Lenses

[meddyliol]

I have a Helios 15cm Sky Scan 2001 telescope which I haven't used for a very long time. It is time for me to start again. The problem is, I have lost the setting up instructions. I have the following bits and am not quite sure what to do with them: A Plossl 25mm lens, a Plossl 10mm lens, a piece marked 2X Barlow but with no lens in it (I assume that it should have). I suspect that there is a lens missing but do not know what. Any help would be appreciated.

Thanks

Brian

[Cornelius Varley]

Hello and welcome to SGL. This manual http://www.opticalvision.co.uk/documents/147.pdf should give you sufficient info on how to set up the telescope and mount (the Skyscan mount is the same as the Skywatcher EQ3-2). The telescope has a focal length of 1000mm so the 10mm eyepiece will give you the highest magnification, 100x and the 25mm eyepiece 40x magnification. The barlow should have a lens at the bottom of the tube. If it doesn't then it is broken.

[meddyliol]

Thanks, do I need the Barlow?

[bingevader]

Depends.  
A barlow has the effect of altering the focal length of the 'scope.

So a 2x barlow will double the focal length to 2000mm.

In turn, this means that any eyepiece you put in it will appear to have doubled the magnification.

So, your 10mm eyepiece will now give a magnification of 200x and your 25mm eyepiece 80x.

[NGC 1502]

Thanks, do I need the Barlow?

Hi Brian.

You don't actually need the barlow, the telescope will be functional with the 25mm and 10mm eyepieces you have, for 40x and 100x as already mentioned.

You can do a great deal of worthwhile astronomy with 40x and 100x, so if it were me, I'd have a few sessions with those, see how you get on.

All the best with restarting astronomy, I've seen the best skies ever from Wales, Ed.

[meddyliol]

Thanks very much. How do you equate a 25mm to 40x and 10mm to 100x?

[Cornelius Varley]

The focal length of the telescope is 1000mm (I believe). To calculate the magnification of an eyepiece divide the focal length of the telescope by the focal length of the eyepiece. So 100/25 = 40 and 1000/10 = 100.

[meddyliol]

On the side of the telescope it states F=750mm so that would give me 30 for the 25mm and 75 for the 10mm. It's all coming back to me (very slowly). I just ordered a 2x Barlow lens  (31.7mm (1.25") Achromatic Super Barlow Lens Eyepiece 2x BA2 for £15.99, might not be the best but a good price I think.  I will need to get an adapter to fit my Nikon D90 to the 'scope as well. Thanks for your help.

Brian

[NGC 1502]

On the side of the telescope it states F=750mm so that would give me 30 for the 25mm and 75 for the 10mm. It's all coming back to me (very slowly). I just ordered a 2x Barlow lens  (31.7mm (1.25") Achromatic Super Barlow Lens Eyepiece 2x BA2 for £15.99, might not be the best but a good price I think.  I will need to get an adapter to fit my Nikon D90 to the 'scope as well. Thanks for your help.

Brian

Oops sorry about my post # 5.   If the telescope is 750mm focal length, then indeed your 25mm & 10mm eyepieces produce 30x and 75x as you mention.

Regards, Ed.