Im Confused

[Buzz Lightyear]

Hi,

There is something i cannot work out with taking photos with direct focus.

I attached my camera 1000D to my Skywatcher 200P telescope with a T-Ring and adapter, ok now i can use the telescopes focuser to get a focused image, but what i do not understand is how do I zoom into the subject?

I will be using a tracking mount.

Does the cameras sensor pick up very faint distant obects without the need for magnification? and i use software to zoom into the subject?

do I find the subject with a eye piece and then make sure the scope is tracking and then replace with my direct camera connection?

so its the magnification bit i do not understand, i have tried finsing answer on web, but no mention of it.

Please forgive my ignorance in advance.

regards,

Paul.

[Merlin66]

Magnification as such doesn't apply to imaging.

It's the scale of the image due to the effective focal length (EFL) that makes thigs appear larger or smaller on the CCD.

Google for CCDcalc (freeware) this will show you various settings.

You can change the EFL by using reducers (makes it shorter) or Barlows (makes it longer) in front of the camera.

Yes, focus and find the target with an eyepiece then swap over.

If you have a good finder, this may be accurate enough to position the object on the CCD.

[Buzz Lightyear]

thank you very much, will look into this further.

[Buzz Lightyear]

quick question, does the 200p being 200mm become a 200mm telephoto lense?

[Merlin66]

When talking about camera lenses it's usual to measure them by focal length rather than aperture. ie 50mm f1.8 etc

Your scope would become a 1000mm f5 telelens.

[RogerTheDodger]

...and you will get ~37x magnification from that

[Whippy]

...and you will get ~37x magnification from that

How did you work that out?

Tony..

[ollypenrice]

How did you work that out?

Tony..

I'm curious about the answer because I agree with Ken that magnification is a meaningless term in imaging. Plate scale (arcseconds per mm on the chip) is the meaningful unit. Or you might be concerned about sampling rate in arcseconds per pixel.

Olly

[Psychobilly]

I got into "trouble" last time I pointed out that magnification is meaningless when it comes to imaging so I think I'll leave it to you gents this time

Billy...

[ollypenrice]

I got into "trouble" last time I pointed out that magnification is meaningless when it comes to imaging so I think I'll leave it to you gents this time

Billy...

Hmmm...

I won't be budging without good reason since I can order a print at any size I like or click to zoom in, etc etc. Or even use a nice projector the throw an image onto the wall on the village church 200 metres away!

Olly

[FLO]

I know where he is coming from. You were not in trouble Peter and there is no need for you to budge an inch Olly.

Some time ago someone asked how to calculate a telescope's magnification when fitted with a DSLR. The correct answer is 'it depends'. But that rarely helps. In traditional photography (I spent two decades in photo retail prior to FLO) we used a ready reckoner. It goes back to the days of 35mm film cameras. If you attach a 50mm lens, take a photo of a landscape, print it to 10"x8", stand in front of the same scenery holding the print in front of you at a comfortable viewing distance, the image will appear 'life size'. The picture scale will more-or-less match the view. If you use a 100mm lens it will appear 2x larger, a 200mm 4x larger, and so on.

So, for a rough guide, divide 50mm into the lens focal length to find its 'magnification'. When digital took over and smaller APS sized sensors became the norm you had to multiply the result by 1.6x, so a 100mm lens gives 2x magnification with 35mm film but 3.2x when using a camera with an APS sized sensor. A telescope with a 1000mm focal length produces 20x when used with a 35mm SLR but 32x with one fitted with an APS sized sensor.

Using this rough guide, an Explorer 200p gives 32x, an Evostar 80 ED 19x and a Megrez-72 14x. The clever and experienced can easily pick holes, and they would be correct, but it doesn't stop it being a popular ready reckoner (at least with photographers) that helps beginners visualise the magnifications of different telescopes.

HTH,

Steve

[brantuk]

Very useful as a rough reckoner thanks Steve

[RogerTheDodger]

As Steve said above, we are talking about "apparent" magnification. Divide the focal length of the scope by the diagonal size of the ccd sensor in mm.

*edit: here is an illustration. I've set two finder rectangles up in CdC - one to the FOV of an SPC900 sensor and the other to the FOV of the 1000d sensor and then centered on M3. If you took a pic with each and printed them both on 10x8" paper then the apparrent magnification would be greater on the smaller sensor.

2nd edit: the Apparent mag of the SPC900 is ~164x