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Jeff Morgan
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Re: CN Report: The Tele Vue NP101 new [Re: starpal]
      #2174353 - 02/07/08 11:21 AM

Quote:

Very interesting since my life long experiece with long f/ratio to short seemed to indicate otherwise viewing the planets.




All things considered, I still prefer longer ratios myself.


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Tom TAdministrator

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Re: CN Report: The Tele Vue NP101 new [Re: Jeff Morgan]
      #2174494 - 02/07/08 12:42 PM

That was one of the main reasons I held off on trying the NP101 myself for such a long time.

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peleuba
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Reged: 12/01/04

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Re: CN Report: The Tele Vue NP101 new [Re: Tom T]
      #2174734 - 02/07/08 02:37 PM Attachment (114 downloads)

Quote:


But after using the NP for a year I have to admit that Paul was absolutely correct in this and I was wrong. The NP is a different beast than the TV101 or the Genesis SDF, and it really is a step up from the TV102.

Tom T.





Hey Tom -

Really a superb job - as always. I could not agree with you more (tee-hee ).

I have really liked the NP101's that I have owned. The NP101 while premium priced, really does represent a near perfect package. When you combine build quality/ruggedness, optics and portabalility I cannot think of another telescope that comes close. Sure, there are others that excel in one area or another, but as a complete package, its alwfully tough to beat the NP101.

Thanks for mentioning our previous conversations on the TeleVue Yahoo group. Those debates were a lot of fun.

Looking forward to finally meeting you in person later this Summer.

Best Regards.


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peleuba
scholastic sledgehammer


Reged: 12/01/04

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Re: CN Report: The Tele Vue NP101 new [Re: Dirtyharry]
      #2174836 - 02/07/08 03:19 PM

Quote:



I confirmed this with a simple but all-telling star test, where I once compared a TV 101 with a TV102. The 102 won hands down and showed a level of SA correction < 1/8th wave ptv.







Harry,

I am sure Tom knows about Abberator...

I am quite comfortable using Abberator and make use of it regularly in my testing... But with all due respect, how fluent are you in performing the star test? Did you make use of colored filters to isolate the wave lengths of light - specifically Wratten 56, 58 and 8? If not, I will tell you its quite difficult to assign a wave front rating - as you have previously done - via the star test. Especially if your not isolating indidual wavelengths. Spherochromatic aberation will be the biggest issue and it will, most certainly, obscure the Fresnal pattern your trying to evaluate. And, I have not mentioned the presense of zones, edge issues, inhomogenity in the glass etc. All will have a detrimental effect and all are unrelated to Spherical Aberation.

The bottom line is that star testing is easy to perform but difficult to interprate even for folks who have a lot of experience. I never, ever, place final judgment on a lens unless I test it indoors, outside and then let an optical genius friend put it through its paces for a couple of nights.

I have owned a TV102 and three NP101's and have been able to evaluate each on a fairly sophisticated optical bench as well as in the field. My TV102 was perhaps 1/6 wave in GREEN light but it was the Spherochromatic abberation that drove me nuts. I would like to know more about the tests you performed on the 101 you had access to .

All three samples of the NP101 I owned had good Spherical corretion - one was nearly perfect in green light. The other two were very good. My biggest objection was that one sample had a very small central zone which wreaked havoc on the star test but was not visble in-focus.

If you could let us know specifics to your testing it would be interesting. I have found that performing a 5 minute star test one night on a scope then, the next day, trying to match it up to Abberator is not really valid in assigning wave front values. Too many variables (other then sperical correction) need to be accounted for and evaluated.

Edited by peleuba (02/07/08 03:49 PM)


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Dirtyharry
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Reged: 11/01/07

Re: CN Report: The Tele Vue NP101 new [Re: peleuba]
      #2175043 - 02/07/08 04:35 PM

Hello Paul,

I'm not an expert on star testing and I agree that star testing is a complex subject, as Suiter's great book exemplifies. That said, I did use a green filter (over a few nights) to reduce the wavelength range and this showed the diffraction rings better both inside and outside focus. It worked much better on the TV 101 than on the TV 102 as I recall. But in essence, the star test is easy to judge. In a word, what you're looking for is perfect symmetry at equidistant points inside and outside focus. An even better test, IMO, is through observing planets under calm seeing conditions. In this respect we both agreed that the view of Jupiter with my buddy's 101 had a slightly softer appearance than that seen through the 102. You see, you don't have to be an optical genius to spot a real difference in performance!
1/6th wave sounds a bit low for some of the literature I've seen on the 102. Less good sample maybe?

Harry.

Harry


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peleuba
scholastic sledgehammer


Reged: 12/01/04

Loc: Southern PA
Re: CN Report: The Tele Vue NP101 new [Re: Dirtyharry]
      #2175159 - 02/07/08 05:35 PM

Quote:


I'm not an expert on star testing and I agree that star testing is a complex subject, as Suiter's great book exemplifies.





