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80mm Triplet with Perfect Edge Stars... possible?

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#1 Pingster

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Posted 02 October 2012 - 07:25 PM

Hi,

Im new to APO triplets. I recently bought this scope:
TS PHOTOLINE 80/480mm F/6 Triplet APO

Being an APO triplet, i assumed it would have perfect edge stars and the image would be completely false colour free. But the edge stars are pretty bad... with brighter ones looking like double stars! And the CA is pretty bad... i see a lot of blue on flared stars.

Why is this?

Can users recommend a triplet between 80mm and 100mm that will give me full FOV perfect stars with minimal CA with my 31nagler & 31 Ethos. My budge would be £1000GBP.

Please only recommend something you own and gives the deserved results and not "i think this one will be good".

Also can i be recommended a finder scope between 60mm to 80mm that again gives perfect stars across the FOV.

Sorry if i seem picky, but 1yr into this hobby... i hate smeared stars and false colour.

Thanks for your time,

Pingster

#2 gustavo_sanchez

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Posted 02 October 2012 - 09:12 PM

I think a triplet should not have any noticeable CA on the stars... maybe you got a lemon.

#3 Pingster

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Posted 02 October 2012 - 09:41 PM

Its an F5. (edit opps it is F6!) So i assums that causes the CA?

#4 orlyandico

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Posted 02 October 2012 - 10:41 PM

i thought it was an F/6

An F/6 refractor of any type will have field curvature, i.e. stars at the edge will not be in focus when stars at the center are in focus. Perhaps you are mistaking that for CA?

#5 orion69

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Posted 03 October 2012 - 01:18 AM

There is no refractor without flattener that has perfect edge stars, especially f/6.

Knez

#6 vi100

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Posted 03 October 2012 - 01:55 AM

Hello,

Take a look at TSAPO65Q. May be a dimmer aperture, but perfect stars and no CA edge to edge. Visit my blog to see images. http://buscandocometas.blogspot.com

Vicente.

#7 cloud_cover

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Posted 03 October 2012 - 02:05 AM

I have to disagree. The author is using the scope visually, not photographically. With the Nagler and Ethos, he should be getting good stats even at the edge. Something is not right if a triplet APO is giving lots of false color at 16x and 36x respectively.
On the other hand, the Televue eyepieces are known to have a bright blue ring right at the edge of the field stop. You can see this best using them in daylight. Could the problem lie in the eyepieces themselves? :)
If you want a truly flat, color free scope, try the NP101. Used it costs starting from USD2000, which is 1250 GBP, approximately.

#8 orion69

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Posted 03 October 2012 - 03:29 AM

Hello,

Take a look at TSAPO65Q. May be a dimmer aperture, but perfect stars and no CA edge to edge. Visit my blog to see images. http://buscandocometas.blogspot.com

Vicente.


TSAPO65Q has integrated flattener, it's Quadruplet Astrograph... ;)

Knez

#9 Binojunky

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Posted 03 October 2012 - 05:17 AM

Regarding colour, maybe a bad eyepiece,or even the diagonal could be causing the problem,DA.

#10 chboss

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Posted 03 October 2012 - 05:27 AM

Good point, visually with the eyepieces he mentions there should be no visible aberrations...
Photographically you need to use a flattner, there is no question.
I would recommend meeting with an experienced observer to exchange views in different telescopes and eyepieces to find out where the problem really lies...

If it is the optic of the refractor, a star test should show if there is a problem.

best regards
Chris

#11 Jon Isaacs

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Posted 03 October 2012 - 05:36 AM

I have to disagree. The author is using the scope visually, not photographically. With the Nagler and Ethos, he should be getting good stats even at the edge. Something is not right if a triplet APO is giving lots of false color at 16x and 36x respectively.



Lateral color is pretty typical in an eyepiece but in a 80mm F/6, the field curvature will be noticeable with an eyepiece like the 31mm Nagler, I see it in my 80mm F/7 FD.

If one wants perfect stars right up to the edge, the NP-101 is certainly a reasonable choice..

