Big Dob Ownership

[A McEwan]

Hi all.

I'm thinking about getting a 10" f4.8 or 12" f5 Dob. I've already got an 8" f6 which is fine but hey - aperture fever!

Thing is, although the actual purchase prices are pretty good at the mo, I'm not sure what accessories are "essential" and what are "optional". I'm thinking Flex-tube, so shroud = essential.

I may be limited in size to the 10" f4.8, so collimation tools = essential or optional? I've been collimating my 8" f6 by eyeball for years but it's very forgiving and rarely strays from perfectly collimated. I doubt an f4.8 tube would be the same...?

If it's an Auto model, powerpack = essential or optional?

Steve has told me that coma correction is essential for photography, but I'll be using it visually or for very minor planetary imaging (if I can be bothered!) so do you agree that it's an optional extra for visual?

Auto Flextube specifically, are there any bugbears with the 10" and 12" models? Anything I should know about in advance? Tracking backlash for example? Anything I haven't thought of?

With thanks in advance,

Ant

[Doc]

Hi Ant.

I don't the model of scope you are getting but I do know a little about big dobs.

Firstly at F4.8 I would say a collimator is essential. If the scope is stripped down or moved alot after each observing session I would get used to collimating before every session. I like the following combination, a collicap and a Hotech Laser.

Depending on how good your scope and eyepieces are in a F4.8 scope you will see some abberations. I have tried the SW coma corrector and it was unusable with my eyepieces, I couldn't get them to focus. The Baader might be better and then the Pararcor even better then that.

Without a Coma corrector I can see astigmatism in mt FOV, it's not to bad and only appears on the outer edges.

Hope this helps a bit.

[brantuk]

I have the 12" flextube and I do get a little backlash with it but not enough to be too troubled. The 10" is smaller and lighter so it might not be quite so pronounced as mine. I have read a good review of the 10" in Astronomy Now and it wasn't a problem.

You'll definitely need a power pack, shroud, and collimator. The Hotech is a sound investment. Most dobs aren't perfect straight out of the box and I've seen many mods to iron out various foibles like balance, secondary adjustment, bearing smoothness, to name but a few.

Do join the "Dob Users" group (click link in my signature). There's loads of ideas and solutions to common problems there. First thing to do is get out there and try the scope first so you get to know it and how it performs.

Nice choice of scope though - the flextubes are smashing vfm and great to use

[WayBig]

Agree with brantuk - shroud (it stops you dropping stuff into the tube), powertank and collimator for starters.

If you're handy with needle and cotton it is probably easy to knock up a shroud

Many here use jump starters or leisure batteries for power as they are cheaper than the branded power tanks

Then the fun really starts...adjustable feet, setting circles, angle guages, cup holders.....

[ollypenrice]

Our 20 inch has a Beacon Hill f4.1 mirror. With good EPs I have never felt the need for a coma corrector, an opinion shared by many guests - or maybe they were just being polite!

I use a laser for speed and because I don't need a second party stationed at the other end.

Olly

[umadog]

I use the Catseye autocollimator for 4.9 Dob. I replaced the centre spot with a Catseye triangle because the factory positioned one was off by about 3 or 4 mm. I use the blackcat to centre the triangle and a Cheshire to centre the secondary. None of this is a big deal when you get used to it. At 4.9 I definitely don't feel the need for a coma corrector, although I do see the coma at lower powers, particular under 100x.

I use a deep cycle marine battery (the deep cycle is important) to run fan and dew heaters.

[Moonshane]

I agree that for visual a coma corrector would not be essential but could improve the view a little depending on your eyepiece quality. Given you scope collection I doubt you'll own poor EPs. I'm no imager but think for imaging planets, you'll be using the higher magnifications which effectively cut out the coma at the edges of the field and therefore you may be ok for planetary imaging with a webcam without a coma corrector? Not sure about this though.

Personally I have used a laser (barlowed) which was good and quick, a 'proper' Cheshire and the cheap and nasty plastic collimation eyepieces that come with Celestron scopes (and maybe also SW too?). I honestly find the latter in combination with a home made collimation cap to be the quickest and easiest way to collimate my 1600 focal length dobs. One thing's for sure, a collimation tool of some kind is essential.

My scopes are not powered (other than fans and soon dew heater on the EPs) so cannot comment on that side of things.

My only other comment would be on the aperture you are going for.