Harry,

I have found Dick Suiter's book to be a wonderful reference especially as it pertains to CATS and Newts. But, IMO it is not the end-all, be-all for APOchromatic refractors...

It makes little mention of Spherochromatism; which is the level of spherical aberration present at a given (single) wave length of light. Spherochromatism makes the star test quite difficult to interpret. It usually manifests itself as making one side of the diffraction pattern "less contrasty" then the other side. In an APO, you can have seemingly different Fresnal patterns on either side of focus yet have a lens with a very high Strehl. I have seen this myself. But, don't take my word for it - go search the literature... Start with reading Roland Christen's essays on the subject.


Quote:


1/6th wave sounds a bit low for some of the literature I've seen on the 102. Less good sample maybe?





I dunno - could be. TeleVue does not publish wavefront ratings or Strehls, so I do not know for sure. I do know that the NP101's I have owned were better absolute performers in every sense. Please note this does not take into account cost. I am just looking at performance on the bench and under the night sky.

Edited by peleuba (02/08/08 11:11 AM)


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JoeBftsplk
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Re: CN Report: The Tele Vue NP101 new [Re: peleuba]
      #2175356 - 02/07/08 07:10 PM

Peleuba,
Your comments on Spherochromatism are interesting. What does it translate to in actual practice while observing?

The reason I ask is that my TV102 sometimes "drives me nuts". I often find that I'm hunting for a good focus on bright stellar objects.

I thought it was astigmatism and asked Televue to check the scope out. They said there was no problem with the scope.

My FS78 doesn't bug me like this, so I don't think it's an issue with my vision. I'm not an expert on star testing and don't wish to become one; but am wondering if this defect you've mentioned could be the source.
Bob


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starpal
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Reged: 08/06/04

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Re: CN Report: The Tele Vue NP101 new [Re: Jon Isaacs]
      #2175931 - 02/07/08 11:08 PM

Quote:



I do believe that a quality rack and pinion is more expensive to manufacture but really that is not the point here, that is really off topic.




Who is to know without looking up some useful price information to make for helpful comparisons? At this point each of us would be operating in essence from opinion. I did try to find something quickly helpful regarding costs of precision racks and pinion gears, last night, but all the producers found stated quotes on request. I can tell you this, I have previously set out to buy precision rack and pinions in industry before and they were not that expensive. As I stated, that being the only moving part to the focuser design itself tells me it is a more cost effective design focuser to use. One that has a leverage advantage as other folks pointed out earlier, but there doesn't seem to be anything more than that that I'm aware of. When I spoke of the NP101's using a R&P as a cost effective measure, who is to say this isn't also true in real life?

Quote:

The point here is that the 101NP uses a Rack and Pinion and you commented that Crayfords were better and this was in indication of TeleVues lack of commitment to quality.




No sir, I never did that. You would unfortunately be reading into this something fully incorrect. I spoke ONLY of cost effectiveness NOT a lack of commitment to quality regarding focuser types. A matter that is indeed on topic, as well, since we are discussing the scope in question. Tom Trusock, the writer of the article clearly pointed out that cost was a part of the equation.

Quote:

Let's not forget that fact about a company keeping a product as cost effective as possible. A question might be why for example would a rack and pinion focuser be used when a Crayford is better and now the industry standard?




I wrote the above through the best of my understandings and my question seems to have been answered in that heavier loads, than I was aware of, are required with todays more advanced levels of astrophotography equipment. I, like others, am willing to accept that "belief" as it does appear to be something very reasonable to understand.

However, this cannot also say the R&P is a better design in itself, overall. I had already brought up commonly accepted points to the contrary by correctly pointing out important distinctions the Crayford design possesses. Why is this relevent to the discussion of the topic telescope? It would be because they make for superior focusing action in other useful areas (no backlash, no play, and a finer focusing action). These are worth pointing out! Just as much so as it is for the distinct advantage a R&P might hold. Users (or potential ones) have varied interests in astronomy. They certainly are not all (or to be) advanced astrophotographers!


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starpal
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Re: CN Report: The Tele Vue NP101 new [Re: Tom T]
      #2176060 - 02/08/08 12:08 AM

[post deleted by spaceydee]

Edited by spaceydee (02/11/08 12:32 AM)


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Jon Isaacs
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Re: CN Report: The Tele Vue NP101 new [Re: starpal]
      #2176347 - 02/08/08 06:56 AM

Quote:

However, this cannot also say the R&P is a better design in itself, overall. I had already brought up commonly accepted points to the contrary by correctly pointing out important distinctions the Crayford design possesses.




