[Hotdog (Anthony)]

Originally Posted by mental4astro

The new maks come with 2" focusers? Interesting. I'd be suspicious that this mod is only to accomodate the larger focuser, rather than an actual mod to the primary too. I'd talk to Bintel and Andrews about that one. Making a larger hole in the primary really impedes its performance. Doesn't make sense to me. I can see the advantage in larger Maks and SCTs, as this hole won't be larger than the secondary obstruction. But in a 6"?

Yeah, it would be surprising for a 6" Mak to manage an image circle covering the full 2" that the focuser allows, but I figured the image circle is probably at least 1.25" otherwise the 2" focuser is kind of pointless. So, unless it is just a gimmick then a 1 degree true field of view should be possible. I need to find out though.

Yes both Maks and SCTs are Cat's, but thier optical technicalities are distinct. You will see that a Maks tube is longer than a SCTs for the same size apeture. This will tell you that the primary and secondary are performing differently between the two styles. The focal reducers for SCTs are designed FOR SCTs. The light path coming from a Maks secondary is a different shape to that from an SCT.

Sure, I realised that SCTs and Maks would need different focal reducer optimised for their respective characteristics. I just couldn't see a reason why it would be much harder to produce one for a Mak than it is for an SCT, which makes the apparent lack of focal reducers designed for Maks a little surprising to me. Unless it's an inverse-telephoto design then a focal reducer will inevitably reduce back focal distance, but that issue applies to any telescope.

"Just because" you live in a light polluted sky enviroment doesn't mean you need to sell yourself short on performance. It is easier to go "up" in f/ratio (with eyepieces and barlows), much harder to go "down" (reducers can introduce unwanted aberrations). It would be a shame to find your f/ratio is too slow in those chances you do get to go to a dark sky.

I'll give you an example. My f/4.5 17.5" dob is out performed here at home (eastern suburbs Sydney) by my f/4 8" dob, and a mate's f/4.5 13.1" dob. It may not seem like much in f/ratio terms, But in a light polluted sky, it made all the difference in seeing DSO's and not! Some may argue that the large apeture of the 17.5" and 13.1" also introduced more light pollution. I beg to differ.

I'm a little confused by this. When you're looking through the eyepiece the apparent surface brightness of an extended object (or the sky...) will depend only on the ratio of the aperture to the magnification, so for a given magnification focal ratio makes no difference and aperture always wins.

Now, for a given maximum image circle size (set by a primary mirror hole, or focuser size, or eyepiece aperture, or whatever) there will be a maximum usable true field of view which will be proportional to the ratio of the image circle size to the focal length of the telescope (to within small angle approximations). Then if you divide your eyepiece apparent field of view by the maximum true field of view you end up with a minimum magnification for the telescope which is essentially just proportional to the telescope focal length. If instead you're limited in the minimum magnification you can use by the maximum focal length of the eyepieces you've got then the minimum magnification would still be proportional to the telescope focal length.

OK, so if you are using the telescope at the minimum magnification then the apparent surface brightness of extended objects will end being proportional to the ratio of the aperture to the focal length, which is indeed the inverse of the focal ratio so the same object will appear brighter in a faster focal length telescope.

This only applies if you're pushing the limits of low magnification though, if you're operating in the range where a pair of telescopes of different focal ratios can match magnifications then all that should matter is aperture.

There are downsides to a faster telescope focal ratio, too. The faster the focal ratio then the worse the aberrations afflicting both telescope and eyepieces, i.e. in order to get the same image quality out of a faster telescope of otherwise equivalent design you'll need better corrected (and therefore more expensive) eyepieces and telescope optics. A relatively simple eyepiece may well be sufficient in an f/12 telescope while the same eyepiece would look awful when fed with an f/4 beam.

I guess I'm struggling to understand how a 17.5" f/4.5 telescope could be out-performed by an 8" f/4 telescope, even in the situation where both are up against their low magnification limit the difference in apparent surface brightness would only be 13% and the bigger telescope would produce that very similar surface brightness over a image over twice a large. Unless the targets you want to see are simply too extended to fit in the larger telescope's field of view I can't see how the small difference in focal ratio would be enough to swing things in favour of the much smaller telescope.

Am I missing something here?

Anyway, given that I've pretty much ruled out a Dob or other big Newtonian on physical size grounds I don't think focal ratio is really going to end up being a deciding factor. As it's going to come down to a choice between an f/12 Mak, a f/10 SCT or maybe an f/8-9 refractor the differences aren't going to be huge in any case. I need to weight up the other relative merits of these telescope types to make my decision.

20X80 binos are fantastic! I'd love to get my mits on a pair. They are also useful from Town too! Very, very useful. I've got a 10X50 pair, and I've been able to see the Sombrero galaxy and M83 galaxy through them, again here from home. Your 20X80 will give amazing views of the sky. Do you have them set up on a tripod?

Yes, I've been really pleased with the 20x80s, especially since I picked them up for just a couple of hundred dollars. They're branded "Sakura" but I think they're just the Kunming United Optics ones you see re-branded under lots of different names, they're BA2 series I think. I found them for sale at a kiosk in a shopping centre mixed in with a load of complete garbage cheap and nasty binos and snapped them up. Optically they're pretty good, especially for the price, but mechanically they're a bit crap. The central focuser is too low geared and has developed a tendency to drift so that they now need almost continual refocusing, and the central bar has come loose so that the binos can rotate about their axis relative to the tripod mount. Despite all that I have had some good views, a few weeks back I got to use them at a really dark site (Siding Spring Observatory) and the Orion Nebula looked great, and scanning around the Milky Way spotting clusters was also amazing. I do generally mount them on a photographic tripod, though the one I have been using is a old cheapie that I've had for many years which really isn't up to the job, far too wobbly especially with the central column extended and not tall enough. In order to get the most I can out of the binos until they finally fall apart completely I've recently bought a much sturdier and taller tripod, a Manfrotto 055XPROB. When the mechanical deterioration of the 20x80s becomes too much I plan to replace them with some 25x100s for even better wide angle, deep sky viewing.

Hotdog, I've got a standing invitation to IIS members to join me at my place to sketch the Moon this weekend, starting tonight (weather permitting). You'd be welcome. See this thread for details:

Thanks, that would be great but I can't make this weekend, I'm going up the coast for a weekend away. Another time maybe.