Fast Scopes

[DSOseeker]

Hello Board,
I have an Orion 10" DOB XT, no encoders. I love the scope. This light bucket has enabled me to see DSO's from my backyard, and stay managable as a grab-and-go scope. My big nag, it's sooo fast, 4.5 focal ratio. I know if I really want to improve the view over my Orion Sirius eyepieces I need TV's. But, does anyone out there have any experience with alternative EP's in a scope that is this fast (4.5). I've been cruising Astromart for some new EP's. There is a good variety of alternatives. I want to make a logical choice for my mirror, and stay Married (if you know what I mean $$). Anyone have any opinions, please share them. I am not expecting TV eyepieces for half the cost, just looking for alternatives people have had success with.

Thanks Everyone,
DSOSeeker

[Dave Mitsky]

I assume that you're looking for eyepieces with larger apparent fields of view than the 50 degrees of the Ploessl design. Unfortunately, wide-field and ultra wide-field oculars that are not corrected for astigmatism do not perform very well in fast telescopes. This means that unless you shell out for Tele Vue Radians, Panoptics, and Naglers or the old series 4000 Meade Ultra Wide Angle line (there have been some troubling reports about the new series 5000 line) astigmatism will be present to a greater or lesser degree at the periphery of the field of view. Many of today's "new" eyepiece lines are merely updated versions of the venerable Erfle or Koenig (or variations thereof) that are now sporting names like Axiom, Optiluxe, Proxima, QX, Stratus, and WideScan. One alternative to Tele Vue is the Speers-WALER but these eyepieces will not come to focus with some telescopes. They are also quite lengthy. The Pentax SMC XL (now discontinued) and XW lines have long eye relief and have fairly good field edge performance but are rather expensive. Burgess and Williams Optics displayed new Chinese ultra wide-field eyepieces at the NEAF last week that may not be all that bad.

Another problem that arises when wide-field eyepieces are used with a fast Newtonian is that the off-axis coma of the mirror becomes evident. The only cure for this is a coma corrector such as the Tele Vue Paracorr.

If seagull-shaped stars at the edge of the field are not an issue, I suggest looking into the University Optics Koenigs or the GTO Proximas. If you don't require a wide-field eyepiece, consider the Celestron Ultimas.

Dave Mitsky

[vet]

you know, a lot depends on how long you've used a fast newt---i prefer f/5.6-6---anything faster? yeah, the coma really goes up. Mr. Mitsky is quite correct on that.
however, as i once pointed out, a 2X barlow will seemingly 'flatten' things out for those who don't wish to fork-out hundreds of bucks. his response was this only made coma 'less apparent'. well, 'less apparent' satisfies many, and at f/4.5, even a TV parracor, or $300 eyepiece seems not 'cost-effective'.

my 1st 'rich-field' was an Optical Craftsmen 6" f/4---coma galore---but i got used to it. vision and the mind are not understood. we adapt. the konigs, even old erfles are great bargains, as he noted, and fast 10" newts? you learn to live with their probs, in most cases. and still enjoy.

[Dave Mitsky]

Originally posted by vet22@Apr 20 2005, 11:38 PM
you know, a lot depends on how long you've used a fast newt---i prefer f/5.6-6---anything faster? yeah, the coma really goes up. Mr. Mitsky is quite correct on that.
however, as i once pointed out, a 2X barlow will seemingly 'flatten' things out for those who don't wish to fork-out hundreds of bucks. his response was this only made coma 'less apparent'. well, 'less apparent' satisfies many, and at f/4.5, even a TV parracor, or $300 eyepiece seems not 'cost-effective'.

Edit
Actually, I said quite a bit more than that, sir. Allow me to make two points perfectly clear: 1) Barlowing does not physically ameliorate coma (although it can possibly result in showing only the "sweet spot" portion of the mirror where coma is not evident) and 2) coma is more noticeable when a low power, wide-field eyepiece is used; Barlowing such an eyepiece will defeat the goal of attaining a wide true field of view. This is not merely my opinion but a consequence of optical physics.

From http://www.lumicon.com/cc.htm (Visual Performance):

The Lumicon Coma Corrector would have minimal performance gain at high visual magnifications with well-corrected, modern design eyepieces. However, the Lumicon Coma Corrector will certainly benefit and improve medium and low power observing with any 20mm to 56mm eyepiece. Wide apparent field eyepieces like the TeleVue Naglers and Panoptics are recommended for use with the Lumicon Coma Corrector. You will need a 2” diameter, 3” long extension tube and/or a 2”-1.25” reducer bushing to use both 2” and 1.25” eyepieces.

