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Equipment Discussions >> Refractors

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Pingster
member


Reged: 06/27/11

80mm Triplet with Perfect Edge Stars... possible?
      #5452597 - 10/02/12 08:25 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


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gustavo_sanchez
sage


Reged: 12/30/10

Loc: Puerto Rico, US
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5452758 - 10/02/12 10:12 PM

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

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Pingster
member


Reged: 06/27/11

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: gustavo_sanchez]
      #5452800 - 10/02/12 10:41 PM

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

Edited by Pingster (10/03/12 12:48 PM)


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orlyandico
Postmaster
*****

Reged: 08/10/09

Loc: Singapore
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5452894 - 10/02/12 11:41 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?


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: orlyandico]
      #5453017 - 10/03/12 02:18 AM

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

Knez


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vi100
sage


Reged: 10/15/04

Loc: Santa Pola (Spain)
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: orion69]
      #5453029 - 10/03/12 02:55 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.


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cloud_cover
professor emeritus


Reged: 08/17/10

Loc: Restaurant at the End of the U...
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: vi100]
      #5453034 - 10/03/12 03:05 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.


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: vi100]
      #5453061 - 10/03/12 04:29 AM

Quote:

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


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Binojunky
Carpal Tunnel
*****

Reged: 12/25/10

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: orion69]
      #5453095 - 10/03/12 06:17 AM

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

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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Binojunky]
      #5453100 - 10/03/12 06:27 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


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: cloud_cover]
      #5453107 - 10/03/12 06:36 AM

Quote:


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


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Pingster
member


Reged: 06/27/11

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Jon Isaacs]
      #5453217 - 10/03/12 08:50 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?


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gustavo_sanchez
sage


Reged: 12/30/10

Loc: Puerto Rico, US
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5453454 - 10/03/12 11:52 AM

Quote:

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...


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Jared
Postmaster
*****

Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5453476 - 10/03/12 12:15 PM

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.


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SteveG
Post Laureate
*****

Reged: 09/27/06

Loc: Seattle, WA
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Jared]
      #5453485 - 10/03/12 12:27 PM

Here is a new scope that has a built in flattener you might be interested in:
http://www.williamoptics.com/telescopes/gtf81_features.php#


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Pingster
member


Reged: 06/27/11

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: SteveG]
      #5453520 - 10/03/12 01:06 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.


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Eddgie
Postmaster
*****

Reged: 02/01/06

Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5453580 - 10/03/12 01:53 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.


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Jared
Postmaster
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Pingster]
      #5453768 - 10/03/12 03:54 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.

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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: Jared]
      #5453952 - 10/03/12 06:16 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.


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dedo
super member


Reged: 06/04/10

Loc: Italy, Rome
Re: 80mm Triplet with Perfect Edge Stars... possible? new [Re: GlennLeDrew]
      #5465434 - 10/11/12 02:04 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.edu/~voltaire/roland/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.


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