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JimP
sage
Reged: 04/22/03
Posts: 443
Loc: South Carolina
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Hello All,
I have a maxscope60 ds with 10mm BF. When I am viewing the Sun at low mag, using say an 18mm eyepiece , the disk and all proms are visible. BUT, I have noticed that as I go up in magnification, say to a 12mm eyepiece, the full solar disk is visible BUT proms along the edge are NOT visible unless the edge of the sun is put into the center of the FOV. A friend suggested that with a larger BF perhaps I could use higher Mag and when the Sun is centered still see all the Proms along the edge. Does anyone have any thoughts/experience here? Thanks very much!
JimP
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Oldfield
Postmaster
Reged: 03/20/02
Posts: 5816
Loc: Hong Kong
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I don't think it helps.
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The Home Astronomer from a city where most people are proud of the light pollution
Toys: Orion 100ED, Ranger with NGF-CM, SM40/BF10, TG-SP II, LXD55, DMK 31AF03, Canon 10x30 IS...
My observation log and ideas My General Blog
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Ralph Marantino
Postmaster
Reged: 05/01/05
Posts: 6330
Loc: Old Bridge, New Jersey
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Niether do I.
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Lunt H-alpha LS100THa/B3400
Rockland Astronomy Club
Member BAA Solar Section
Central Appalachian Astronomy Club
Astronomical League
DSPST,PST & 70mm Coronado Cal K telescope.2.2A
Zeiss 2 inch prism diagonal&Zeiss Zoom EP
2inch APM Herschel Wedge in a 94mm F7 Brandon
14.5 inch f4.2 Starmaster Dob(Zambuto mirror)
TeleView Genisis SDF with 1000 Oaks Glass filter
Zeiss 63mm f13.5 refractor 1000 Oaks Glass filter
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BYoesle
Pooh-Bah
Reged: 06/12/04
Posts: 1115
Loc: Goldendale, WA USA
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This sounds a bit odd Jim. My guess is that there may be some non-uniformity in the contrast of your telescope-filter system (more common with internally double stacked systems [i.e. the 'sweet spot'] than externally double stacked), and if prominences are visible at low power along the limb in one part of the field, they may be less so in another part of the field. This would tend to be exacerbated by increasing the magnification.
I have found that proper collimation of both the focuser and the objective are critical to achieving optimum field uniformity with ultra narrow band H alpha filters. You may want to check on this and re-evaluate the situation.
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Bob Yoesle 
You are a child of the universe,
no less than the trees and the stars...
And whether or not it is clear to you,
no doubt the universe is unfolding as it should.
Desiderata
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Keith Howlett
scholastic sledgehammer
Reged: 03/06/07
Posts: 977
Loc: Northumberland, UK
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Jim,
That's a great scope, I used to have one. 
The external + internal etalon double stacks have a sweet spot in the fov where you will get better views. It is likely that is what you are seeing rather anything to do with the BF.
Cheers,
Keith
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Ralph Marantino
Postmaster
Reged: 05/01/05
Posts: 6330
Loc: Old Bridge, New Jersey
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come to think of it I usually rotate the front 90mm etalon on the Bob o scope 90mm for best daily results. 
--------------------
Lunt H-alpha LS100THa/B3400
Rockland Astronomy Club
Member BAA Solar Section
Central Appalachian Astronomy Club
Astronomical League
DSPST,PST & 70mm Coronado Cal K telescope.2.2A
Zeiss 2 inch prism diagonal&Zeiss Zoom EP
2inch APM Herschel Wedge in a 94mm F7 Brandon
14.5 inch f4.2 Starmaster Dob(Zambuto mirror)
TeleView Genisis SDF with 1000 Oaks Glass filter
Zeiss 63mm f13.5 refractor 1000 Oaks Glass filter
Itty Bitty Solar Radio Telescope.
LHRIES Lite Solar Spectrometer
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