For more on coma correctors see the following URLs:

http://www.cloudynights.com/reviews3/GSO.htm

http://www.astronomics.com/main/Coma_Corre...91SQQKE1/Page/1 (http://www.astronomics.com/main/Coma_Correctors.asp/catalog_name/Astronomics/category_name/T6C0K0CAFKES8M95XW91SQQKE1/Page/1)

http://binoculars.com/products/Vixen_Coma_...00SS_27431.html (http://binoculars.com/products/Vixen_Coma_Corrector_R200SS_27431.html)

http://www.alpineastro.com/optical_accesso...accessories.htm (http://www.alpineastro.com/optical_accessories/optical_accessories.htm)

I don't care how much or how little someone spends on telescopes, eyepieces, and accesories but I do care about presenting the facts accurately to the amateur astronomy community.

Here is my response to vet22's criticism in the post he mentioned:

I believe that I answered the OP's question regarding appropriate eyepieces as it was stated. For those of you who are unaware, Barlowing an eyepiece does not eliminate the coma from a fast mirror, it only makes it less apparent since the resultant true field of view is smaller and may fit within the so-called "sweet spot". Using a Parracor does eliminate coma but that means spending a healthy sum. Whether this expense is "worth it" depends largely on individual tastes.

From the FAQ about Collimating a Newtonian telescope web page:

"Any good eyepiece gives a very sharp view near its focus - that is, in the center of the field of view. Towards the edge, however, all eyepieces cause more or less unsharpness of star images. This is mainly due to astigmatism (I won’t explain that here) of the eyepiece - it is not the mirror’s fault - but shows up worse with a short focus mirror (with a low focal ratio). For most eyepieces, the coma of the primary mirror gives much less contribution to the unsharpness.

Edit

The "sweet spot" in the focal plane around the optical axis, where coma has limited effect even at high magnifications, could be surprisingly small. Also surprisingly, a large telescope mirror does not have a larger "sweet spot" than a small mirror - the diameter is a only a function of the focal ratio of the primary mirror, not its size. This table gives the diameter of the "sweet spot" where coma is less than 1/14 wavelengths RMS, and the Strehl ratio is lowered by no more than 0.2 (this corresponds roughly to the rather obsolete Rayleigh "quarter-wave" criterion of diffraction limiting).

Focal ratio Diameter (mm)
f/4 1.4
f/4.5 2.0
f/5 2.8
f/6 4.8
f/8 11
f/10 22

At the edge of this "sweet spot", coma may be noticeable in good atmospheric seeing, but within a circle of half this diameter, it would be very difficult ever to detect it (and the requirements for "good" seeing are stricter, the larger the mirror is). The small size of the "sweet spot" is of course one price you must pay for the convenience of a telescope with a large mirror, yet short, manageable tube, but it may not be as serious a drawback as it appears. In a "fast" f/4.5 telescope with high magnification, giving an exit pupil of 0.5 mm (this is 50x per inch of aperture, a reasonable upper limit at least for small telescopes), an eyepiece of "standard" field (some 50 deg apparent) has a true field of approximately 2 mm, the same size as the "sweet spot". But obviously, with short focus telescopes (low f/ numbers), centering the "sweet spot" within the eyepiece field of view is very much more critical than with the smaller, "slower" telescopes that were common before the "Dobsonian revolution". The "eyeballing" collimation, even today often found in manuals, may be good enough for a 6 in. f/8 telescope and yet fail badly when applied to a modern, large and fast instrument."

Dave Mitsky

[vet]

Originally posted by Dave Mitsky+Apr 21 2005, 02:04 AM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Dave Mitsky @ Apr 21 2005, 02:04 AM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-vet22@Apr 20 2005, 11:38 PM
you know, a lot depends on how long you've used a fast newt---i prefer f/5.6-6---anything faster? yeah, the coma really goes up. Mr. Mitsky is quite correct on that.
however, as i once pointed out, a 2X barlow will seemingly 'flatten' things out for those who don't wish to fork-out hundreds of bucks. his response was this only made coma 'less apparent'. well, 'less apparent' satisfies many, and at f/4.5, even a TV parracor, or $300 eyepiece seems not 'cost-effective'.

Edit
Actually, I said quite a bit more than that, sir. Allow me to make two points perfectly clear: 1) Barlowing does not physically ameliorate coma (although it can possibly result in showing only the "sweet spot" portion of the mirror where coma is not evident) and 2) coma is more noticeable when a low power, wide-field eyepiece is used; Barlowing such an eyepiece will defeat the goal of attaining a wide true field of view. This is not merely my opinion but a consequence of optical physics.

