How can I know the magnification my DSLR will have with the scope I buy?

[AlphaOrionis]

Hey guys, I'm considering getting myself an Orion ED80 f/7.5 to begin astrophotography. 

I've never connected my DSLR on a telescope before, and Im wondering how the magnification is calculated. If I were to get the proper equipment to connect the DSLR to the telescope, as I understand it, there would be no eye piece to magnify. I have a TeleVue 2x 1.25'' barlow and I heard its possible to use a barlow with a dslr, but I'm wondering if it might be enough to get decent pictures of Saturn/Jupiter. 

How can I know the magnification I will get?

Thanks!

[AlphaOrionis]

Edit: Also related to this purchase... All of my equipment is 1.25'', and the description of the telescope says it accepts both. Will I lose light by using a 1.25'', or will it make no difference?

[Leveye]

Stellarium is great for that. You can put in your sensor size and scope size and aperture. You then can put a red image size box around your target that will show you what you will get and can compose the shot also.

[kalasinman]

Loose light with 1 1/4"? Probably not if you use an APC or smaller sensor.

In imaging, it's not mag but pixel scale, but ignoring that, just take the diagonal measure of the imaging chip in mm and divide the FL of the scope in mm by that.

[AlphaOrionis]

Stellarium is great for that. You can put in your sensor size and scope size and aperture. You then can put a red image size box around your target that will show you what you will get and can compose the shot also.

Hey that actually worked! Ive used stellarium to measure normal eyepiece and telescope FoV but didnt know it had a camera option. Thanks! 

This leaves my last question. Does using a 1.25'' adapter on a 2'' telescope reduce the light by .75''? 

Cheers!

[sgazer]

as you've found, it's not so much magnification, but field of view. Also try ccdcalc, it will tell you your field of view with various scopes/lenses and cameras and also overlay DSO images to you can get an idea of scale.

1.25" tubes are ok until you get really wide angle eyepieces and large camera sensors like your DSLR. You'll get some vignetting (darkness) around the edges and corners of the image, but no effect on the light at the centre. You really need 2" to connect your DSLR.

Most people use what's called prime focus. Connect your DSLR directly to the scope with a DSLR to 2" tube adapter, no lens or eyepiece. The scope becomes your lens.

[jambouk]

Use this winderful website:

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

James

[Carina Lass]

Formula is focal length over the diagonal size of your sensor equals magnification.

There is no light lost in the use of smaller diagoanal or adaptor.

[ronin]

Hi, When imaging by prime focus you do not get a magnification as such.

The scope "simply" creates an image size that is proportional to the object size and the telescope focal length.

The formula is simple school level:

Size = Scope focal length x subtended angle of object. (Thats a good approximation, the real one is a bit more involved and would make almost no difference)

So using say the Pleiades and giving that a size of 1 degree and a scope of 1000mm we get:

Size = 1000x (Tan 1) = 1000 x 0.0174 = 17.4mm

A 500mm scope would deliver an image size of 8.7mm

The difference being the image brightness on the sensor, that relates to the f number.

Leave that for later.

You will get told about pixels and angles, never sure where this sits. I looled it up one day and found values from 1 to fractionally below 6. That spread seems too big for it to be definitive. The SBIG site said 1 arc sec per pixel (I like ratios of 1:1 ), it then threw that out the window when it discussed planets. They said because it is brighter, so I wonder if the brightness is the dominent factor.

For Jupiter you will likely be able to add a barlow to the optical train, and I suspect a 3x or even up to a 5x. Jupiter is bright enough to have the final image increased (I was trying to not use the term "magnified" and I managed it ). You will how ever need a good barlow or powermate.

Jupiter and Saturn are usually imaged by a webcam, so using a DSLR will be a slightly different approach. Do not expect Jupiter and the moons to appear on the same image, Jupiter tends to get exposed before the moons. So it is either Jupiter and no moons, or, moons and a big white blob.

Check the forward and backward focus with a DSLR the focuser does not not always allow the image to fall on the camera sensor.

You may get some CA on the images, the 80 is still an ED and Jupiter and Saturn are both bright, so be aware of that.

[kalasinman]

In the same sense that a 27mm eye piece in a 750mm FL scope will produce 27x, a Nikon APC DSLR at prime focus will give about the same size image.