Esprit 100 or 120?

[texta]

Hey all,

Getting close to pulling the trigger on some astrophotography equipment - finally! I've narrowed down my choice to the excellent value-for-money Sky-Watcher Esprit. But I can't decide which model to get - the 100ED or the 120ED.

I can afford either (plus I know there's a heap of other gear I'll need to get). The 100 isn't actually that much cheaper than the 120 in the whole scheme of things - $3200 vs $3800. That was a bit surprising.

There's the field of view difference of course, but there's also a noticeable f/stop difference - the 100ED is f5.5 and the 120ED is f7.

I am concerned that with the reduced field of view on the 120, there'll be some targets that just won't fit. (I'll likely be using an 8300 sensor.) My question to you experts there is - are there many of these? I know that Andromeda just won't fit (in either really - perhaps with a FR on the 100). And if I use one of the lovely quiet (but small) Sony sensors, then it may well knock out some really useful targets.

Also, with the f7 on the 120, that means extra exposure time. I'd like the time to be minimal, not being the sort of person who wants to spend the whole night out there. (I know, I know, I can see you shaking your heads.) :p

Ultimately, I'd like to go deeper as well, for galaxies and far nebulae. The 120 would be more useful for that, but would it be good enough? I would likely get a reflector of some sort longer term for those, but would either of these be useful in the meantime?

That's a few questions. Sorry to throw all that at you! But hopefully someone can help me with my (terrible I know) ;) quandary.

Thanks all!
m

[Camelopardalis]

Mark, as you've noticed they have different attributes so it really comes down to what you value the most.

The 100 is almost a f/stop faster than the 120, but the 120 should resolve more detail and collect more faint light due to its larger aperture. A fast 4" scope probably wouldn't be my weapon of choice for small, faint objects, but some test images I have taken have pleasantly surprised me. At 840mm focal length vs 550mm the 120 is always going to win in this respect. Throw a small, sensitive sensor in the mix and it should be interesting, but a longer focal length instrument still will likely do better.

There's also size and weight to consider, so it depends on what mount you're using. The 100 surprised me when I got it. Didn't expect a fast 4" scope to be the size it is. The dew shield is huge.

There aren't all that many targets as large as Andromeda, Orion nebula, Eta Carina nebula...another solution would be to throw a larger (full frame) chip at the 120 which would give it a larger FOV than the 100 with APS-C sized sensor like an 8300. I recently attempted Andromeda and shy of some faint extension, it just about fits, see here http://www.iceinspace.com.au/forum/showthread.php?t=138095

My feeling is that the sensor you choose and how dark your imaging sky is will have just as much effect on how quickly you collect good data.

[raymo]

The 120 collects 33% more light than the 100, so the sub lengths
should be similar, and as was stated, the 120 has better resolution.
raymo

[Slawomir]

Hi Mark,

I would pick 100mm f5.5 refractor over 120mm f7 one for astrophotography, especially when starting off with this hobby. Faster f ratio results in a better SNR (for the same apertures and with the same exposures) meaning capturing fainter stuff, so that could compensate for the smaller aperture. Also, shorter focal length would be less demanding in terms of accurate guiding (more arcseconds per pixel).

I would not worry about losing too much detail with smaller aperture. For example, this is a photograph of NGC6334 captured with a 4-meter telescope on a top of a mountain in Chile:
http://apod.nasa.gov/apod/image/0803/catspaw_noao_big.jpg

And the same nebula recorded with a 10-cm telescope on a mass-production mount in Paddy in Brissie: http://www.astrobin.com/full/201490/C/?real=&mod=

Not nearly the same level of detail of course, but nonetheless the much smaller telescope (1600 times less light being collected per second) does not compare too badly IMO and it allows for fairly satisfying astrophotography.

Having said that, you could always get a reducer for the 120mm refractor, and a 120mm refractor would be a more versatile and potentially more capable telescope of the two.

Just my five cents :)

Slawomir

Might be worthwhile having a look at this PDF: http://www.stark-labs.com/craig/resources/Articles-&-Reviews/ImageSampling_Fratios_SNR_RTMC.pdf

EDIT: there is an FSQ85 currently for sale and advertised on this website...

[Amaranthus]

Raymo (bless him) has it wrong here -- sub length for astrophotography has absolutely nothing to do with aperture (but it does [kind of], along with focal length, affect the image scale...). The only relevant term for exposure length is the f/ratio. The Esprit 100 at f/5.5 will put more photons on your sensor per unit time than the f/7 Esprit 120 (indeed, equivalent subexposures will require 62% less time on the f/5.5 optics). Dunk is correct that the larger aperture will have better resolving power, but this can be 'defeated' by careful use of dithering and drizzle stacking. If this is your first astrograph, I'd go with the 100 @ f/5.5