Just recently I got the following email asking me my thoughts on whether to choose a Skyquest XT8i or the XT10i….
Hi, I have had a look at your website which is very informative. I recently sold my skywatcher explorer 200mm f/8 telescope as I do wish to purchase the orion skyquest xt10i. Although not a beginner with telescopes I wouldnt say i’m an intermediate either! The skywatcher was actually a gift but I wish to purchase a dobsonian mount with a computerised star finder, so the xt10i has caught my eye and been recommended numerous times.
May I ask, I have read on a few occasions that the xt8 would be a better purchase in terms of magnification due to having a better focal length but less aperture?
Basically, I do want to do some basic astrophotography as featured on your website, discover deep space objects, study the moon and the planets in the easiest way possible, with the best instrument possible for less then £1000. surely the xt10i fits that bill?
Thanks in advance for any advice, Stephen
My answer was as follows:-
As a general rule of thumb the maximum magnitude of a telescope is 2x the aperture in millimetres, or 50x the aperture in inches.
So the 8inch XT8 should yield 400x maximum, whereas the 10inch is 500x due to the better resolution it can offer. But “seeing” in the UK is rarely good enough to provide stable images at high magnitudes. The standard 10mm plossl eyepiece shipped with the scopes would give 1200 focal length divided by 10 = 120x mag for both the XT10 and the XT8 since they both have the same focal length of 1200mm.
If you bought a 2x Barlow this would give you 240x mag which is still well within the limit of either scope. One of my EP’s is a 13mm Hyperion, which can take two FTR’s (fine tuning rings), to decrease its focal length down to about 8mm.
The tuning rings give higher magnification effectively turning the one 13mm (92x) eyepiece into 4! With 14mm FTR gives 10.8mm (111x), 28mm FTR gives 9.2mm (130x), and both together gives 8.1mm (148x). With 2xBarlow I then have effectively 222x, 260x and 296x.
So its still well within the limits of either scopes resolution. It’s only if you try using say a 4mm EP + Barlow that you start to get nearer to the possible limit, but at such high magnitude, the “stickiness” of the dobsonian system on Teflon pads becomes too big an issue to move the scope in tiny amounts, making viewing rather impractical. No sooner have you moved the scope about, and got the planet/object centred, then the earths motion makes it march right out of view again within about 20 seconds! This is what drives XT owners to fit lazy-susan bearings for the azimuth, and ensure the scope is well-balanced in Altitude axis, for smooth motion when chasing objects.
Hopefully you will have got the idea from my web site that Aperture is the primary consideration, not magnitude. Dobsonians are better suited to wide sweeping low power taking in breathtaking awesome views of the night sky, the milky way, clusters, nebulas. More aperture allows you to view DSO’s (deep sky objects) such as galaxies, but bear in mind these are generally still only faint smudges to the observers eye on all except the brightest nights.
The magic of observing galaxies is really the knowledge that you are seeing them “naked” and can make out their shapes, and sometimes dust lanes in them. And of course ensuring you’re at a dark sight with your eyes iris’s well adapted to the dark and wide open straining to see all the photons possible.
You can’t see colour.
You probably know this with your owning the Skywatcher, so I won’t go on too much.
The Leo Triplet is very nice in a wide view.
The extra inches of the 10inch make a BIG difference in brightness and clarity when compared to my 6inch SCT, and I believe will be markedly different to the XT8 also.
The XT10 is f4.7, and therefore “faster” than the f5.4 of the XT8, ie. in photography terms it can expose a photo faster due to the extra light it gathers, and hence also appears brighter to the human eye.
Really the only question I think you should be asking yourself about is whether you plan to transport the scope about very much? The XT10 will lay across the back of my rear car seat, but the OTA and the base are reasonably heavy. I’m young and fairly fit, but if you need to move it far or have back problems this could be a deciding factor.
If you find the price of the XT10 reasonable I would say go for that.
I think you can get the Intelliscope for the XT8 as well, if you decide to buy the XT8 instead.
Of course the other thing you might consider which is just over £1000 is the latest goto dobs like the XT8g, the motorised Goto version which tracks the viewed objects, therefore making higher mag observation more practical.
But then you’ve blown your main budget in one go, before allowing you to get any additional accessories like a Barlow, red-dot finder (I get on much better with one of these than the actual finder scope), laser collimator, moon filter (she’s painfully bright), etc.
One of the main additions I recommend getting soon is a nice big 2inch SWA (super wide angle) low power eyepiece. Looking into the milky way with that on a good clear night is just amazing. So many thousands of stars, deep, deep, deeper.