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F/4 orion optics uk dob


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hi everyone,

thinking of selling my Skywatcher Skyliner 350P FlexTube GOTO to downsize and upgrade for a 12" Orion Optics Uk dobsonian with a 1/10pv mirror. but being F/4 I think I need a Coma Corrector or a paracorr. the thing is I have no idea how they work, are they like filters that screw on the end of the eyepieces or does it fit into the focuser and the eyepieces fits into that?

the eyepieces I be using most will be Explore Scientific 82 degree 24mm. Tele Vue Nagler  type 5 31mm eyepiece. SkyWatcher Nirvana UWA 7mm.



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Some look just like a Barlow. Put the corrector into the focuser and then the eyepiece in the corrector. The Altair Astro CC and it's clones are like that and you can pick them up for about £60 s/h. The TV Paracorr and ES ones fit similarly, but have tunable tops so you can accurately tune it for different eyepieces to get perfect correction, and of course are much more expensive. The Baader Multi Purpose CC screws into the eyepiece like a big chunky filter. It also has spacer rings to help tune it to different eyepieces. The rings can be fiddly and easily lost in the dark. Like with eyepieces, you pay more for better correction.

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Why are you downsizing is it just for the 1/10 pv mirror. I ask because I downsized once from a 16" dob to a 12" dob and it was rubbish. The views were no where near as good. Yours is slightly different as it's only 14" to 12" but then you gain the better mirror. All I'm saying is you might be disappointed.

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Mainly because I would like something a lighter and quicker to setup but still want some good size aperture. Plus I always fancied Orion optics dobs.

I’ve got to the stage where It puts me off setting the 14” scope up even though it does not take long to do so. 

My house has steps going at the front and I have to go up steps in the back garden so carrying the Ota itself is fairly heavy.

also I don’t want second scope as it becomes more clutter in the house.

there are pro’s and con’s to both scopes.

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I couldn't agree more Tony, I got rid of my beast of a scope due to having a heart bypass and it was just to heavy and not being used, but I wasn't really happy with the 12" replacement and just longed for my old scope back. Orion Optics make excellrnt optics so you could be on to a winner fingers crossed.

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An alternative could be to keep the big dob, put it on wheels and make some ramps for your steps.


I have just one awkward step to get my 14" dob down, but with a ramp and wheels it is so easy now. I really wouldn't want to have to lug my dob around without those modifications.


Good luck whatever you decide. :thumbsup:

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Thanks for the idea Pete,

The problem with my garden is the step are about 4 foot high and not very wide. The front off the house is the same but wider steps.


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5 minutes ago, seven_legs said:

Thanks for the idea Pete,

The problem with my garden is the step are about 4 foot high and not very wide. The front off the house is the same but wider steps.


I see, it makes a lot if sense to have something manageable that you will get more use out of even at the expense of a little light grasp. :thumbsup:

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Plus, when going to dark site I put my gear in Robs(newty) car and he has upgraded to Celestron CPC 1100 and it’s a tight squeeze. Going to a star party with tents and stuff is impossible now lol

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Hi Tony,

I fairly recently upgraded my Dobsonian from a 10" F6.3 to a 12" F4.

Both are OO VX with the 1/10 wave optics. I sold the 10" and put the money towards the 12" (the 1/10 optics were a free upgrade in the 12)


I'll run through a few pros and cons, but before i do, if it would help you, you would be welcome to check out my OO 12 F4 anytime.

Pop over to me in Derby if you are free and you can have a play. The kettle is always on.


I'll say now that the 12" F4 is as big a solid tube scope as i would want to handle. Remember OO do both a 14" and a 16" in a solid tube. They are not for the feint hearted.

Despite being 400mm shorter, the 12" F4 has a lot more mass than my old 10"


My main reason for upgrading was i wanted to be working at a 1200mm focal length rather than a 1600mm fl

And the 12" F4 is the largest aperture i can use.


