Getting an Orion Optics uk dob- but standard or longer version?

[Laurie61]

The high mag view of planets/moon will not be affected by going for the shorter tube as this is down to optical quality, good collimation, scope cool down and seeing conditions. So a view at 200 x in both would be basically the same.    

[neptune1]

Thanks Laurie61, helpful to know. You have the longer tube, may I ask how tall you are (you don't have to answer!)

[Laurie61]

Thanks Laurie61, helpful to know. You have the longer tube, may I ask how tall you are (you don't have to answer!)

I am 5ft 10 in old money   I think if I was using the 5.3 I would be fine without a box. I also use mine on a tracking platform which lifts the scope higher than just using the dob mount sitting on the floor. I actually lifted my dob base originally as I preferred the scope a little higher to save bending over when viewing targets lower to the horizon.  This is my scope with the extension legs fitted, when I started using the platform I removed them as the viewing height is about the same. 

and on the Tracking platform

[neptune1]

Thank you, that's a cool piece of kit. I think I'm going to be too short to manage it.

[Laurie61]

Thank you, that's a cool piece of kit. I think I'm going to be too short to manage it.

There is no point buying something you would not be able to use,  an f4 would be the safe choice size wise

[neptune1]

Yes I think so too, might have to suck up paying for coma corrector then... I have to remember yours is a custom made super tall one. What does it measure top to bottom please (tube alone)? Just so I can get a feel how much bigger it is than the 12L. Sorry for all the Q's.

[Scooot]

Yes I think so too, might have to suck up paying for coma corrector then... I have to remember yours is a custom made super tall one. What does it measure top to bottom please (tube alone)? Just so I can get a feel how much bigger it is than the 12L. Sorry for all the Q's.

Not sure I'd buy the coma corrector from the off. I've read that coma doesn't bother some people and at higher magnifications looking at say planets it's not so prevalent. But someone with an f4 scope would be better to advise.

[Laurie61]

The tube is 1.75 m long 

[Peter Drew]

If you are relatively inexperienced I would definitely recommend the longer version. The F4 is really a specialist imaging telescope, one of our regular members has one, it gives great photos in short exposures but is a pain to get collimated well enough for good planetary visual performance. I would prefer to stand on a box rather than fuss with the collimation.

[Scooot]

I think OO scopes hold collimation quite well. I originally used my cheshire to collimate my scope. But now every time I use it I put a Hotech laser in to fine tune it. I do this with the paracorr in place. I might need to tweak the secondary mirror and very occasionally the primary, to ensure the light beam is on target. This is very easy. I haven't used the cheshire for months, I don't see why it should be any harder at f4, perhaps finer adjustments. I agree that its a tougher learning curve, but I wouldn't want to have to stand on a box. -

[cosmological]

I recommend you buy a shortish stepladder to go with that scope, every scope should come with one

I specially bought a small stepladder for my scope even though my scope is shorter then the one above, it goes wherever the scope goes.

[John]

With regard to collimation, at F/5.3 the "sweet spot" of alignment around the optical axis where collimation is good enough to achieve diffraction limited views is 3mm in diameter. At F/4 this drops to 1.4mm. At F/8 you have a generous 11mm to play with !

So achieving and maintaining good collimation is more exacting at F/4 than F/5.3. But it's clearly not impossible as a number of our members own such scopes, or even faster ones !

[Peter Drew]

Keeping one's feet off the ground during frosty weather is worth the effort. 

[Scooot]

That's interesting john.

I've been unsuccessfully googling to try and find a table or formula to work out the sweet spot at different focal lengths. However I did find one article which seemed to imply that accurate collimation was less important if you used a Paracorr. The implication being that it is mainly coma that's introduced if you miss the sweet spot and the paracorr tightens up the views across the whole FOV. I don't know if this is true but would be interested in others' views. If it is true it would make it less of an issue when choosing faster scopes if you intended to buy a paracorr anyway.

[John]

Mis-collimation reduces contrast and resolution as far as I am aware.

[Naemeth]

Thanks guys. You're all a lot taller than me, I'm a bit worried I'll need a step up to the big one...

What eyepieces would you recommend if I went for the F4, as a startIng point/

You won't need a step for the longer one. The eyepiece should be at about 5 foot from the ground, gives you plenty of wiggle room .

[webboid]

I have a 12" F4 OOUK dob & use it as my preferred scope. Apart from some of the comments made above regarding better eyepieces & collimation demands for me the biggest advantages it gives are: Very easy to carry & transport, I can sit on a lowish stool & even observe at the zenith sat down. Coma can be a little distracting at times. Accuracy of collimation is important but rarely have I found the seeing good enough to make such a difference when viewing planets. I have only once seen detail around the GRS but that night the seeing was very good. It even looked salmon pink that night.  Sitting whilst observing is so much better than standing. I enjoy observing so much more & for longer now because of that advantage of the shorter tube. 

[scarp15]

Yes an adjustable observers chair, so you can get a lofty perch whilst observing at higher locations is nice. 

[Moonshane]

I am making a massive assumptions so feel free to (virtually at least) throw something at me. I have had the 12" f5.3 and loved if but it's a lump to move about and surprisingly bulky compared with the 12" f4 I now have (although I do also have a 16" f4). the 12" f4 is really very good at everything including planets. I do have a paracorr but found the much cheaper Altair/GSO a very good/close second. without a paracorr there is a lot of coma but this is not a massive problem unless fussy. better quality eyepieces help a lot - even Televue plossls (maybe £50 used) though work well.

assuming you are a little less tall than me (I am 6'3") and (forgive me) a little less strong then me, I'd recommend the 12" f4.

my most used eyepieces are 8mm and 10mm Radian, 13mm Ethos and 26mm Nagler. Whilst not suggesting you need these expensive options, it gives an idea of the sorts of magnifications I use.

hope this helps. I'd agree with the quality upgrade too if you are buying new - I tend to buy them used.

[Moonshane]

ps I just use a standard cheshire and also a home made observing stool. I found a 4" riser for my telrad useful.

[Scooot]

You won't need a step for the longer one. The eyepiece should be at about 5 foot from the ground, gives you plenty of wiggle room .

I don't think you've allowed for the gap between the bottom of the tube and the ground, mines about 200mm but it depends on where its balanced. So that would mean the top of the tube would be about 5 8. So not much wiggle room for a 5 5 person. -

[AlexB67]

There you go to get the extra few inches, problem solved

[Naemeth]

I don't think you've allowed for the gap between the bottom of the tube and the ground, mines about 200mm but it depends on where its balanced. So that would mean the top of the tube would be about 5 8. So not much wiggle room for a 5 5 person. -

Good point, I suppose it depends how the 'scope is made .

[neptune1]

ALexB67- I would definitely not need a stepladder in those boots! Might break my neck though when i get the heels caught in the mud.

So I ordered an f/4 on a dob mount in the end with the 1/10 optics. It's coming with an angled finger and a plate to mount at a later date. The chap assured me it would only need an occasional tweak so I'l hold him to that!

[Moonshane]

I think that's a great choice.