Inward travel focus

[steviebee]

Not sure if is the right section, so please move in need be.

I have a SW Startravel 102mm, I like the scope a lot and for views of the Orion Nebula etc  it is very good. The problem I am having is it does not have enough inward travel for focus. I use a 2" diagonal which is a SW one and a SW LET 28mm lens. With this set up it is not quite in focus when fully in. I also use a 12mm and 10mm 1.25 lens. I use the 2" - !.25 reducer, again there does not seem to be enough inward travel with this set up. If I use a 1.25 diagonal it is fine focus being about 50% out. My question is if I was to remove about 5mm of thread at the back of the scope would this be detrimental to the scope at all? It would I feel give me the extra inward travel it needs I have attached pictures to show you the thread. I am competent enough to remove the thread just wanted to seek your views. If not is there anything else you could suggest.

 

TIA>

Steve.

[johninderby]

The usual thing is to simply shorten the OTA tube a bit. I’ve done this on 102 and 120 refractors with no problems.

[steviebee]

Thanks for that johninderby, I will give this some thought. How much have you taken off? I was thinking about 5 to 10mm.

 

[johninderby]

Took 10mm off the 102 and 20mm off the 120 but on the 120 had fitted a crayford focuser which was longer than the stock one so needed a bit extra off..

 

 

[steviebee]

Thats great, thank you. I assume where I have arrowed is where I remove it from?

 

Regards

 

Steve.

[Davey-T]

Just be aware that there's a reason for it being the length it is made if you move the focuser too far up the tube it will cut into the light path.

Dave

[johninderby]

10mm off shouldn’t affect anything and yes that’s the point to shorten the OTA tube. I used masking tape and marked the amount to be taken off at a few points and wrapped a piece of heavy card around the tube and marked a line around the tube. For cutting used a junior hacksaw with a fine blade. Don’t forget to draw short lines up from the original mounting holes before cutting to help with locating the new mounting holes.

[Bruce Leeroy]

16 minutes ago, steviebee said:

Thats great, thank you. I assume where I have arrowed is where I remove it from? 

No I don't think he did, if you remove the threaded part you arrowed you will not get any extra inward travel. I think what your hitting is the end of the focusing rail which you should see if you look underneath....rack the focuser all the way out then look underneath for the rail that guides the focusing tube in/out.

 

Well you may get the extra inward travel come to think of it but to test if it will be enough you can completely loosen of the screws from under the focusing wheels and make that final 5-10mm gap close up.

[steviebee]

Bruce Leeroy, thank you. I have just loosened the screws from under the focusing wheel and it has now gone in all the way. Sod's law it is overcast now so I will not be able to test it. It certainly looks as-though it has given an extra 10mm of inward travel. This is what I love about this forum full of people only too willing to offer help and advice. Thanks again to you all

 

Steve. 

[steviebee]

Just been out and tested it. I can actually see 4 moons of Jupiter, something I have never seen before with the ST102mm. OK Jupiter has a bit of a tail/flaring that moves around as I position my eye on the EP, but a heck of a lot better. 

 

Again many thanks to you all for your help.

 

Steve.