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Hi. I've recently gained a used skywatcher 114/500 telescope, with a 25mm eyepiece (giving a magnification of x20) which I have been using to look at Jupiter, weather permitting. The resultant view is pretty small and not sharply focussed, but I can clearly see the planet (as a smallish dot) and three of its moons (as smaller dots). Hooray!

 

I have just bought an Ostara x2 Barlow and an Ostara 6mm Plossl, as a way of giving me magnifications of x40, x80 and x160 (approximately). I went out tonight, and although extremely cloudy there were enough breaks in the cloud to briefly test my new purchases. First I tried the Barlow and the 25mm. As expected I could see the same view but ever so slightly larger. The crispness, or lack of, was about the same. However, when I tried to use the 6mm, with or without the Barlow, I was not able to focus the image. It remained out ofd focus no matter what I did.  Admittedly I only had a few minutes to try before the clouds descended completely, and rain ended play, but it did seem pretty impossible to gain a decentish focus. 

 

Is this likely to be due to a need for collimation? I got the impression when I collected the telescope that it had been stuffed in an attic for a long time and hadn't been used in years. Also I drove about 5 miles with it in the boot, and I carry it in and out several times a night sometimes, as the weather is variable. I haven't collimated it yet because I don't know how to do it. So I would think it would definitely benefit from beingcollimated.

 

But my question is, does this inability to gain focus at higher magnifications sound like it could be down to collimation issues?

Thanks in advance for any helpful replies.

 

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Posted (edited)

What sort of star/airy-disc shapes do you get either side of focus?

If round then collimation isn't likely to be too far out but in any case I'd expect you to still be able to reach focus. I assume you were seeing an airy disc or image full of fuzz while trying to focus? Or was it just blackness? It's possible that the Ostara needs further in-travel to reach focus compared to the 25mm you have. Perhaps try in daylight on a bright distant target and see if you can find focus at all and where relative to the 25mm the focuser is set to.

Edited by DaveL59

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6 minutes ago, DaveL59 said:

What sort of star/airy-disc shapes do you get either side of focus?

If round then collimation isn't likely to be too far out but in any case I'd expect you to still be able to reach focus. I assume you were seeing an airy disc or image full of fuzz while trying to focus? Or was it just blackness? It's possible that the Ostara needs further in-travel to reach focus compared to the 25mm you have. Perhaps try in daylight on a bright distant target and see if you can find focus at all and where relative to the 25mm the focuser is set to.

I'll have to try again to be sure, and the weather has gone bad so can't right now, but I seemed to be getting a fairly roundish disc with a dot in the middle. I was quite brief so it's hard to say. Also I am very new to this, so I'm not sure about terminology. 

 

However, it seemed to be out of focus by a lot, then when I turned the focusser in one direction it got more in focus, then at the point when it should have popped into focus  it didn't, then when I continued in the same direction it got more out of focus again. 

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ahh ok, sounds like maybe you overshot, easy to do. with higher mag you need finer movements to hit the focus perhaps. At least is sounds like you aren't limited by focuser travel. For sure the weather isn't helping at the moment, now it's cleared a bit the planets are behind tree's here plus quite a breeze so I've not played at all tonight.

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7 minutes ago, Jm1973 said:

I'll have to try again to be sure, and the weather has gone bad so can't right now, but I seemed to be getting a fairly roundish disc with a dot in the middle. I was quite brief so it's hard to say. Also I am very new to this, so I'm not sure about terminology. 

 

However, it seemed to be out of focus by a lot, then when I turned the focusser in one direction it got more in focus, then at the point when it should have popped into focus  it didn't, then when I continued in the same direction it got more out of focus again. 

That certainly sounds like you are able to reach focus with the eyepiece, as you are getting to a minimum size then it is increasing again.

Was that on Jupiter still, or did you try on any stars? It’s well worth doing a star test, try on Polaris as it is about the right brightness and doesn’t move much.

Jupiter at first glance may not show much, especially if the seeing is poor. Give it another go when you’ve got clearer skies and see how you get on. First thoughts are that collimation probably isn’t far off, so I wouldn’t fiddle with it until you have had a longer session.

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1 minute ago, DaveL59 said:

ahh ok, sounds like maybe you overshot, easy to do. with higher mag you need finer movements to hit the focus perhaps. At least is sounds like you aren't limited by focuser travel. For sure the weather isn't helping at the moment, now it's cleared a bit the planets are behind tree's here plus quite a breeze so I've not played at all tonight.

Ah, hopefully that is the case. It was all rather hurried, trying to catch gaps in the cloud. 

 

I'll try again when the skies are clearer and try to be a bit more methodical. Thanks for the replies.

Clear skies.

 

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Posted (edited)
7 minutes ago, Stu said:

That certainly sounds like you are able to reach focus with the eyepiece, as you are getting to a minimum size then it is increasing again.

Was that on Jupiter still, or did you try on any stars? It’s well worth doing a star test, try on Polaris as it is about the right brightness and doesn’t move much.

Jupiter at first glance may not show much, especially if the seeing is poor. Give it another go when you’ve got clearer skies and see how you get on. First thoughts are that collimation probably isn’t far off, so I wouldn’t fiddle with it until you have had a longer session

That was Jupiter. The sky was full of clouds and three just happened to be a gap where Jupiter was. I'll try it on a star as well when the weather gets a bit better.

 

Thanks for the reply. 

Edited by Jm1973

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