Help: Versatility in a Barlow
Posted 27 December 2012 - 06:37 PM
Celestron 70EQ 1.25" focuser, 10mm Plossl (90x)
Orion XT8i 2"/1.25" focuser 11mm ES82 (109x)
The max mag for the 70EQ is allegedly around 155x (unless someone with time in the cockpit can suggest otherwise?)
With a 10mm EP, a 2x barlow is probably pushing it beyond useful at 180x. Therefore, a 1.5x barlow would seem to be a more appropriate option there, at 135x.
With the XT8i/11mm, the 2x barlow option is well within max recommended mag, at 218x.
So, I had been aiming to put a 2" 2x barlow on the XT8i, but I'm reluctant to get a separate barlow for the 70eq, which is more limited by its 1.25" focuser. Wanting to effectively share the barlow across both scopes, I realize this could be entering the realm of a meaningful compromise. I need some help thinking thru the best available options to satisfy the desire to provide a 2x barlow option for the XT8i, while also providing a 1.5x-ish option for the 70eq.
Let's assume I will NOT be putting a barlow onto both at the same time.
I have rebooted the notion of a 2" barlow for XT8i, since I'm unlikley to barlow a 2" EP on the XT8i anytime TOO soon (I own no 2" EP's... yet). So, a 1.25" would be useable on both scopes 'now'. I think 2x is too much for the 70eq, lest I also get a middling EP that won't surpass the 155x range on the 70EQ, when barlowed at 2x (let's rule that out for the moment).
So... what options do I have to provide a 1.25" barlow that can deliver 2x for the purposes of the XT8i and 1.5x-ish for the purposes of the 70eq?
I see a GSO 1.25" 2x barlow (no idea yet if good or bad), which appears to offer the interesting capability of unscrewing the barlow element and attaching to an EP, delivering something in the vicinity of 1.5x magnification. At what cost is that feature? What (negative) impact does it have on the attached EP? Diminished eye relief, field of view, etc? Or is it fairly benign?
If there isn't a reasonable compromise, that's 'fine'. I just need to know that before I go out looking for separate 2x and 1.5x options.
Also, any specific insights that can be provided on the quality of the budget-minded barlows on the market (like the GSO 1.25", or their 2" 'ED' version, for example)?
EDITED NOTE: If there isn't a decent compromise option, perhaps the better solution is to just not barlow the 70eq at all, and maybe get something like a 6mm expanse clone EP to round out the higher mag end of things?
Thanks in advance for an insight you can provide.
Posted 27 December 2012 - 10:23 PM
Barlows, like eyepieces, have a specific focal length. If the field stop of your eyepiece is located right at the intended length of the Barlow, you get the stated multiplier. The Barlow's barrel is designed to located an eyepiece at the "correct" location. If you put the lens element of the Barlow directly in the end of the eyepiece, you'll get a lower multiplier like 1.5x, like you state. If you put more distance between the Barlow and the eyepiece (through a diagonal, for example) you'll get a higher multiplier.
I can't promise that there isn't a negative effect to mounting the Barlow lens element outside of it's intended focal length, but I don't think there is.
To me, the solution is to buy a 1.25" 2x Barlow. You can use it as the regular 2x (diagonal, Barlow, eyepiece), use it as 1.5x (screw the lens element into the end of your eyepiece), or as a 3x-4x or so (put the diagonal between the Barlow and the eyepiece).
Posted 27 December 2012 - 11:46 PM
In a lot of small scopes, even using the barlow in front of the eyepiece (2X) can be too much magnification.
Some (definitely not most) barlows have a lens which, unthreaded from the barlow's tube, can be threaded into the eyepiece like a filter to produce 1.5X. In that case, the 1.5X is almost always usable and rarely produces a magnification duplication.
Look for that feature in the barlow, because it is not common. GSO brand (and the various brands GSO is sold under) usually do.
Posted 28 December 2012 - 01:06 AM
With that being said, I remember trying to use ES82 11mm with the bottom element of the Shorty 2x in my XT8i and found that it would NOT reach focus. FYI, at that time, I was using the stock focuser on the XT8i.
Posted 28 December 2012 - 07:54 AM
I am a big fan of the 1.25 inch GSO 2x Barlow and frequently use it as a 1.5x Barlow.
Posted 28 December 2012 - 10:16 AM
Lately I've sold almost all my other Barlows and have just kept these two. I had 2x, 2.5x, 3x and 5x Barlows but hardly ever used them. Out they went. In general, I'd rather not fiddle with Barlows in the field. But I did just order a Baader 2.25x Barlow for my Baader Zoom. That's a special case!
Posted 28 December 2012 - 12:54 PM
If the 10mm + 1.5x element attached doesn't fare too well in the 70eq (and I assume using the barlow at 2x will just be too much for that scope), then I'll work on getting some different EP's for it. Starting with a 6mm expanse clone (like the Agena EWA series) and perhaps eventually replacing the 10mm plossl with the 9mm expanse clone. Hopefully the barlow works out ok, and I can stall any EP purchases solely for the purpose of more magnification on the 70eq.
I'd prefer to pour money into the XT8i and add one or more low/mid power widefield EP's in the near(er) future. My daughter will be spending far more time behind the XT8i than the 70eq, but the 70 is for her personal use whenever she wants, doesn't need help setting up, so I'd still like to make sure she has some of the essentials covered. Right now, some added magnification beyond 90x seems in order.
If/when I get any 2" EP's, I will then consider one of the GSO clone 2" ED 2.x barlows, until then I can utilize the 1.25".