Lense/eyepiece issues

[Shayner2005]

Hi all!

I'm having issues with my lenses on my celestron 102slt. I bought a celestron x-cel 3x barlow to use with the stock 9mm and 25mm eyepieces. However, I can't get it to focus with either combination of lenses with the barlow; I can get close, but it runs out of focus the other way, almost as if I don't have a fine enough focus control. I've tried different targets with each of the stock eyepieces with the barlow. I've been trying to get a good look at mars right now, but I can't get more any detail at all. Even with Jupiter, it seems I cant get any detail with any of the combinations either.

Thanks

[Peter Drew]

The 3x Barlow is a pretty powerful unit to use in conjunction with a small F12 telescope, the 9mm eyepiece, Barlow combination would produce well over 400x magnification, way too much for virtually any object.  How does it do with just the stock eyepieces? they should be adequate although not of the highest quality.    🙂

[Louis D]

The 102slt is an f/6, so it should be fine with the 25mm + x3 Barlow = ~8mm.  It could be that the focuser doesn't have enough in-travel for that Barlow.  Do you have the focuser tube retracted all the way when the object image is smallest?  You could try removing the diagonal and using the scope in straight-through mode to see if it will come to focus.  You could also try putting the Barlow ahead of the diagonal to see if that combination comes to focus.  It will yield more than 3x in that configuration.

[Shayner2005]

2 hours ago, Peter Drew said:

The 3x Barlow is a pretty powerful unit to use in conjunction with a small F12 telescope, the 9mm eyepiece, Barlow combination would produce well over 400x magnification, way too much for virtually any object.  How does it do with just the stock eyepieces? they should be adequate although not of the highest quality.    🙂

The stock eyepieces do alright looking at the moon. They leave something to be desired looking at anything else in my opinion. With the 9mm I can see Jupiter and the gallilean moons, although the planet itself is too bright, and just barely make out Saturn's rings. Mars appears as just a red dot.

 

1 hour ago, Louis D said:

The 102slt is an f/6, so it should be fine with the 25mm + x3 Barlow = ~8mm.  It could be that the focuser doesn't have enough in-travel for that Barlow.  Do you have the focuser tube retracted all the way when the object image is smallest?  You could try removing the diagonal and using the scope in straight-through mode to see if it will come to focus.  You could also try putting the Barlow ahead of the diagonal to see if that combination comes to focus.  It will yield more than 3x in that configuration.

I want to say that the focuser was in the middle, but I could be mistaken. I tried the straight-though mode, but it didn't seem to be any better. I did not try putting the barlow before the diagonal.

[Louis D]

6 minutes ago, Shayner2005 said:

I want to say that the focuser was in the middle, but I could be mistaken. I tried the straight-though mode, but it didn't seem to be any better. I did not try putting the barlow before the diagonal.

If the focuser was in the middle of its travel range when the image was the most compact, you may have issues other than limited in or out focus.

Can you point your scope at a distant tower during the daytime to test out the various combinations?  Ideally, it should be at least a couple of miles away to better simulate infinity focus as at night.

[Shayner2005]

1 hour ago, Louis D said:

If the focuser was in the middle of its travel range when the image was the most compact, you may have issues other than limited in or out focus.

Can you point your scope at a distant tower during the daytime to test out the various combinations?  Ideally, it should be at least a couple of miles away to better simulate infinity focus as at night.

I usually do that to calibrate the spotter scope, but I usually only use the 25mm lense and that looks ok. Even when I just look at the moon, everything is nice and crisp with both the 25mm and 9mm.

[Peter Drew]

Some confusion this end!  I looked up Celestron 102 SLT and it came up with 102mm F12 Maksutov.  There are obviously various models available!

This of course changes my original advice and there may be completely different issues causing your problem.  I still think the 3x Barlow would be a bit on the high side, a 2x would be more useful. I agree with what Louis D suggests in the first instance.   🙂

[Louis D]

Apparently, it's a Mak in the UK and an achromat in the US.  However, I did find that Harrison Telescopes sells the achromat version:

https://www.harrisontelescopes.co.uk/acatalog/celestron-nexstar-102-slt-telescope.html

[Mr Spock]

If the Barlow is the type with the elements all in a nose piece, you can unscrew the nose piece and screw it on to the diagonal instead. It's worth a try.

[Shayner2005]

12 hours ago, Mr Spock said:

If the Barlow is the type with the elements all in a nose piece, you can unscrew the nose piece and screw it on to the diagonal instead. It's worth a try.

I don't think I can unscrew it. When I had done the math, the 9mm eyepiece was ~70x magnification, at 3x I figured I'd be around 200x. 

 

Appreciate the advice. Any tips on eyepieces I should try for planetary and some DSO viewing?

Edited October 8, 2020 by Shayner2005