Jon

#12 Pingster

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Posted 03 October 2012 - 07:50 AM

I am purely visual only atm. Arh, so the bad edge performance is refered to as field curvature, when the inside and outside stars focus point is different. Thats exactly what i am getting with my 31nagler and 17 ethos. I tried focusing on the edge stars, then hoped my eyes would adjust for the middle. But it seems my ageing eyes at 37yrs are not up to the job.

So the NP-101 will resolve field curvature. Looks like the scope for me. Ill see if i can find a used unit her in UK.

Another question, the quadruplet, so they flatten the image even for visual use as well? You use it visually like any normal triplet and can swap eyepieces without having to mess with eyepiece distance spacers?

#13 gustavo_sanchez

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Posted 03 October 2012 - 10:52 AM

Its an F5. So i assums that causes the CA?


In your original post (at the top of the thread you said that you bought a F/6 80/480 scope... now you say is a F5... Are you sure you know which scope you got? I don't know any 80mm F/5 triplets... but I know some 80mm F/5 achromats...

#14 Jared

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Posted 03 October 2012 - 11:15 AM

Basically, all refractors (except those with integrated flatteners) have field curvature that is proportional to the focal length. The radius of curvature is generally around 1/3 of the focal length of the scope. So the shorter the focal length, the worse the field curvature. It's not a quality issue--just a simple fact. Obviously, a small, fast scope will have much worse field curvature than a larger or slower scope.

The false color you are seeing is likely lateral color introduced by the eyepiece, not by the telescope itself. You can find an explanation/definition here:

http://spie.org/x32874.xml

Most quadruplet designs such as the Tak FSQ series or the TeleVue NP series (short for Nagler Pezval) use the additional element(s) to reduce field curvature. As a result, your stars would not show astigmatism at the edge of the field (the double star look). They are still usable visually, though some consider them overkill.

It is normal, by the way, for the eye to lose some of its ability to accommodate as we age. No matter what your age, though, matching up an 480mm focal length refractor with a 31mm Nagler is going to be a tall order without a flattener of some kind.

One other option you could consider if you can't afford (or don't want to afford) a used NP-101 or Tak FSQ is to look for a refractor with a longer focal length. A used Televue 102, for example, would have a much flatter field than your 80mm simply because it has a longer focal length. It's also no harder to mount and use than an NP-101. You'd get much better edge performance (perhaps even good enough to make you happy with that 31mm Nagler) without the expense of an astrograph. The trade off would be very slight on-axis color on really bright objects such as the limb of the Moon, Venus, etc.. Very few observers find it objectionable, though. Nothing wrong with an NP-101 visually, though... This is just a way you could save a little money and still get top quality.

By the way, the only advantage to a triplet is better color correction across a wider range of wavelengths at a given focal ratio. It's extremely important for astrophotography. It does not necessarily result in higher quality or correction for any aberrations other than chromatic aberration. It certainly doesn't affect field curvature.

#15 SteveG

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Posted 03 October 2012 - 11:27 AM

Here is a new scope that has a built in flattener you might be interested in:
http://www.williamop...1_features.php#

#16 Pingster

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Posted 03 October 2012 - 12:06 PM

Thanks for all your replies. Each very helpful. You are indeed right the scope is an F6. I confused it for my dob F ratio.

Today during the day i took it out and found some flat surfaces to try it on. Each test was done using a 31nagler.

1. Place the nagler direcly into the focuser, straight thru view. Focused the middle, then looked at the edges. Very good. I am suprised te entire field FOV looks to be very sharp. Only the extreme edges vastly differ in focus (95% outwards).

2. Put in my new Baader Erect diaginal, focus. View a touh less sharp, but again the field is generally all in good focus. The edges require just a touch of fine adjustment to be perfect, then my eyes easily adjust for the middle. Very good.

3. Then i put in my 2yr old Williams Optics Dieletric mirror diaginal, the one i used when starting this tread. Focus middle, very nice sharpness.... But to my complete suprise... The edges are terrible! More colour and very out of focus! This totally suprised me. I did not expect this result. A erect diaginal having better edge than a mirror. Infact i never ever reade of a mirror diaginal adding field curavture!