An 8" scope gives 50 square inches approx of mirror, a 10" gives 79 square inches of mirror (158% of the 8") and a 12" gives 113 square inches (226% of the 8" and 143% of the 10"). If I were you I'd definitely favour a 12" on the basis this will give you more of the wow factor than the 10" based on the comments of others (I have never used a 10") and the above rough calculations.

[umadog]

Actually, MoonShane makes a point. At F4.9 you need good eyepieces. For instance, Baader Hyperions are well thought of and there are plenty of good reviews. My experience with the 13mm, however, was very disappointing at F4.9. Loads of astigmatism in the outer third of the field (not coma, I know what coma looks like). Others have commented on this also. Don't forget that Hyperions aren't exactly cheapo eyepieces.

I now use only use TV eyepieces on the F4.9. So comments regarding no need for a coma corrector are true, but it does depend on the eyepieces. TV Plossls can be had at reasonable prices on the second hand market and there are probably other eyepieces which will play nice at <F5. Point is that you need to do your research. Either try them first with your scope or make sure you're buying from somewhere with a good return policy.

[ollypenrice]

I have found a couple of used Radians, both perfect, for a hundred pounds a throw. Like umadog I'm TV all the way and especially for the Dob. I also agree that 12 inches is a quantum step for many of the finest objects.

Olly

[Sam]

I agree with what's been said already. Collimation is key and it's what will make the difference between getting fantastic views and worrying if you bought a dud scope. The big dobs are unforgiving with even minor collimation issues so it's worth spending the time getting it right. I usually use the collimation cap to get things as centred as possible and then do the fine adjustments on a star and this seems to give excellent results. I used to use a laser collimator but I don't think my laser was very accurate.

Eyepieces get a rough time from a big dob so it's worth spending a bit on some quality gear.

[A McEwan]

Thanks for the info. Eyepiece wise I should be ok as I have a selection of Pentax XLs and an XW30, a couple of Meade S4k UWAs, a Vixen LVW22 and a Negler 13mm T6. Also a smattering of various orthoscopics and a Siebert Starsplitter. I tend to avoid "cheapie" eyepieces these days.

Astigmatism is not a word I wanted to hear as I know I have some in my observing eye, so to have it exagerated in the scope is not a good thing.

In fact some of the comments have made me realise just how easy a time I've been having of it with the f6 Dob I have - it's so easy to collimate and holds collimation exceptionally well.

I may reconsider and hold out for a big dob of f6 or so to make life easier on myself and to ensure it actually gets used.

This is all still going around inside my head - nothing definite decided yet, but really thanks for the info you've given me.

Ant

[umadog]

Ok, look, I went from F6 to F4.8 and it's really no big deal at all.

Collimating is exactly the same. It isn't "harder" at F4.8. The mechanics and theory are the same and it's just as easy to get it spot on. The only difference is that you have to get it spot on and that your mirrors need to be mounted well so that it stays collimated as you move the tube in elevation. e.g. the spider vanes needs to be taut and the mirror springs need to be firm. Also, if it's a truss OTA, it needs to be well made. That's it. Don't let anyone scare you away from a fast scope because of collimation. Of course, don't forget that ~F5 isn't generally considered super-fast nowadays. That's generally reserved for < F4-F4.5.

Your eyepieces sound fine. They ought to be well corrected and won't show astigmatism. I use Radians, Panoptics, a Nagler, and a TV Plossl. They're all sharp apart from a little coma at the longer focal lengths. I have Meade smooth-side 9.5 mm Plossl and that looks good too. There is no astigmatism coming out of the back of the eyepiece so there should be an interaction with your eye. There is still a touch of coma, of course, but at F4.8 there isn't much and you can easily zap it with a corrector if it bothers you or interacts with your own astigmatism (which it may).

There is no huge reason you should shy away from F4.8 in favour of an F6 10" or 12". Going to F6 will cause the focal length to jump from 1.46m to 1.83m at 12" so maximum true field with a 31 mm Nagler (for reference, it's about the biggest tFOV you'll get) goes down by over a 1/3 of a degree: 1.74 to 1.36 degrees.

[PeterW]

Woa! Bigger than 16" and you'll be wanting faster then f5 unless you love climbing tall ladders in the dark. The rest of the astro community is pushing sub f4 and not finding significant problems that cannot be resolved. Shorter and easier to transport and use. I remember seeing a 22" f6.... A very tall bit of kit!

PEterW