Remaining ON TOPIC means discussing the NP101 focuser rather than the generic issues of as to whether the Crayford is better than a Rack and Pinion focuser. Clearly that is a personal choice and best discussed in the appropriate forum. There are many ways to skin a cat and the question here is whether or not the NP-101 has an effective focuser.

I think this is the appropriate starting point:


Quote:

All TV scopes use a proven rack and pinion focuser, unique to TeleVue products. Frankly, I think it's one of the best focusers on the market. For telescopes that have a greater depth of field it works wonderfully. On the faster scopes however (like the NP series) a two speed focuser is definitely appreciated. Enter the Focusmate. The Focusmate is an add on fine focuser for, well, nearly any of the telescopes sold by TeleVue in recent history. (This is just one of the things there is to like about TV. When they design their equipment they do it with several goals in mind, including - but not limited to - backwards compatibility, as well as with an eye to future products.)

The Focusmate sports a 6:1 planetary reduction gearing and is easily installed by the end user. While perhaps not as aesthetically pleasing as the a StarLight Instruments FeatherTouch, it was designed to fit with other accessories within Tele Vue’s Imaging System line. The Focusmate works and it works well. It mates up to an optional motor drive which can be controlled by hand or computer. One of the definite pluses to the Tele Vue system is that the motor is quickly and easily disengaged (without tools) if one wishes to focus manually - you simply loosen a thumbscrew and rotate the motor out of the way.




Looks to me like the NP-101 has a high quality focuser that is effective though the gear reduction unit is not as attractive aesthetically as the Feathertouch.

Jon Isaacs


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Doug D.
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Re: CN Report: The Tele Vue NP101 new [Re: Jon Isaacs]
      #2176377 - 02/08/08 07:39 AM

Gee, I even like helical focusers....! It depends on the scope, the application, and the likely demands a user puts upon it. A blanket statement that Crayfords or R&Ps are superior - end of story - is really a bit silly. I have a Moonlite on my Dob and I love it, but pop a 31 Nag in there and it slips easily. Does it mean the Moonlite is junk or that an R&P is "best"? Of course not. They both work fine within their intended specs, set points, price points, and respective performance ranges.

As for the NP101, I guess I agree with Tom that most might find the Focusmate less than attractive. Personally, I kind of like the look. I have Focusmate and driver on both my 76 and NP101. They are a delight to use for high mag work (and webcam imaging). My only "complaint" is that I think they are a little pricey - particularly the rebranded Orion focusmate driver.


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Tom TAdministrator

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Reged: 02/26/02

Re: CN Report: The Tele Vue NP101 *DELETED* new [Re: starpal]
      #2176509 - 02/08/08 09:26 AM

Pete Said
Quote:


What about flatness across FOV?





From http://www.cloudynights.com/item.php?item_id=1749

Quote:


Other characteristics of the design include: improved color correction (in a Petzval, color is reduced from that of a comparable focal ratio doublet by some 30%), and flat fields. As implemented by TeleVue, the NP series allows for stunning widefield views with no field curvature.





Remainder of post currently unavailable as part of a good faith effort to clear up a misunderstanding.

Edited by Tom Trusock (02/10/08 04:19 AM)


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jtpowers
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Re: CN Report: The Tele Vue NP101 new [Re: Tom T]
      #2176585 - 02/08/08 10:11 AM

Hi Tom,

I've heard or read somewhere that shorter focal length (i.e. Tak's Sky90) refractors are more sensitive to seeing/atmospheric variations. Perhaps it's mainly true for short f. ratio doublets...?

In your experience with the TV102 and NP101, does this hold for the f5.4 NP101 vs. the f8.8 TV102? Or is it offset in the NP101 because the front elements are a longer focal length (F12 or something)? Or, is it offset as a function of lens design, element number (as it contributes to lens design), etc...? I've always wondered about this, especially as the seeing here in Boston is typically atrocious.

Thanks!


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Tom TAdministrator

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Reged: 02/26/02

Re: CN Report: The Tele Vue NP101 new [Re: jtpowers]
      #2176708 - 02/08/08 11:17 AM

Hi John,

Jeff Morgan linked to a nice article by Bryan Greer on just this topic:

http://www.fpi-protostar.com/bgreer/seeing.htm

T

Edited by Tom Trusock (02/08/08 01:06 PM)


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peleuba
scholastic sledgehammer


Reged: 12/01/04

Loc: Southern PA
Re: CN Report: The Tele Vue NP101 new [Re: JoeBftsplk]
      #2176861 - 02/08/08 12:29 PM Attachment (84 downloads)

Quote:


Your comments on Spherochromatism are interesting. What does it translate to in actual practice while observing?





Hi Bob,

Spherocromatism will not cause bright star points to become less star like. The effects of spherocromatic aberration can be seen more easily when looking at bright planets or by performing the star test.