I shall do the same tests tonight if the skies are clear.

#17 Eddgie

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Posted 03 October 2012 - 12:53 PM

Field curvature is almost guarenteed to be the issue with this scope when using modern wide field eyepeices.

Daytime testing may show the 31mm Nagler to look much sharper than another eyepecie, but the 31mm Nagler may still not present pinpoint stars at the edge of the field at night.

A point source is one of the most difficult targets you can use when checking for off axis abberations. It is without mercy.

So, don't be surprised if you still can't get the view to be pinpoints all the way across at night.

There is also this. Someone not so long ago in this exact same forum was concerned that his new fast APO was not giving very sharp images. These fast scopes, when used with eyepeices like a 31mm Nagler, will start to reveal astigmatism in the observer's eye for most observers. It becomes very difficult to keep the field sharp because so much of your eye pupil is being utilized that any abberations in your eye lens will start to show. When the exit pupil is smaller than about 2 or 3 millimeters, even observers with astigmatism may not need their glasses, but when the exit pupil is 6mm or 7mm, even people that don't have astigmatism bad enough to require glasses may start to see stars less sharply.

I could not even keep the entire field of a 102mm f/9 ED scope all in focus when using a 31mm Nagler. Nor could I keep the entire field in a Televue 101 perfectly focused. Part of it was my eye, part of it was field curvature from the scopes.

For the same size true field, it is easier to keep the field all in focus by using a 68 degree afov eyepiece. For example, a 22mm Nagler and a 27mm Pan give about the same size true field, but if you are having trouble keeping the entire field in focus, it will be easier to do with the 27mm Pan. So, for the same size true field of view, a longer focal lenght eyepiece is the best way to reduce the effects of field curvature of the scope. Of course you give up some of the "Spacewalk" offered by the bigger apparent field of the Nagler, so you have to ask yourself which is more important, the wider apparent field, or a field that is more pinpoint all the way across..

I love pinpoint fields. I personally would rather use a Panoptic given the same size true field than a Nagler so I can keep the field as perfectly presented as possible. That to me gives more of a spacewalk experience than looking at a field that is populated with lots of abberated stars.

We each make our choices though, and many people only care about the center of the field.

You appear to be growing more discerning with respect to off axis performacne, and you may indeed find that if you want both Nagler/Ethos views and pinpoint stars to the edge, you may need to go to a different scope, or ad a flattner to your current scope.

#18 Jared

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Posted 03 October 2012 - 02:54 PM

I've got no explanation for your follow-on test. I don't see how a flat mirror can cause field curvature--even a defective mirror. Spherical error, sure, but field curvature and astigmatism? Doesn't make sense.

#19 GlennLeDrew

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Posted 03 October 2012 - 05:16 PM

I'm also baffled by the reported worsening of field curvature when a decent mirror diagonal is installed. One thing which raises flags for me is if the testing is done under conditions of varying brightness and/or differing exit pupil diameter. For one thing, a 31mm eyepiece on an f/6 telescope in bright daylight is going to yield a better looking result due to the smaller eye pupil stopping down the system. If we take the observer's pupil as 2mm, the actual effective aperture of the scope is 80 * (2 / 5.17) = 31mm @ f/15! The resulting increase in depth of field will significantly lessen visible de-focus due to field curvature. And other aberrations are lessened as well.

Do these tests when your pupil is at least close in diameter to that of the exit pupil being used.

#20 dedo

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Posted 11 October 2012 - 01:04 PM

Is the WO dielectric 1,25"? Most dielectric diagonals have issues at their edges, and generally a 2" is preferred when dielectric. A normal prism (not an erecting one) delivers more often better results on refractors. Again, given the fact that a refractor has a closed tube, a normal, quartz diagonal may be preferable as it more likely doesn't have issues at the edges.

More here:
http://geogdata.csun...dielectric.html

By the way, I own the same scope and use it with a Nagler 3-6mm and a Swarovsky 7.7-23.1mm with....a 1,25" dielectric WO diagonal, but as for my eyepieces I don't search very large fields...With those EP I find the field curvature more than acceptable but have a FOV very small compared to what the 31mm gives to you. Colour correction is excellent on very bright stars.

