing in registax (ie read the guides, which i didnt / havent yet) you can get a better processed image.

Nick

[leelee970]

Hi Paul,

This link may help.

http://www.12dstring.me.uk/fov.htm

Lee

[Langy]

Nick, I'd be happy with an image like that from a web cam. I've got a cheap one on my desk as I type which I'm about to play with and see how much I can strip it back. The only issue I may have is that it's one with the LED so the board may be too big to fit the 1.25" Eye Piece socket.

Lee that's a great site in your link, should be published more and gives a nice demonstration of what you can see (well hopefully).

Thanks again!

[IIxFUSIONZxMMx]

Hi I use a skywatcher 130p. Last night I took a photo of the moon using the 2x barlow and 25mm lens and my xboxlive webcam (not modified) I just held the cam to the eyepiece and got this photo  might be worth getting hold of an xboxlive webcam

Thanks Mark