Barlow question.

[doug mc]

I have a 2x Meade shorty barlow. It requires quite some intrafocal adjustment to reach focus. Just wondering if the GSO 2.5x apo barlow is the same or close to parfocal for standard plossl eyepieces. I don't have the budget for Teleview units. Thanks.

[doug mc]

Just discovered that the GSO 2.5x apo barlow requires a fair amount of inward focus , so i will give up on that one. Any recommendations? It's a newt so a long barlow would be ok.

[Merlin66]

By design, the Barlow lens has to be positioned inside the prime focus, by roughly half is focal length.
(How do you measure the focal length of a negative lens like a Barlow? Well measure the free lens diameter say 25mm and then draw a circle on a card twice this diameter (50mm) point the lens towards the Sun and when the image fills the circle measure the distance from the card - this the the focal length of the lens)

[astro744]

Consider a Tele Vue Powermate when budget allows as they are close to parfocal. See http://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_app

[ab1963]

To be honest after having 3 Televue 2x powermates 2 2.5 powermates 1 4x powermate 2 2x barlows and 1 3x barlow you can’t beat single focal length eyepieces for visual I keep thinking I need a Barlow but the truth is I don’t so stick to the focal length you need in stand alone eyepieces for short focal length Delos and for longer ES 14 100mm and 21 Ethos and you will be happy without the messing around with barlows this is the costly lessons I’ve learned about barlows

[Merlin66]

Andrew,
I understand you circumstances....
I must be honest and say by far the greatest use I have of Barlows and Powermates is in imaging.
The PM gives excellent results.

[N1]

Same here, I find them awkward for visual but pretty essential for imaging. I have a 1.8x TMB and a 3x GSO ED, and the TMB needs to go quite a bit further in than the GSO.

[doug mc]

ab1963, you have a very good point. Will look at Starguider type eyepieces.

[ab1963]

I have viewed through the starguider ep’s and they are very good value I just need a little more fov but it’s each to his own and know a few people that 60 degrees is the sweet spot for them

[barx1963]

For visual use, I have found that you usually end up using no more than 2 eyepieces 90% of the time. In my 12" dob, I had a 24mm Panoptic and a 13mm T6 Nagler. Had a bunch of other EPS that almost never left the box. On 2 separate occasions I acquired barlows, hardly ever used them and never satisfactorily.

In my 20" dob I now have a set of Ethos EPS (10,13,17 and 21). My 17mm is the EP i use maybe 90% of the time with very occasional use of the 21mm for big galaxy fields or the 13mm if a bit more power is needed for a small PN or galaxy. Very occasionally, the 10mm would come out. I had an 8mm but sold it as it just never got used.
To be honest, if I just had the 17mm I would still be perfectly happy.

Keep it simple!!

Malcolm

[doug mc]

Ended up with a Meade vintage 2x barlow. Glass looking good. I removed the lens, blackened the outside edge and flocked the inside barrel. Comes to focus with only a few millimeters inward travel using a plossl. The barlow purchase was here from fellow IIS member.