From http://www.lumicon.com/cc.htm (Visual Performance):

The Lumicon Coma Corrector would have minimal performance gain at high visual magnifications with well-corrected, modern design eyepieces. However, the Lumicon Coma Corrector will certainly benefit and improve medium and low power observing with any 20mm to 56mm eyepiece. Wide apparent field eyepieces like the TeleVue Naglers and Panoptics are recommended for use with the Lumicon Coma Corrector. You will need a 2” diameter, 3” long extension tube and/or a 2”-1.25” reducer bushing to use both 2” and 1.25” eyepieces.

For more on coma correctors see the following URLs:

http://www.cloudynights.com/reviews3/GSO.htm

http://www.astronomics.com/main/Coma_Corre...91SQQKE1/Page/1 (http://www.astronomics.com/main/Coma_Correctors.asp/catalog_name/Astronomics/category_name/T6C0K0CAFKES8M95XW91SQQKE1/Page/1)

http://binoculars.com/products/Vixen_Coma_...00SS_27431.html (http://binoculars.com/products/Vixen_Coma_Corrector_R200SS_27431.html)

http://www.alpineastro.com/optical_accesso...accessories.htm (http://www.alpineastro.com/optical_accessories/optical_accessories.htm)

I don't care how much or how little someone spends on telescopes, eyepieces, and accesories but I do care about presenting the facts accurately to the amateur astronomy community.

Here is my response to vet22's criticism in the post he mentioned:

I believe that I answered the OP's question regarding appropriate eyepieces as it was stated. For those of you who are unaware, Barlowing an eyepiece does not eliminate the coma from a fast mirror, it only makes it less apparent since the resultant true field of view is smaller and may fit within the so-called "sweet spot". Using a Parracor does eliminate coma but that means spending a healthy sum. Whether this expense is "worth it" depends largely on individual tastes.

From the FAQ about Collimating a Newtonian telescope web page:

"Any good eyepiece gives a very sharp view near its focus - that is, in the center of the field of view. Towards the edge, however, all eyepieces cause more or less unsharpness of star images. This is mainly due to astigmatism (I won’t explain that here) of the eyepiece - it is not the mirror’s fault - but shows up worse with a short focus mirror (with a low focal ratio). For most eyepieces, the coma of the primary mirror gives much less contribution to the unsharpness.

Edit

The "sweet spot" in the focal plane around the optical axis, where coma has limited effect even at high magnifications, could be surprisingly small. Also surprisingly, a large telescope mirror does not have a larger "sweet spot" than a small mirror - the diameter is a only a function of the focal ratio of the primary mirror, not its size. This table gives the diameter of the "sweet spot" where coma is less than 1/14 wavelengths RMS, and the Strehl ratio is lowered by no more than 0.2 (this corresponds roughly to the rather obsolete Rayleigh "quarter-wave" criterion of diffraction limiting).

Focal ratio Diameter (mm)
f/4 1.4
f/4.5 2.0
f/5 2.8
f/6 4.8
f/8 11
f/10 22

At the edge of this "sweet spot", coma may be noticeable in good atmospheric seeing, but within a circle of half this diameter, it would be very difficult ever to detect it (and the requirements for "good" seeing are stricter, the larger the mirror is). The small size of the "sweet spot" is of course one price you must pay for the convenience of a telescope with a large mirror, yet short, manageable tube, but it may not be as serious a drawback as it appears. In a "fast" f/4.5 telescope with high magnification, giving an exit pupil of 0.5 mm (this is 50x per inch of aperture, a reasonable upper limit at least for small telescopes), an eyepiece of "standard" field (some 50 deg apparent) has a true field of approximately 2 mm, the same size as the "sweet spot". But obviously, with short focus telescopes (low f/ numbers), centering the "sweet spot" within the eyepiece field of view is very much more critical than with the smaller, "slower" telescopes that were common before the "Dobsonian revolution". The "eyeballing" collimation, even today often found in manuals, may be good enough for a 6 in. f/8 telescope and yet fail badly when applied to a modern, large and fast instrument."

Dave Mitsky [/b][/quote]
i see no contradictions, and pointed out coma in fast newts may be 'liveable'---i certainly never suggested 'barlowing' would be of use as a 'low-power' tool. most don't have 'unlimited-budgets', so, to me there is always a point of 'diminishing returns'. the 2004 review of the f/5 hardin mentioned the coma with its 32mm wide-field, but that did nothing to lower the reviewer's wide-field pleasure, or enthusiasm.

this world is full of people with more $$$ than time---so naglers, pentaxes, takahashies, etc., were made---there's a market---but priced well beyond a newbie's need. as most old 'star-hoppers' know, a blurred M-57, at the edge of field, is still seen, and you move the 'scope. you get used to it. to me the problem below f/5.6 does not = the cost of expensive gear---often more than a decent telescope. we all have our opinions. any 'truths' are arrived at by each user, as their need requires.