2x Teleconverter with Canon 200 f2.8 L for DSO work.....

[stan26]

Guys/Gals, just wondering if anyone has tried this, and had any sucess.......

At the moment my longest good quality (APO) Focal Length is my little 200mm f2.8 L lens, which is absolutely fantastic at wide field work, but struggles with alot of objects. Whilst im not looking to point the camera toward the smaller targets, galaxies etc, I would like to image the "larger" DSO at a bit more FL. For example, M42, M45, the rosette etc are all framed a tad too wide for my liking at 200mm, I want to get a bit more up close and personal. I have worked out that I basically want to double my FL to 400mm.

I have recently simplified my AP setup by moving away from guided EQ mounts and moved over to Astrotrac (now my only method of tracking the night sky), so naturally im geared up for widefield work, and obviously have weight limitations.  

I initially started looking at small APO's on the market. I think somthing like a William optics ZS71 (f5.9) would do the job quite nicely, but I would require a flattener/reduced (f4.8) and this would set me back around £460 all in.

However, I could buy a Kenko 2 x converter for the L lens which would give me a 400mm F5.6, with (in theory) a nice flat field, for £130........

You see where I'm going here.....

Any thoughts...

(edit: im working on a very tight budget here, and sturggling to justify to myself and the missus the need to spend nearly £500 on "another"  telescope)

Stan.

[allcart]

Sent you a PM Stan .

[Andy_E]

No experience with the Kenko, but I have used the sigma 1.4x in conjunction with the 400mm USM L lens with good results (day time photography though). I see no reason this wouldn't work... you'd end up effectively with my lens (400mm f5.6). I find myself only using it on rare occasions as I have to switch to manual focus, but your 500/450D should achieve autofocus at f5.6 for daytime photography as well if that interests you (obviously not so important for AP).

[Psychobilly]

There are quite a few different series of  Kenko convertors some of them don't modify the reported aperture so the older bodies will try to autofocus and sometimes manage it on very high contrast targets... Not an issue for Astro use as you will be manually focusing...  Image quality varies across the different ranges as well...

Peter...

[stan26]

Thanks guys, actually my Sig is out of date, I'm actually using a modded 1100d for astro and a 50d for daytime.

Whilst I appreciate that the overall iq will decrease and the af will struggle, the only thing I'm really concerned about is the amount of light reaching the sensor with the added optical elements. With regards to this I guess a similar focal length similar speed doublet APO will always out perform it, or is this not the case?

Regards

[Andy_E]

Taking you up to f5.6 + the light loss from the extra elements yes the APO will outperform in terms of light gathering power to the chip. However, I doubt a doublet APO will match your lens in terms of image quality, swings and roundabouts I suppose.