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Richard136

Wide angle eyepieces in SCT / long focal length scopes

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I am considering buying a long focal length telescope (e.g. SCT or similar) but am concerned about the narrowness of field that may result.

What are you real-world experiences using wide angle eyepieces in long focal length telescopes for visual observing?

I assume I could use a wide angle eyepiece to compensate for the scope's long focal length, but I also assume that at some point the combination of wide angle EP and long focal length scope will break down, giving a manner of unwelcome effects on the image.

Are the flatter-field scopes (eg Edge HD) really that much of an improvement? Would a focal reducer benefit me?

Basically I'm looking for confidence that I can do wide(ish) angle viewing on a SCT without the image quality breaking down too much. Unfortunately I don't have access to a scope / eyepiece combo to test this myself...

Cheers -- 

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Well, I've just bought myself a Orion Optics OMC200, which has 4 metres of focal length, so know what you mean - the largest FOV I can get with my current eyepiece collection is 0.56 degrees, only just enough to frame the Moon.

What eyepiece you choose will be down to budget - are TeleVue Ethos out of reach? (around £300 second hand, around £450-£600 new)

I wouldn't bother with a focal reducer - it can cause vignetting with wide angled eyepieces.

Edited by Naemeth

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Hi Naemeth

Thanks for the response.

£300 for an eyepiece is too much for me!

What EP do you have at the mo to give the 0.56 degree field? Any unwelcome effects from it?

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I use a C8, with a more modest 2030mm focal length. With a 2" visual back and 40mm TMB Paragon EP (same as Skywatcher Aero) I got a pleasing 1.3 deg FOV about 50.8x. I now have a Vixen LVW 42mm which gives 1.33 deg FOV (biggest possible) at 48.3x. This is enough for almost all DSOs, except for a handful of very large ones. I use my Nagler 31T5 more often with its "mere" 1.18 deg FOV, which gives excellent views of most DSOs (great for scanning Markarian's chain). The LVW lets me fit the double cluster in with some room to spare, the Pleiades also just about fit. For real wide field, I have my bins and 80mm F/6 frac.

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Hi Richard,

You don't say what size SCT you are considering buying specifically and each scope / eyepiece combination will provide different results. You'll need a 2" diagonal and a low power 2" wide angle eyepiece to achieve the widest possible FOV in an SCT. I have a C11 with a LARGE focal length of 2800mm, the largest FOV I can get with it is just under 1 degree with my 2" 36mm Baader Aspheric 72 degree eyepiece

2800 / 36 = 77.8

72 degree / 77.8 = 0.93

In real terms this is enough to easily to frame the moon, it also allows me frame the Orion Nebula nicely which is also fairly big. Not enough for the entire Andromeda Galaxy but easily enough to frame the central bulge and make out some of it's elliptical shape. Globular clusters and most of the smaller DSO's and galaxies will easily fit in the FOV, although the larger ones including many of the open clusters (of which I have little interest personally) will not.

I've don't have a focal reducer (yet) although I've read conflicting reports on whether this offers a wider FOV for visual use, at least not when used with a 2" diagonal. I imagine someone else can help with this point though....? If someone confirms I definitely would see a benefit it will go straight on my shopping list!! :grin:

The Edge HD is more beneficial for imaging rather than visual as I understand it although I'll happily stand corrected if I'm wrong about this.

James

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A focal reducer takes the 2" maximum image circle (about 47mm) and maps into (roughly) a 27mm image circle (maximum size for 1.25" EPs). As it cannot grab more than the 2" image circle available at the end of the OTA, it does not show more of the sky.

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A focal reducer takes the 2" maximum image circle (about 47mm) and maps into (roughly) a 27mm image circle (maximum size for 1.25" EPs). As it cannot grab more than the 2" image circle available at the end of the OTA, it does not show more of the sky.

Thanks Michael, you've saved me a few £££'s there ! For now anyway.... :grin:

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Hi Naemeth

Thanks for the response.

£300 for an eyepiece is too much for me!

What EP do you have at the mo to give the 0.56 degree field? Any unwelcome effects from it?

50mm LV :) - my 24mm Panoptic gets close at 0.41 degrees. No unwelcome effects from using it on my other scopes - I just haven't received my OMC200 yet ;).

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Thanks Michael, you've saved me a few £££'s there ! For now anyway.... :grin:

The reducer is useful for imaging, however, but then imaging is even more expensive than observing

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Check out this link: http://www.12dstring.me.uk/fov.htm

Switch to Visual, Select the SCT Scope you have (or want to buy) in the left pull Down menu.

Keep barlow on No Barlow.

Then on the right pulldown menu you can Select a variarity of eyepieces.

Then Select the widefield Objects to give you a reasonable acurate idea of the FOV you will get with the SCT and a specific eyepiece.

