Trouble focusing without barlow and 35mm extension

[paolot]

Hi all

After some trouble with my HEQ5 Pro Mount (which turned out to be a power supply - thanks to everyone in the mount section who helped with that!) I thought I was finally ready to go!

Aligned my finder scope with a tree not too far during the day time - but had trouble focusing. Thought it might be because it was too close.

Took my scope out, and had Orion in a perfect spot to have a look at the Orion nebula. Once I located Rigel in my finder scope, it was sitting as a large blob of light in my eyepiece. As I tried focussing, it became smaller, but then once I reached the end of my focuser, it remained a blob of light, and I could still see the shadow of what holds up my secondary mirror. I tried going in the opposite direction, but it only got larger, until it disappeared and all I had was a giant black hole.

I then put the 35mm extension on it and at the top end of the focuser (all the way out) it was almost in focus. I then put a 2x Barlow on it, and at about half way through the focuser, I managed to get it perfectly in focus.

I took everything off, puller out my laser collimator and made some minor adjustments (just off, probably because I did it inside, then brought it out and must have knocked it slightly). Put it all back together but still could not get it to focus without the 35mm extension and 2x barlow.  

I managed to get a good look at the Orion nebula, but because of the focussing issues, couldn't use the scope to its full potential.

The scope is an 8inch Bintel refractor (believe f/4). I was using a 25mm plossel eyepiece, and then a 9mm eyepiece. With the Barlow both worked - without, neither did.

Just in case I'm doing anything wrong with collimating it, I looked through the eyepiece and slightly moved the secondary mirror so that it was circular. I then put the laser in, pointing towards the back of the tube, and adjusted the secondary mirror so that the laser was pointing in the centre hole (it is marked). I then adjusted the primary mirror so that the laser coming back through the eyepiece hit the centre hole in the collimator. 

Anyone have any thoughts on why this might be happening?

[ronin]

Collimation and the focal position are not related, so altering the collimation does not move the focal plane in or out. So leave that alone.

2 thoughts occur:

What is the scope ? If it is the 200P variety then there is usually a 2" adaptor in the focuser and that has to be removed to use a 1.25" eyepiece or else you cannot get it to focus. Seems that it looks like it should be there but it shouldn't. Remove it and try again.

If you bought it used then did the previous owner do AP? If so then they could have moved the mirror further up the tube to enable a DSLR to achieve focus. So then it may not allow an eyepiece to achieve focus.

My bets are on (1) it seems a common problem as everything looks correct and the last idea people have is to start taking bits off.

[paolot]

Thanks for the reply Ronin.

Yes, it's BT200. Pretty sure I know what your talking about - the eyepiece fits fine and I've borrowed a couple of 2" eyepieces to try out (and tried lol) so I don't think that's the problem. I took the extra part out and the 2" fit right in.

I bought it new so I doubt that the second part is the problem either

Any other thoughts?

[paolot]

I've just been doing a bit of googling (spent yesterday doing it and found nothing - today a bit more luck).

Could it be that I need a bigger extension? And does the amount of extension I need change depending on what eyepiece I'm using?

[Peter Drew]

If without the Barlow the focuser has to be extended outwards to almost get the object in focus then yes, you will need a longer extension to reach the focus. With just the eyepiece and the focuser fully extended try withdrawing the eyepiece from its holder. If you then find you can get a focus then the extra extension needed would be equal to  or slightly longer than the amount by which you pulled the eyepiece out. Hope that makes sense. 

[Astro Imp]

Thanks for the reply Ronin.

Yes, it's BT200. Pretty sure I know what your talking about - the eyepiece fits fine and I've borrowed a couple of 2" eyepieces to try out (and tried lol) so I don't think that's the problem. I took the extra part out and the 2" fit right in.

Any other thoughts?

When using 2" eyepieces you leave the 2" fitment in the focuser and when you change to 1.25" eyepiece you remove the 2" holder and replace it with the 1.25" holder.