ICEINSPACE
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04-05-2020, 10:42 PM
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Registered User
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Join Date: Feb 2007
Location: Mt Tamborine
Posts: 587
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Barlow question.
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.
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05-05-2020, 08:26 AM
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Registered User
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Join Date: Feb 2007
Location: Mt Tamborine
Posts: 587
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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.
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05-05-2020, 10:10 AM
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Registered User
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Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,904
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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)
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05-05-2020, 04:08 PM
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Refractors-That’s It
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Join Date: Jan 2015
Location: Rangeville, Toowoomba
Posts: 440
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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
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05-05-2020, 05:04 PM
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Registered User
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Join Date: Oct 2005
Location: Junortoun Vic
Posts: 8,904
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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.
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05-05-2020, 06:24 PM
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Registered User
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Join Date: Oct 2013
Location: Dunners Nu Zulland
Posts: 1,661
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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.
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06-05-2020, 08:59 AM
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Registered User
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Join Date: Feb 2007
Location: Mt Tamborine
Posts: 587
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ab1963, you have a very good point. Will look at Starguider type eyepieces.
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06-05-2020, 09:19 AM
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Refractors-That’s It
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Join Date: Jan 2015
Location: Rangeville, Toowoomba
Posts: 440
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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
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07-05-2020, 07:15 AM
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Bright the hawk's flight
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Join Date: Nov 2008
Location: Mt Duneed Vic
Posts: 3,978
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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
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23-05-2020, 02:35 PM
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Registered User
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Join Date: Feb 2007
Location: Mt Tamborine
Posts: 587
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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.
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