Help converting GSO 2" 2x ED barlow to 3x?

[Benboy]

Hi all.

I posted this in the equipment section but thought I would try my luck out here.

I'm wondering if I can convert my GSO 2" 2x ED Barlow to roughly 3x by simply adding a spacer? If so what size spacer do you think would be required to get approximately 3x?

Thanks

Ben

[asimov]

It's hard to say, but I sometimes use a 100mm extension tube with my 2X barlow but it's not quite as big as using my 3X....(Talking jupiter here for example).

The televue site has a chart online that will tell you more than I could.

[casstony]

Assuming the focal length of the barlow is ~97mm (measured from centre of the objective to the end of the barrel) you'd need to add another 97mm to achieve 3x magnification.
ie. doubling the focal length of a 2x barlow will give 3x magnification.

http://www.astunit.com/tutorials/barlow.htm

You could get approximately the needed separation by screwing the barlow element into the nose of a 2" diagonal and inserting the barlow barrel into the eyepiece end of the diagonal. If you wanted more separation you could draw the barlow barrel out of the diagonal a little before tightening the set screw.[edit: I see you have a dob so scratch the diagonal idea]

[Benboy]

Thanks guys. That's exactly what I needed to know. Hmmm...that's gonna be one big barlow!!!


Ben

[bmitchell82]

I don't know if it will work quite like that as the image will get bigger you will start hitting vignetting big time. could i suggest you try getting a power mate? that way you don't have a 200mm barlow with things all over the show. safer for your EP's and less stress on the focuser.

My saying is Do it once Do it right Save the heart ache!

[astro744]

May I ask why you need 3x in 2" format? If you are Barlowing anything over 32mm you will end up extending the eye relief to much. This is very noticeable with a 40mm eyepiece and 2x Barlow and provides for difficult viewing since the eye has to be so far back.

A Powermate is a good option for both visual and photographic use since it wont throw your eye relief out any further than it is and wont give any vignetting. These are available in 2.5x (1.25"), 5x (1.25"), 2x (2") & 4x (2"). The 2" ones are heavy especially the 4x.

Tele Vue make a 3x 1.25" Barlow that is nicely priced and of exceptional quality. If you need 2" then what magnification do you seek and perhaps just get an eyepiece to give you that magnification.

What size and focal length eyepieces do you have?

[Benboy]

bmitchell82 - Yeah, I don't think I will be doing that massive barlow thing - after I found out how big it would have to be!

Astro744 - My reason for asking was really just for the odd occasion when I might want to push my scope a little bitter harder than normal - perhaps on the moon or on nights of exceptional seeing. It wouldn't be worth me buying another barlow or powermate just for those odd occasions - but if it was easy enough to do a simple barlow extension it would be worth it.

I am only just starting out and slowly purchasing gear - currently I have the 25mm and 10 mm that came with my scope plus a 6mm and 4mm sterling plossl. I have also just recieved what I thought would be my first "real" eyepiece- a baader hyperion zoom. The 2" barlow was bought because the hyperion can be used in 2" mode and the other eyepeices can be used with the adaptor and then when I buy a 2" eyepeice in the future I would already have a barlow to use.

I appreciate the input from you guys -


Thanks

Ben

[astro744]

Your eyepieces will give the following powers with 1200mm focal length:
(2x Barlowed powers in parentheses).

25mm = 48x (96x)
10mm = 120x (240x)
6mm = 200x (400x)
4mm = 300x (600x)

8-24 Zoom
24mm = 50x (100x)
16mm = 75x (150x)
12mm = 100x (200x)
8mm = 150x (300x)

I can see you want higher powers with your zoom but 300x on a 6" f8 telescope is about the limit for everything except perhaps the Moon. I can see only a Tele Vue 3.7mm Ethos SX can satisfy you giving 324x and 0.3deg field of view. Wow!:) (A Tele Vue 3-6mm Nagler zoom would also be a good alternative as would 3, 4, 5mm Radians currently on 3 for 2 sale, or just get the 3mm).

I know these eyepieces cost more than the 'scope but either can also be used on a short APO refractor should you ever get one.

To stick to budget what about the Tele Vue 3x 1.25" Barlow and the 10mm eyepiece you already have giving 360x. You could also use the 8-24 zoom in 1.25" mode with the 3x Barlow.

[asimov]

Ben I did a quick test with you in mind last night. Bear in mind I don't do visual, I'm an imager. I'm getting larger image scale with my Meade APO 2X barlow + extension tube than with my Televue 3X barlow. I can do some measurements (from 2X barlow elements to camera sensor) If you'd like, but I'd doubt the extension tube itself is much over 100mm in length.

This was with the 11" SCT on jupiter. Either the math is wrong or I'm not actually getting 3X the image scale with the 3X barlow:shrug:

[Benboy]

Wow! We've just had a big shake up! There is just one advantage I can think off being in a huge earthquake (I'm in Christhchurch, NZ). When the city loses all power at night (no street lights etc - no light pollution) you get amazing views of the sky without having to drive to the middle of nowhere!

Back to business...
astro744- my intention is to use the barlow with the zoom. I realise it is pushing it to go beyond 300x but it would be just for on the moon and on the occasional exceptional night of seeing. I've already tried viewing the moon with the 2x barlow on my 6mm and 4mm plossl and can see the potential for pushing the zoom at least a tad harder. At this stage it wouldn't be worth buying another eypiece or barlow just for that tho - not until my astro budget is stocked up again.

asimov - yeah 100mm sounds about right - i don't think i need to have it exactly 3x was just wanting to push the mag up a bit more.

Thanks guys

Ben