Help for a new boy...

[SquashyDave]

Hi

I'm very new to this and to astronomy in general.

I just sent this e-mail to a Skywatch supplier. Can anyone else here help me out meantime though?

Thanks everyone - nice to make your e-acquaintance.

****************

Please forgive the naivety of this e-mail and the questions I am about to raise - I am fairly new to astronomy and the minefield of optics available for this hobby.

Yesterday I had a Skywatcher PS200 delivered and spent the evening constructing it.

Tonight, with some of the planets visible, I got to use it.

I was expecting to see planets as big as a penny piece in my hand through the viewfinder. I looked at Saturn and Mars. Mars was about the size of a flea, and Saturn just a speck with its rings not even visible. (Yes, I was looking at the right objects - not stars!)

I have been led to believe that seeing Saturn's Cassini rings, deep space objects such as other galaxies, and the other planets, like Jupiter in all their colourful glory, is what is possible. At the moment, I have a device that produces images no bigger than the rubbish reflector telescope I had had up until now, albeit with a lot more ease, thanks to the Dobsonian mount.

Please would you advise me as to what I need to do to see the planets as something more than tiny dots? I have a 10mm lens, and a 25mm wide field lens included in the kit. Both seem to produce the same sized image of the planets.

I have a Canon 300D digital camera with a telephoto lens. Can this be attached to produce larger images of the objects I am looking at?

[umadog]

If Saturn looked the way you describe then I doubt you were looking at Saturn. The rings are clearly visible in a 2" refractor when they're as "open" as they are now. If going from a 25 mm to a 10 mm eyepiece doesn't alter the apparent size of the object then you're definitely looking at a star not a planet.

Saturn has an apparent angular diameter of about 0.005 degrees (so you can't resolve it by eye, obviously). The moon is 0.5 degrees. So if you magnify Saturn 100x it will have the same apparent angular diameter as the moon naked eye. That is how big it will appear. If you're able to obtained a focused image (which it sounds like you are if you can see stars as sharp points) then is nothing that could be wrong with your telescope that would cause Saturn not to appear that size if it is magnified appropriately. Your eyepieces are adequate for this task.

You will *subtle* colouring on Jupiter (like different shades of milky coffee). All deep space objects will appear in gray-scale. There will be no colour in DSOs.

You won't be taking photos of DSOs with that scope. You'll see details on planets more easily naked eye then you will by taking photos through the scope.

Finally, with a telescope the size of yours, the amount you can magnify the object is limited by the atmosphere not the instrument. In practice you won't be able to magnify more than about 180x to 300x on most nights due to atmospheric turbulence. A larger instrument would not be able to magnify more since the atmosphere is the limiting factor. So expect to Saturn about twice the size of the full moon on an average night.

[nightfisher]

I think umadog summed it up pretty well there

[Cornelius Varley]

I think what you are experiencing is a classic example of a beginners expectations exceeding the capabilities of the telescope. Many beginners expect to see Hubble quality views through the eyepiece and are disappointed with the reality. If you check out this Field of View calculator you can select your telescope and eyepiece, then select an object to view and the calculator will show you the size of the planet etc you will see in the eyepiece. Even at 240x magnification (10mm eyepiece + 2x barlow) Saturn is relativity small.

Peter

[SquashyDave]

Hi Umadog

Thank you for the information - very helpful. I agree that my expectations were a little high.

I've looked at the Field of View site recommended to me by Peter (Cornelius Varley), and that helps me understand my expectations a little better. What I still don't understand is how the images shown on this site, of say Jupiter, using a Canon 300D (which I have) are more like I would have expected with the scope... That makes no sense. Is that with the camera attached to the scope?

Also, you say that I could expect images twice the size of the full moon? If that were so, I'd be delighted.

Regards,

Dave

If Saturn looked the way you describe then I doubt you were looking at Saturn. The rings are clearly visible in a 2" refractor when they're as "open" as they are now. If going from a 25 mm to a 10 mm eyepiece doesn't alter the apparent size of the object then you're definitely looking at a star not a planet.

Saturn has an apparent angular diameter of about 0.005 degrees (so you can't resolve it by eye, obviously). The moon is 0.5 degrees. So if you magnify Saturn 100x it will have the same apparent angular diameter as the moon naked eye. That is how big it will appear. If you're able to obtained a focused image (which it sounds like you are if you can see stars as sharp points) then is nothing that could be wrong with your telescope that would cause Saturn not to appear that size if it is magnified appropriately. Your eyepieces are adequate for this task.

You will *subtle* colouring on Jupiter (like different shades of milky coffee). All deep space objects will appear in gray-scale. There will be no colour in DSOs.

You won't be taking photos of DSOs with that scope. You'll see details on planets more easily naked eye then you will by taking photos through the scope.

Finally, with a telescope the size of yours, the amount you can magnify the object is limited by the atmosphere not the instrument. In practice you won't be able to magnify more than about 180x to 300x on most nights due to atmospheric turbulence. A larger instrument would not be able to magnify more since the atmosphere is the limiting factor. So expect to Saturn about twice the size of the full moon on an average night.

[Cornelius Varley]

The size of a planet compared to the size of a DSLR sensor is quite small. If you replace the DSLR with a webcam the scale of the image becomes greater. Infact the diameter of the planetary disc remains unchanged but because of relativly small size of the webcam's sensor the image becomes greater in relation to the chip size.

Peter

[umadog]

"Also, you say that I could expect images twice the size of the full moon? If that were so, I'd be delighted."

It is so, roughly speaking. Just look up on the current size of the planet's disk (it will be listed in arc seconds) and multiply that by the magnification. The resulting number is how large the object will be. Remember that the full moon diameter is about the width of your little finger at arm's length, so it's smaller than you might think. The planets will still look small through a telescope, therefore.