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Focusing a Celestron Nexstar 6SE


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Hi,

Tonight is my second attempt of using my Celestron Nexstar 6SE and 3rd time ever using a telescope! I have it set up looking at the moon right this minute but it's just a massive blurry white blob and I assume its a focusing issue.

Does anyone know what I might not be doing?

Sorry if there isn't enough info, I don't know all the lingo yet!

Thanks

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Assuming you have put in the mirror diagonal into the visual back and and eyepiece in at the opposite end, you simply need to focus with the rubber knob, rotating it either left or right, until the object you are looking at comes into focus (for example, the moon).

Brinders

Edited by Brinders
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I already gave that a go and spent some time spinning it in both directions but its doesn't appear to make any difference.

I have the diagonal in place with a 2x barlow (sorry if that's not the correct term) and a 25mm eyepiece.

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When I first used my 8SE I had to turn the focussing knob an aweful lot more than I expected, like 30 or more twists! Maybe worth a try? I'd lose the barlow to begin with as well - you may find it easier to focus without it to begin with. Sorry if you've already done that! otherwise, not sure what to suggest... :(

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Ok I have made a bit of progress. I removed the barlow and the eyepiece and managed to focus it just from looking at the moon on the diagonal. Then I put the eyepiece back on and the moon went blurry again.

James F - I can see the whole of the moon when looking down straight into the diagonal.

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It really should come to focus with the eyepiece then. You've tried running the focuser from as far clockwise as it goes to as far anti-clockwise as it goes? Even if you went through focus and out the other side I'd expect you to notice the image getting better defined and then less defined again.

James

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I suggest you turn the focus knob one way to the end, and then gradually turn it in the other direction. A SCT like the C6 has a lot of focus travel, it may be the case that you haven't reach the focus yet. I remembered when I first got my C6, it took me a while to find the focus. Otherwise take it to a local astronomical society and see if they can help.

Also, you can't really focus the scope without an eyepiece in place, because the focal plane position will be very different.

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Ok thanks, I'll have another go on the next clear night we have.

To be honest I'd set it up during the day and get a distant object in focus then. That way when it comes to the next clear night there'll be less faffing about and more observing :)

James

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And if you're still stuck, post a rough idea of where you are. Devon's not a huge place, but I'd not want to have to paint it. It's quite possible there's someone nearby who would be able to help you out.

James

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Just want to point out, a SCT's ability to go massively over focus is a feature rather than a drawback especially when you use binoviewers. My C6 and C925 are the only scopes I own that can reach focus with a 2" diagonal and no barlow when I use a binoviewer.

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I just assumed the SCT's go so far over focus due to their large f/l. My 8SE has a f/l of 3032mm.

I thought an SCT's focus range is related to the their moving primary focusing mechanism. The secondary mirror magnifies the primary mirror by 5x, and I assumed it also magnifies the focuser (primary mirror) movement by 5x.

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I thought an SCT's focus range is related to the their moving primary focusing mechanism. The secondary mirror magnifies the primary mirror by 5x, and I assumed it also magnifies the focuser (primary mirror) movement by 5x.

I dont understand the mechanics of scopes but i meant that i assume (in my mind) that the focusing of the SCT is so wide due to how the mirrors are set out and the long f/l.

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The reason I don't think SCT focus range is related to its focal length is because large dobsonians have similar focal lengths to SCTs. However large dobs use the same focuser as smaller newtonians so they must have the same focus travel range as their smaller cousins.

It's my theory anyway.

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The reason I don't think SCT focus range is related to its focal length is because large dobsonians have similar focal lengths to SCTs. However large dobs use the same focuser as smaller newtonians so they must have the same focus travel range as their smaller cousins.

It's my theory anyway.

That's all true. I'd not considered that fact. So why do SCT's have such a wide focusing range either side of perfect focus. Optically.............is there a need for it, or is it just a design thing.?

For some reason, one night when i went to use my SCT the focus was wayyyyyyyyy out. I dont remember leaving it like that (i'd have no reason to leave it out of focus). Maybe my niece was messing with it. I had to turn the focusing knob about 30-40 times full circle to get focus back.

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It's probably related to the fact that you have multiple optical components working together rather than just the single one in a newt. In a Mak or SCT you have the corrector, the primary and the secondary, all of which have their own optical characteristics. Optics gets a pile more complicated when you have multiple components working together because the characteristics of the system change as you change the distances between the components. When you move the primary mirror in a Mak or SCT you're changing the distances between the corrector and primary and the primary and secondary so there's a fair bit going on.

It only struck me relatively recently that whilst the focal plane of a newt is a fixed distance from the primary and you move the eyepiece in the focuser tube to bring it to coincide with the focal plane, with a Mak or SCT the position of the eyepiece is fixed and what you actually do is change the focal length of the system by moving the mirror until the focal plane ends up where the eyepiece needs it.

James

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Actually, if you like, a Mak or SCT doesn't have a wide range of travel either side of focus. It has a wide range of possible focus positions and an eyepiece placed anywhere in that range can be in focus with the mirror in the correct position. Because the focal length changes however, it's entirely possible that the image size will also be different with the same eyepiece in different places.

James

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As pointed out previously, you need to forget about the Barlow. And do make sure that the diagonal is properly in place, then put the 25mm eyepiece in. I assume that you haven't tried any sort of alignment but have simply pointed it at the Moon, or where you think the Moon is. I suggest that you also make sure that you have first aligned the finderscope - red dot I assume - properly during the daytime. You should then be able to find the Moon. As for the alignment method I suggest you try a one-star alignment with Betelgeuse. It's nice and red so you know you found it rather than something else.

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