Skywatcher 250PX - focusser question

[John]

My used 205PX came with a R&P focusser rather than a crayford which I think the latest ones are fitted with as standard. My focusser does appear to be collimatable though which I have not seen before on a Skywatcher R&P.

The question I have though is on the point of focus of the scope. I was pointing the scope at a tree top about 150 feet away to align the finder. I found that I had to use the 2 inch adaptor and the 1.25 inch adaptor to get eyepieces to focus, ie the focus point seems quite away from the side of the tube - around 4-5 inches from the tube. When I tried the 2 inch eypiece that came with the scope, I could not get it to focus on the tree top at all - not enough outward focus

Is this normal with these scopes ? - I had expected the focus point to be closer to the tube than that.

Thanks,

John

[GazOC]

I don't know if this will help John but I have a view from my house extending about 100 metres and my 14" won't come to focus but works fine at infinity......

[FLO]

Is this normal with these scopes ? - I had expected the focus point to be closer to the tube than that.

It isn't unusual to need a focus extender for scopes of this type, particularly when using 30-40mm 2" eyepieces. The difficulty for the manufacturers is that imagers want the focuser mounted close to the tube so their DSLRs come to focus but observers need lots of outward focus travel for their larger eyepieces. If they simply supplied a longer draw-tube to keep both camps happy, it would protrude into the light-path when imaging. Correcting for lack of outward-focus with a focus extender is easy but overcoming insufficient in-focus is not. Hence the compromise.

[GazOC]

Pah, bleddy imagers!!!

[FLO]

[John]

I don't know if this will help John but I have a few from my house extending about 100 metres and my 14" won't come to focus but works fine at infinity......

Thanks Gaz and Steve,

I did wonder about the point you make above Gaz - I may well see a big difference when I look at astro stuff through it - when the "beep" clouds go away that is

As usual I'm probably worrying unecessarily - I always seem to get a bit paranoid when I get a new scope

John

[GazOC]

Hopefully thats it John, my Dob will not come anywhere near focus on terrestrial targets but is fine with every eyepiece I've tried it with, both 1.25" and 2" at infinity.

[themos]

The Type 2 Nagler 12mm would not focus on the moon when fully seated in the 2" focuser of the 300P. I had to either use the 1.25" adapter or pull the eyepiece out a bit (1/3 of inch?) and then it could focus with the focuser tube fully out.

[Kaptain Klevtsov]

150 feet is far too close to test the 'scope for astro stuff. Point it at something proper far away, it'll work fine.

Kaptain Klevtsov

[John]

150 feet is far too close to test the 'scope for astro stuff. Point it at something proper far away, it'll work fine.

Kaptain Klevtsov

I usually use this particular tree top to get my finder roughly aligned with the scope. I recall that my C5 and C8 SCT's could focus down to 20 feet or so but I know that the SCT focus mechanism has far more travel than a newt. I must remember not to try birdwatching with my 10 inch dob - unless the birds are over a mile away and flying upside down that is

John

[John]

150 feet is far too close to test the 'scope for astro stuff. Point it at something proper far away, it'll work fine.

Kaptain Klevtsov

Yep - the moon has just focussed beautifully for me with the focusser about halfway through it's range of travel - can't argue with that !.

Thanks for the re-assuring words folks.

John