One of the strong points of the TV102 is it's relatively deep depth of focus due to the longish focal ratio. If your hunting for "best focus" in the TV102 while your other scopes seem to snap, I think something is wrong. I think your FS78 is around F/8 so it should have a similar depth of focus.

Astigmatism is pretty easy to detect and, if bad enough, will prevent a star from focusing down to a point of light. Rather then a point, the star will appear in as "cross" pattern.

I have done some modeling and have attached a jpeg illustrating astigmatism. The image on the left is that of a star at high magnification through the TV102 with no aberrations - its an essentially perfect view. The middle image depicts the same star but with 1/4 wave of astigmatism. The right most image is the same star but with a half wave of astigmatism. Notice the cross pattern.

Is this what you see?


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Tom TAdministrator

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Reged: 02/26/02

Re: CN Report: The Tele Vue NP101 new [Re: peleuba]
      #2176876 - 02/08/08 12:39 PM

Hi Bob,

I think Paul already mentioned this, but don't forget about the effects of seeing. If it's happening some nights and not others, I suspect that's probably a real good bet.

And as an added clarification about spherochromatism I'd include the fact that it's inherent in the design and really to be expected of any moderate speed doublet. One way (there are others) to correct for spherochromatism is by using extremely long focal lengths and I'm guessing a Petzval falls in here because of the speed of the front doublet.

FWIW, the effects of spherochromatism are mainly seen out of focus and can fool the user into thinking they have a telescope with some spherical aberration. This is why filters are appropriate for use when testing APO's.

TMB has a good paragraph on spherochromatism here:
http://geogdata.csun.edu/~voltaire/tmb/definition.html

To remain on topic, I'd add that like Paul, I too find the NP101 to have the lowest amount of spherochromatism of any apochromatic refractor I've ever looked through.

T


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Tom TAdministrator

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Reged: 02/26/02

Re: CN Report: The Tele Vue NP101 new [Re: peleuba]
      #2176881 - 02/08/08 12:41 PM

Quote:

Quote:


But after using the NP for a year I have to admit that Paul was absolutely correct in this and I was wrong. The NP is a different beast than the TV101 or the Genesis SDF, and it really is a step up from the TV102.

Tom T.





Hey Tom -

Really a superb job - as always. I could not agree with you more (tee-hee ).

I have really liked the NP101's that I have owned. The NP101 while premium priced, really does represent a near perfect package. When you combine build quality/ruggedness, optics and portabalility I cannot think of another telescope that comes close. Sure, there are others that excel in one area or another, but as a complete package, its alwfully tough to beat the NP101.

Thanks for mentioning our previous conversations on the TeleVue Yahoo group. Those debates were a lot of fun.

Looking forward to finally meeting you in person later this Summer.

Best Regards.




Hi Paul,

Me too!

BTW - did you manage to get your hands on that NP101 we discussed this summer?

T


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JoeBftsplk
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Re: CN Report: The Tele Vue NP101 new [Re: peleuba]
      #2177126 - 02/08/08 03:00 PM

Paul & Tom,
Thanks for the responses. I'd guess the third (rightmost) image is closest to what I see. However, to Tom's point, it doesn't seem to be an issue all the time. That, plus the scope and diagonal went back to Televue with the specific complaint and they could find no evidence of astigmatism. Thanks for verifying that spherochromatism is not likely to be the cause. Could simply be conditions like Tom said.

I recently installed a Feathertouch focuser on the TV102. Maybe that will help on difficult nights. Thanks to our NY snowbelt weather I haven't had a chance to try it yet.

Would like to go side-by-side with an NP101 or maybe a TSA102 sometime. Might give me a better feel for how the TV102's performing. The FS78 is almost always a perfect gem, but that may be due to its size more than anything else.
Bob


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Tom TAdministrator

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Reged: 02/26/02

Re: CN Report: The Tele Vue NP101 new [Re: JoeBftsplk]
      #2177141 - 02/08/08 03:06 PM

Hi Bob,

I can't speak to your specific sample, but if if makes you feel any better, I owned a TV102 for many many years, and it was a very very good optic.

As per the FT focuser - if you installed it, one other thing you might want to check is the collimation of the scope. If you somehow managed to miscollimate it while installing, that could contribute to the "finicky" feeling on a scope. (In this case it would most likely be a misalignment of the focuser with the lens. Frankly, I doubt this happened, but it is a possibility I guess.)

T


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naglertized
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Re: CN Report: The Tele Vue NP101 new [Re: Tom T]
      #2177575 - 02/08/08 06:21 PM

Tom,

Does the NP101 impart any color tonality, such as being "warm" or "cold"? Does color tonality differ between the NP series and the other TV APO's such as the 102?

Thanks


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