Lens or Telescope? a dilemma!!!

[dp297 (Dimitris)]

I am looking for a Nikon 300mm f2.8 lens (which goes for about 900 dollars) which I would like to use for widefield, perhaps narrowband AP due to its fast f.

Of course the dilemma is...a Takahashi 60CB goes for about the same amount. Its a bit slower and even with a reducer (which will cost of course raising the price tag) it can reach f4.2

Any opinions?

[E_ri_k (Erik)]

I'm not an expert, nor do I know much about Nikon lenses, but I found that only a few camera lenses were really good for astrophotography when I was doing a bit of widefield stuff. My expensive Canon L series lenses introduce ALOT of coma, which was a bit disappointing. Found my 100mm macro lens ($1000 worth) is awesome at f2.8-f4 ish. Do a google search. There was a website listing lenses best suited to astrophotography, but it slips my mind.
Hope this helps.
Erik

[Wavytone]

From an optical perspective most good refractors are considerably sharper than camera lenses of similar focal length, and once you go past 500mm or so, camera lenses start to become insanely expensive and a good refractor is a better buy.

The downside is they're not much use for anything other than astrophotography as they have a fixed aperture and limited focus travel.

[bojan]

I do agree with Wavytone...
However, there are cheap alternatives - try some of old, M42 lenses.. for Zenit (russian made).
My Tair-11A (135mm, F2.8, stopped down to F4) is way better than any Canon lens I've had in my hands for testing.

Also, have a look at this post: http://www.iceinspace.com.au/forum/s...4&postcount=13

There are some links to what I have done in the past, resulting in lenses that are good enough for astrophotography, but not too expensive.

[Wavytone]

Dimitris,

One simple way to find out what camera lenses are like is to put them on a tripod aimed at an artificial star (a distant bright reflection of the sun off something small will do nicely) and hold up a short fl eyepiece (4...8 mm is needed, longer fl won't show you much) to take a look at the image.

Most camera lenses are very far from diffraction limited, ie. the image won't be a dot.

While it may be OK for photography generally, the reason this matters is that by smearing the incoming light across a wider area the image of a point source is dimmer than it would be if concentrated in a diffraction-limited image. This is why a diffraction-limited telescope objective will outperform most long telephoto camera lenses.

IMHO the Zenit lenses were pretty awful, poor copies of Takumar lenses from the 1970's; optical design has advanced significantly since then. At least choose something more decent for astronomy like the MTO 1000mm f/10.

[alocky (Andrew lockwood)]

If you google about a bit, there's a page I recall where someone has actually taken some astrophotos through a good APO (FSQ or TV NP) and a 20k Canon or Nikon 500mm pro lens. I seem to recall he favored the lens for sharpness, aberration control and colour. But after spending that kind of cash on a lens you'd provably be able to convince yourself it worked better than the VLT...
Cheers
Andrew.
found it !
http://www.samirkharusi.net/televue_canon.html

[bojan]

To illustrate a bit more, attached are the images of Alpha & Proxima Centauri taken with Tair-11A some time ago (Canon 400D, ISO1600, 10x30sec, Tait-11A @F5.6, 100% crop)

[astroboy]

Not quite sure why this started a conversation about a 300mm F2.8 and ended up with a 135mm @5.6 and a F10 MTO but anyway the 300mm F2.8 will need to be stopped down to about F4 for good results on FF camera .
I was happy with a Canon 300 F4L IS @F5 which are quite light weight and reasonably cheap .
It just comes down to what you will find the most use for a camera lens or a scope .