Canon 300mm telephoto lens

[delphinusuk]

Hi,

I was wondering how limited I would be if I used a 300mm telephoto for astrophotography to start off with (attached to a canon 40d)?

Am I right in thinking it would be ok for imaging wide field objects, e.g., nebula and clusters but no good for (much deeper) deep sky objects, planetary or lunar photos?

I think I read somewhere that a 300mm focal length is just below the cusp where I would need more advanced tracking (than an astrotrac say) to get good images?

Thanks in advance for any help, Barney

[pete_l]

Hi,

I was wondering how limited I would be if I used a 300mm telephoto for astrophotography to start off with (attached to a canon 40d)?

Am I right in thinking it would be ok for imaging wide field objects, e.g., nebula and clusters but no good for (much deeper) deep sky objects, planetary or lunar photos?

I think I read somewhere that a 300mm focal length is just below the cusp where I would need more advanced tracking (than an astrotrac say) to get good images?

Thanks in advance for any help, Barney

I quite like using long lenses for astrophotography. It's surprising just how big some targets are, and a 200mm or 300mm lens lets you get the whole thing in one frame. In addition I quite like having some "context" around an object - kinda like having a frame around a picture.

The only thing you may need to look out for is that some large lenses have a tendency for the focus ring to slowly slide down the body of the lens when it's pointed high into the sky.

[Psychobilly]

Micopore surgical tape is good for locking off focus and/or zoom settings on camera lenses...

I have stopped buying "single use" scopes and started buying camera lenses...

If its a canon compatible lens then you can use remote manual focus controlled from a computer...some aps like Astrophotography tools (APT) allow FWHM based AF...

Peter...

[delphinusuk]

Thanks - sounds encouraging...!

Btw what does FWHM mean?

[Psychobilly]

sorry it stands for Full Width Half Max and is (in this case) a messure of the size of a star image on a sensor... which is linked to the accuracy of the focus...

[delphinusuk]

Ah, ok thanks. Automated focus sounds good... I've been struggling to see objects on my display on the back of the camera when using live view.

Think I already need some micropore tape for my 17-85mm lens that came with the camera!

Also what does someone mean when they talk about 'fast optics/lens'? Does a fast lens have a low focal ratio?

[Psychobilly]

With camera lenses yes... with lenses the focal length is "fixed" and its the size of the aperture that varies...

Primes will generally be faster thsn zoomd at the same fl and be opically simpler design with less glass elements...

[Mr TamiyaCowboy]

I run with a Canon EF 90-300mm none IS telezoom lens on a canon 350D body.

i cannot fault my ef lens except the high F range (F4.5-5.6). lens creep can be a problem, i overcome this by keeping a small loosing rubber band close by the focus ring.

focusing can be a pain in the brain. i tend to auto focus on either the moon or jupiter, then flip off auto and fire away. exposure times will vary but i push the limit at 5s on Dso without tracking. just be sure to nudge tripod/mount to re-centre it every couple shots.

do a test to find the start of star trails, take shots at 1-3-5-10-20-30s then zoom in to shots. i found 5s i started to trail so i dropped down to 4s and that seems good. i tend to shoot the moon at around 1/100s -2500s at a high ISO and F5.6, with DSO i also shoot at ISO1600 but use a short focal length @90mm and F4.5

so far i have captured the moon, jupiter, vega, M42 orions nebula all with the 90-300mm.