#21 greju

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Posted 12 October 2012 - 02:51 PM

I think this meets all of the OP's criteria.

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#22 greju

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Posted 12 October 2012 - 02:53 PM

Oh yeh, and it's a Bushnell. :grin:

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#23 sg6

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Posted 12 October 2012 - 03:45 PM

At that short a focal length you will get curvature and even a triplet has it's limits. Simply you cannot go making them shorter and shorter without something running beyond it's capabilities.

All that the new glass types do is make that limit a bit lower down they do not eliminate it.

The lens have spherical faces and they are not ideal the tighter they get the more errors you get.

Also sure I have read of that scope and a few problems were reported. Someone here got it for imaging and returned it eventually.

For visual I would always look for f/7, make life easy don't push the components, if you are imaging then you will need a flattener almost immaterial of the scope.

#24 Pingster

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Posted 14 October 2012 - 12:31 PM

Hey guys thanks for the addition feedback. I have a
2in WO diaginal and so is the coorect image baader. At night both diaginals give approx the same amount of smearex edge stars. But oddly duing the day the baader with no doubt has the flatter field. Very odd and suprising.

As a few members have stated... Fast APOs all suffer field curvature. Its not avoidable.

I contacted a very popular telescope website in the UK, i asked for "a 60mm to 80mm refractor that can produce a perfectly sharp FOV using 31nagler and 17ethos, budget being 1600USD (1000GBP)".

I was told they have nothing in theor range that would be suitable, that really suprised me. They recommended another site.... Who gave me the same reply an they recommended another site. Againntheu said nothing in their range! I was very suprised. Is it truely that difficilt to make a refra tor for 1600usd that has a perfectly sharp FOV. Unbelieveable.

I find this extremely odd. As i have a pair of cannon 15x50is bins. They have a perfectly sharp fov to the edge! And no false colour. So binoculars companies can give you somthing that provides two barrels with edge shape stars and no colour for $2000usd. Yet it seems we cant get one perfect tube for 1600.

The only scopes that seem to provide a perfect fov are in the 5k region... Ie TV NP1-01

#25 Eddgie

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Posted 14 October 2012 - 04:56 PM

The only scopes that seem to provide a perfect fov are in the 5k region... Ie TV NP1-01


Well, if you are including all scopes in the word scopes then the good news is that you can get a Celestron EdgeHD 8" for less.

I have one, and it gives pinpoint images to the field stop when using a 31mm Nagler.

But if you are limiting yourself to refractors, depending on visual accomodation, it may be very difficult. A bigger, slower scope will make the field curvature less a factor.

But I have an EdgeHD 8", and for the same size true field, it is as sharp as my 6" APO. Of course I can get a 2 degree true field in the 6" APO, but only a 1 degree field in the EdgeHD 8". But a stunningly good 1 degree field.

So, now you have to ask yourself the hard questions... What do you value in a view.

For a decade, my message to this forum has been that telescopes can differ far more in their off axis behavior than in their center of field performance.

And the advice that I would have would be for anyone considering a major purchase would be to try to famaliarize themselves with the off axis behavior of the telescope and how it will behave with different eyepeices.

And there is another important tip.. Another message that have been repeating many years..... The effects of field curvature are reduced as the focal length of the eyepeice gets longer.

If you change a 31mm Nagler for a 35mm Panoptic, the field will only be very slightly smaller in the Panoptic, but the lower magnification will mitigate some of the field curvature.

A 41mm Pan will have a slighly larger true field, than the 31mm Nagler, but once again, because of the much lower magnification, the effects of field curvature from the scope will be greatly mitigated.

I can't use a 31mm Nagler in my C14 because I can't accomdate the field curvature of the scope.

But if I plug in the 41mm Pan, I can almost completly accomdate the field curvature of the scope.

For the same size true field, the longer the focal lenght of the eyepeice, the less visible the field curvature of the scope will be to the observer.

I read this in a book (Telescope Optics). And guess what.. It really works that way...






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