SW ST120

[steviebee]

Hi, I have the SW ST102. I think its a nice little scope, but I am having trouble getting inward focusing on brighter stars, it just seems as though it needs to travel in a tad further to obtain focus. I have fitted the Crayford Dual Speed Focuser but this has made no difference. I was wondering if the SW ST120 has the same problems with inward focus. Or perhaps a filter/filters would help to reduce the glare of the brighter objectsglare? I dont really want to go to the exspence of another scope ( I think SWMBO would hang, draw and Quater me) I am using a SW 2" Dialectric Diagonal with the SW LET 28mm eyepiece which I find comfortable. I have used other  1.25 eyepieces but they all seem to need that little bit more inward focus. Any ideas or simple fix's. Or is it just an inherant problem with the SW ST102mm.

[Dave In Vermont]

I doubt a new telescope is the answer here. Most likely something is being overlooked.....

Is all well-seated? Is anything also attached to the optical-pathway that could add to the focal-travel needed? Could the scope reach focus before you attached the Crayford? Go over it all step-by-step. What eyepieces are you trying to use that won't focus?

We'll get you sorted - so don't panic.

Dave

[Moonshane]

Can you upload a photo of how you set up? Can you focus on faint stars and the moon?

[BXRO]

Hi, ensure that your EP is correctly seated in the focuser, my first mistake was to fix the eyepiece slightly proud of the focuser and the travel of it wasn't enough to bring it to focus

[Knighty2112]

Have you tried a 1.25" diagonal in the scope? I remember a while back someone had issues achiving focus with a Crayford fitted to a ST120 scope. The issue was I believe the 2" inch diagonal had a 1.25" adapter that reduced the inward focus when using the diagonal with the 1.25" diagonal because it added about a quarter of an inch approx when fitted into the diagonal, thus reducing the inward focus by a quarter of an inch . My 2" diagonal that I use on my ST120 (with a Crayford focuser too) has a 1.25" adapter too, but this doesn't change the position where focus is achieved when using either 2" or 1.25" EPs as the 1.25" adapter sits flush to the diagonal and doesn't make the EP sit any higher in the diagonal than a 2" EP. 

[Cosmic Geoff]

I have the Sky-watcher Startravel 102. I have not had any trouble with the focusing. TBH while this is a portable and easy to use wide-field scope it's not one I'd throw money at in the way of dual speed focusers etc as there is a noticeable amount of chromatic aberration. Check that you are getting the full range of focuser movement - there is nearly 2" with the original focuser and mine sits at 1/2" 'out' with the original diagonal and 25mm eyepiece.

[happy-kat]

Surely a quick fix if you need more inward focus travel is use a short extension tube, these do not have any optical elements , assuming the new focuser is what has shortened the optical path. 

[Knighty2112]

49 minutes ago, happy-kat said:

Surely a quick fix if you need more inward focus travel is use a short extension tube, these do not have any optical elements , assuming the new focuser is what has shortened the optical path. 

An extension tube is used to give you more outward focus than the focuser itself can achieve. Fitting an extension tube before the diagonal and EP in this case will move the focus point even further away. 

[happy-kat]

yes good spot I got it the wrong way around.

[moriniboy]

See if you can find one of these, may be enough to allow you to focus with the 1.25's

[Knighty2112]

14 minutes ago, moriniboy said:

See if you can find one of these, may be enough to allow you to focus with the 1.25's

Not seen these before. Good idea. Should do the trick.  

[steviebee]

3 hours ago, Moonshane said:

Can you upload a photo of how you set up? Can you focus on faint stars and the moon?

I will try to upload a photo tomorrow or Friday. Yes faint stars and the Moon no problem.

[steviebee]

4 hours ago, Dave In Vermont said:

I doubt a new telescope is the answer here. Most likely something is being overlooked.....

Is all well-seated? Is anything also attached to the optical-pathway that could add to the focal-travel needed? Could the scope reach focus before you attached the Crayford? Go over it all step-by-step. What eyepieces are you trying to use that won't focus?