When observing with a newtonian i;m more interested in low power, wide field than high power planetary.

If i want to look at planets i have other scopes for that.


Another reason for the upgrade was more to do with me and my preference for 2 eyed viewing.

My binoviewer (for Newtonians) is a big bit of kit. So big that when using it with the longer 10" i had major balancing issues.

I had to add extra weight to the primary end, which was a pain at times.

No such problems with the 12, and even though the primary weighs only 5.6kg it actually feels heavier, and any combination on the focuser won't tip the scales.

My Earthwin binoviewer is performing well and gives me 3 magnifications per eyepieces with the powerswitch (@ 1.2x, 1.9x, 2.4x) giving me a low power of 60x with my 24s.


Regarding collimation, i find the F4 no harder to collimate than the F6.3

You will probably need some correction for coma if you go down the F4 route.

I brought a second hand Paracorr 2 a few weeks ago, but i've not had chance to use it yet.

FYI, you collimate with the Paracorr in place in the focuser. It add 1.15x to the scopes FL (so an F4 becomes an F4.6)


Other pros and cons with OO.



Great optics

Lightweight tubes (VX Range)

Bombproof Dob mount works well.

Easy quick setup

Well designed and good quality mirror cell.

The 12" F4 is much more transportable than the longer 12" F5.3 or 10" F6.3 and will go across the back seat of most cars


The more recent OO focusers are better, but older OO focusers are crap. (Buy a moonlite)

The secondary spider and in particular the holder are cheaply made, and are of poor quality.

VX tubes are a bit flimsy

Cheap, crappy finderscope. (Get a Telrad)

Tube can slip in the rings, and they have to be kept tight. They are not terribly well made.

Mirror cooling fan is an afterthought (crap quality)

Tube end covers are crap. And i mean crap.

Expensive compared to competitors.


And this......remember with an F4 you may (probably will) have issues with longer focal length eyepieces.

I cannot use my N31 with it. Way too big an exit pupil and the obvious sight of the secondary + spider in view.

I wouldn't recommend anything more than say an ES 20 / E21 for low power (if you want 100º fov)











Edited by Bino-viewer
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Thanks for all the info rob that’s great.

would love to pop around sometime and have a look. I am on nights at the moment but finish Friday.

its a shame that that the Nagler 31mm is unsuitable for the f/4 as my tel vue nagler type 5 31mm is my most used eyepiece and my favourite. I wonder if the f/5.3 would be better but like you I like my wide field views.

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It would certainly be better in the F5.3, but thats a big,big scope.


When i say its (N31) unsuitable for an F4 others may disagree a bit.

But you have a 7.75mm exit pupil, so unless you have eyes like a bushbaby you are going to miss out on those photons.

I have pretty small pupils and at 50 they are only going to get worse.

But everyone is different. The exit pupil on the F5.3 is nearly 6mm. Still too big for me. Even a 21mm Ethos (5.25mm) may be too big


My N31 currently does not get used a great deal. 

I really like it too, but for me trading it for a E21 makes some sense.

Ironically my N31 worked really well in my old OO 10" F6.3


BUT.....check out this 'astronomy tools' eyepiece fov calculator.

If i was to trade my N31 for an E21 i'd have a wider fov with the 12" F4 than with the 10" F6.3

I've also added in an ES 20/100 and 25/100 to compare.

Take a look. I'm using M31 Andromeda as a guide.....


12" F4 31N

12" F4 ES 25/100

12" F4 E21

12" F4 ES 20/100

10" F6.3  31N



Edited by Bino-viewer
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  • 2 weeks later...

Did a comparison of the Nagler 31 in Tony's Skywatcher, compared to the OO 12" f4 and f5.3 (also put my scope on there to show how wide field you get).

The exit pupil is 6.88 with the skywatcher, and I always found the views great in it , although I am not sure what 7.75 would be like. On the longer tubed OO scope f 5.3 it's 5.81, so maybe the f5.3 would be best and more practical.


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