Focuser upgrade help needed!

[jamespels]

Hi all,

the time has come to replace the tired focuser on my TAL 2 (6" F8 reflector). The existing one is a 1.25" r&p.

I mainly use the scope for visual work (it is fantastic on moons and planets, splitting doubles, etc.) but want to use it for some AP work (again, mainly lunar and planetary - I know F8 isn't ideal for feint objects).

I have manged to stretch the budget to "up to £200" and the GSO / TS 2" 1:10 monorail looks good. However, I would like to get as much change as possible from this and there are quite a few single speed Caryfords around £100 (I have seen several people recommend the Revelation 2" one).

I have heard elsewhere that fine focus becomes less important as the focal ration increases - is this correct and would it offer me a significant advantage? Also, I understand that the main benefits of the monorail over the standard Crayford are arrying capcity and stability under load - is this something I would notice with, say, an NPC900 webcam?

Is there anything I have not considered here that would make a big difference if you were making this choice, and are there any other focusers I should be considering in this price range?

Thanks,

J.

[SiriusB]

For me, dual speed would be a luxury at f8, a necessity at f5, that's purely for visual though. I suspect you'd be glad of a dual speed if you're intending doing a spot of imaging.

Broadly speaking the range of critcal focus is inversely proportional to the square of the focal ratio.

In plain English that means an f8 has nearly twice the focus latitude of an f6.& four times that of an f4.

Hope that helps.

[philj]

Hi James

Speaking from experience I have fitted a twin speed crayford to an 80mm F15, yes F15:) and its a delight to use.

As you say long focus refractors dont really need them BUT it helps tremendously teasing out fine detail on lunar and planetary. OK it all depends on teh quality of the original R&P, if you have a good un then there wont be much advantage but there will still be some.

I used an Antares which is OK for the price, lower end crayfords, the 1970s scope which I bought for 70 squids didnt really warrant a moonlite:D so I went for the Antares. Its a little limited on the handling capacity, eg its eyes water with a 2" filter wheel, diagonal and 2" eyepiece but then alot of sub 200 quid focusers would.

I say go for it, if your into plabetary then youll get the use out of the finer focus

Philj

[jamespels]

Thanks both!

Moonlight and FT are obviously out; GSO/TS as I said look good for the monorail; Rev seem like a good bet for the Crayford - anybody else out there I should consider? I have heard Sky Watcher do not compare so well with these two, for instance, but am I dismissing them unreasonably?

I am hoping there will be a few different focusers at psp2011 I can play with so I won't make a final decision until after that but you know how it is when you have money waiting to be spent on something!

[Moonshane]

I have an Antares and a Revelation (both dual speed) on my dobs. they are very well built and good value for money but having seen a monorail it looks like the build quality is better and the design also less susceptible to slippage. in your position I'd get a monorail dual speed or wait for a used single speed moonlite.

[Moonshane]

ps at least one of mine will be at PSP of course so you can have a butcher's then.