We'll get you sorted - so don't panic.

Dave

Hi Dave, I am sure it is something I am doing/not doing. I will get a photo of my normal setup on here. Then I am sure someone will spot what I am doing wrong.

[Moonshane]

Sounds like you need to let the scope acclimatise outside before use. I normally suggest 5 mins per inch of aperture.

[steviebee]

27 minutes ago, Moonshane said:

Sounds like you need to let the scope acclimatise outside before use. I normally suggest 5 mins per inch of aperture.

I always do. I normally get it set up in daylight! But it is always atleast an hour before I go outside.

[steviebee]

Here is a picture of my normal setup!

[spaceboy]

I was going to say I had the ST102 with dual speed focuser and now have a ST120 with dual speed and neither of them had any problem focusing. That was before I seen you had the piano black dual speed. My guess this is slightly longer than the standard dual speed due to the ability of it to rotate. The only suggestions I can think of is either 1) try to pick up a standard DS focuser 2) revert to 1.25" diagonal to shorten the optical path or 3) go with a Revelation 2" quartz diagonal as these have a low profile 2" - 1.25" adapter but no use to you if 2" EP's don't focus either.

[Moonshane]

What I don't understand though is why it's only bright stars that won't focus well

 

 

[spaceboy]

11 minutes ago, Moonshane said:

What I don't understand though is why it's only bright stars that won't focus well

 

 

My guess because it's an achromatic refractor. I love to use refractors and I can live with CA in achro's but there's no denying fast achro's on bright stars can fall short of expectations. Poor seeing and an abundance of chromatic aberrations may be the culprit ???

[Knighty2112]

Get a bahtinov mask for the scope. They are relatively cheap to get, and I have one that fits all my range of refractors to get correct focus. I normally focus on One of the brightest stars in view, and set it with that. Does the scope focus OK on the moon? If so and it focuses OK on other stars then I fail to see what the problem is as the focus when achieved is the same for all astronomically objects, unless you are doing imaging and having focus problems there? 

[spaceboy]

Or simply remove the diagonal and put the ep directly into the focuser. If you can't reach focus then your never going to. I think it is more a case of CA (purple fringing) than focus issues. If it is you could try a semi apo filter but at f/5 you'd be wasting your money. 

[steviebee]

Firstly thank you for all your comments, and suggestions. I am now typing this rather Red faced. I have had problems with the ST102 since I took the lens's out to clean as they were filthy ( bought 2nd hand). On another thread on SGL it was mentioned about putting  a lens in back to front, and I had. Sightly, well a lot better but still not enough inward focus. I then read about lens pinching, yes I had tightened them a tad tight ( the pit falls of being in heavy industry most of my working life using BIG spannners lol). Still not a lot better. I then read on another thread about the spacers between the lens's making sure these were in the right order. Yes I had got two of the spacers in the wrong way round. What a differenece. I can now start to enjoy the scope again. Although I am thinking about getting the ST150 refractor :-).

[Moonshane]

Brilliant post and you did very well to sort everything out. You now know the scope intimately and will presumably have no issues with collimation of a newtonian if you ever get one! We all make mistakes like this and well done for being resourceful enough to find out and brave enough to sort 

[steviebee]

HI Moonshane, thank you. I am never one to give up on something. I do have a SW 200 Explorer, really pleased with that, it lives in France under mostly clear, dark skies :-). I have collimated it already after its long drive to South West France, so she is no stranger to me either. I'm learning all the time. I did put the ST102 down for a while because of the 'focus' problem, but the last few nights ( once the clouds cleared) have been really enjoyable again, even with the light pollution in my garden.  Even got a neighbour and his family interested now after he see me setting it up in daylight. so that is agood thing as well. Now all I have to do is be a good boy for a few years and perhaps SWMBO will let me have a 16" Dobby for when we retire to France :-)))

 

[laudropb]

Glad to hear you have sorted out your problems. This has been a very instructive post and you have done well to see it through to a happy conclusion.