I really like that website and have it bookmarked. It has gotten a lot of praise over the years. As the imaging mode is also very helpfull for astro photographers.

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Be aware that website is a little inaccurate. It shows the 42mm LVW as having a 65° fov when in fact it's 72°.

The 42mm LVW gives the maximum view on my C9.25. I can just get the Pleiades in - quite a feat for an SCT :)

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I have found having the FR installed focusing is a lot easier and the view is different, i will make some notes next time the clouds go away, a with and with out on my wider EP's.....

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Because the scope becomes faster, there is more "snap" to the focus, and the reducer also corrects for curvature (and coma, if memory serves), so it could give somewhat different views. I just don't like faffing around with the focal reducer to change magnification halfway through observing (worse than using a Barlow in terms of hassle).

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Because the scope becomes faster, there is more "snap" to the focus, and the reducer also corrects for curvature (and coma, if memory serves), so it could give somewhat different views. I just don't like faffing around with the focal reducer to change magnification halfway through observing (worse than using a Barlow in terms of hassle).

I have made the changing of bit some what easier with the aid of Baader Click Locks, it still needs the focuser twiddled but nothings perfect....

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I have found having the FR installed focusing is a lot easier and the view is different, i will make some notes next time the clouds go away, a with and with out on my wider EP's.....

That's interesting. As I said in an earlier post I've read conflicting reports on this but if there is a noticeable difference visually (wider FOV) it may well be worth the investment. Are you using 2" or 1.25" eyepieces? And what size diagonal do you use?

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I would not go too mad with focal length as this does feduce the FOV a great deal, I have a 12 inch LX and that has a F/L of 3 meters, I also have a Mak from Skywatcher at 2700mm F15 . The build of the Meade is better but in my view the star airy disc is better in the Mak and in Jonathans OMC will be suberb.

I would say my most used eyepiece between the two is a 35mm Panoptic, though a 41mm also works but not quite so well on the Mak, sadly both cost a good deal of wonga. At these focal lengths a lesser eyepiece will also work well, 35mm of 40mm Areo from SW or the 40mm 34mm Meade SWA or the same from ExSc, with either I can get a close to 1 degree, though Naemeth with his fabulous OMC200 is going to struggle for FOV, however this is a suberb quality planetary scope, I wish I had one.

Alan

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I've read quite a lot of threads over the years about getting the max FoV from an F/10 SCT. By the time focal reducers, 2" diagonals and long focal length eyepieces have been considered I'm often tempted to suggest that it's simpler (maybe less expensive too) to buy a used 8" F/6 dob for £200 or even less to use when a wider field is needed.  

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I've read quite a lot of threads over the years about getting the max FoV from an F/10 SCT. By the time focal reducers, 2" diagonals and long focal length eyepieces have been considered I'm often tempted to suggest that it's simpler (maybe less expensive too) to buy a used 8" F/6 dob for £200 or even less to use when a wider field is needed.  

Indeed - you don't buy a Mak or SCT for things like M31 and M45. I'm fortunate enough not to have a Mak / SCT as my only scope.

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I've read quite a lot of threads over the years about getting the max FoV from an F/10 SCT. By the time focal reducers, 2" diagonals and long focal length eyepieces have been considered I'm often tempted to suggest that it's simpler (maybe less expensive too) to buy a used 8" F/6 dob for £200 or even less to use when a wider field is needed.  

yes your right but it won't fit in here....

DSC_9906.jpg

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I've read quite a lot of threads over the years about getting the max FoV from an F/10 SCT. By the time focal reducers, 2" diagonals and long focal length eyepieces have been considered I'm often tempted to suggest that it's simpler (maybe less expensive too) to buy a used 8" F/6 dob for £200 or even less to use when a wider field is needed.

Over time I came to that same conclusion with the Mak John. It is a dedicated lunar/planetary and small DSO scope, and does these things very well. Regardless of what you do, you will never get widefield views from it, so I think it best to concentrate on what it is good at. I ended up with a set of TV Plossls and also used an 18mm BGO which worked very well. Jonathan has one or two other scopes for widefield ;-)

Stu

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It's an interesting point having about a second scope for viewing wider fields. Cost wise it's probably cheaper than buying the extra gear to "force" an SCT to do wide field. Square peg, round hole, maybe?

I am coming round to that way of thinking, but then there is the problem of storing / transporting two scopes. (I don't have an obssy, unlike the rather excellent looking one in the photo posted by Tinker!)

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John,

100% correct!!!!!!

40mm eyepiece, lets not go mad 40mm Aero, 130 quid.

Diagonall, lets go for the Reveltion, I think 90 quid.

Reducer lets be reasonable S/H 65 quid.

then you need a 2 inch back for the SC, 30 quid.

There is 315 pounds, a nice scope can be got for that.

Alan.

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