1951 USAF resolution test chart – Knowledge and References - 1951 usaf resolution test chart

 2024-08-28 12:59:18

I'm not into imaging though, visual observing only but my heaviest eyepiece / diagonal weighs around 3 lbs and the Moonlite copes with that easily enough.

I work in optics, nothing at all to do with graphics, and also know what optical quality is, measure down to 1/10 green wavelength which your graphics gets no where near.

The Starwave is a Semi APO the Build Quality is excellent however the optics are not an Astrograph. I had one of these and was disappointed but only due to it been advertised as an astrograph. As a Semi APO it is very good.

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... Nyquist limit. What actually happens when our input signals cross the Nyquist Limit? Figure 1 depicts the sampling of a sine wave as it rises in frequency ...

Olly Penrice is testing one of these currently and so far has good things to say, also have look through this thread http://stargazerslou...atic-refractor/

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But its also made me think (and I found a thread about exactly this) of what I may be dangling off the end of my scope... I have a Canon 7D and it is rather heavy. The Skywatcher Apo's seem to have a rep for not being able to hold heavy cameras, so may need to spend a lot more to get a rear end that is good enough...

A bad image does bother me!! I would way rather view a crisp view of a cluster that was smaller in the eyepeice than a large ever-so-slightly fuzzy or purple tinged cluster. Just me I'm afraid

I own and use both an Achromatic and an Apochromatic 120mm refractor and when used visually there is a slight difference. However this is only really noticeable on very bright objects like Venus. I am quite happy with the Achromat for visual work - usually the Moon. For imaging I can't tell them apart for white light Solar imaging with a DSLR - but for DSO long exposure imaging they are a world apart!! The APO winning hands down every time.

Long focus achromats don't give the very low power wide views but perform better at medium to high power. In fact at F10 and above can give a lovely sharp image.

Don't get involved in DS imaging with an achromat. The camera reaches deeper into the short violets than does the eye and the different wavelngths have different foual points. SInce focus is everythibng (or nearly!) in DS imaging... it is vital to have an apo.

I was just looking and reading about the Altair you linked to above; I noticed the Starwave ED80 was significantly cheaper. Is that simply because it is a doublet and not triplet lens design? Not igoring you advice in the above post just querying...

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Owing to spherical and chromatic aberration the achro will not produce the quality that the apo will. A 120 will produce both CA and SA. So if quality is paramount then really the apo is required.

I have read the technical diference between these two types of refractor, but do you think the difference would be noticeable to a beginner/untrained eye?

Dont get me wrong its a good piece of glass for the price, but if you want to take it a bit seriously the issues do start to come up.

Achromatic telescopes do not have spherical aberration. If it does, that is a fault in manufacture. Send it back. Chromatic aberration yes, they have that, but the amount depends on focal RATIO, not focal length, so an F/15 will show so little as to be hardly noticeable, in amateur sizes, but at F/5 it will be noticeable. Spherical aberration, Never!

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Excellent comments, thanks folks; again, plenty of food for thought but I get the sense that particularly for a beginners scope, a good acromatic may be best... I'll look through both if I can.

An optical microscope is used with multiple objectives attached to a part called revolving nosepiece. Commonly, multiple combined objectives with a different ...

I think you will be hard pushed to get a better scope for its price than one of these with reducer and gubbins to connect your camera,

Quality of image also has a lot of other contributing factors, other than colour correction. Not all scope optics are created equal, regardless of their colour correction characteristics. An argument could be made that in general Apo's produce better overall images (talking visual again) because extra effort has most likely gone into their design and manufacture. But in reality, that is not really the case.

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Sep 22, 2018 — In the center, where the intensity is high, it looks as "very blue" — much like those 465 nm blue indicator LEDs we can see everywhere today, ...

For imaging you need a flattener / reducer, in my experience you have to get the matched one to save lots of hassle with spacing.

A few things to note - this is my first scope and ultimately I will be looking at imaging over visuals (although appreciate that may not be on this scope) and I have a 'good' eye and I'm fussy over image quality (work in graphics and animation production so I know good quality when I see it!)

Yep, good call, I was just looking at exactly that... seem good quality scopes but would need to spend a bit more for the focuser.

Bandpass filters are used to select certain wavelengths from a broader spectrum light source. Filter material may reflect or absorb the filtered wavelengths. / ...

Both refraction and polarization have several applications in everyday life. Applications of refraction and polarization of visible light are well known, for ...

the price looks big vs the Skywatcher, but when you replace the focuser and get the flattener to bring it up to match this scope, I think this will still be a better choice.

Dont get me wrong its a good piece of glass for the price, but if you want to take it a bit seriously the issues do start to come up.

I work in optics, nothing at all to do with graphics, and also know what optical quality is, measure down to 1/10 green wavelength which your graphics gets no where near.

I own two achromat refractors and in the end you get used to working around the problems. A lot can be done with a minus violet filter and post processing - the main point of course is that the Achromat are a darned sight cheaper, so if your budget doesn't stretch to an Apo then the Achromat is a good choice.

As a side note I must say this forum is incrediably active, knowledgable and positive compared to the forums I usually view. Nice one all

As a beginner I would go with the achromat and see how you get on with it. You can get filters designed to reduce any colour fringing that you get - but as I said, I personally don't find them necessary - its a decision that you have to make for yourself.

However, beauty is definitely in the eye of the beholder. I might have very different opinion if I looked through Dirk's kit, and if I was an imager too.

Most chinese refractors have a degree of spherical aberration, usually a touch of undercorrection. Thats why the most popular type of Chromacor (a CA / SA corrector made in the Ukraine) was the one that added 1/6 wave over correction - combining it with the usual undercorrection gives a virtually null corrected scope as well as one that shows 90% less CA.

My 80mm apo gives nice sharp views across most of the field, with no colour issues. My TAL100rs achromat give sharp images across pretty much the whole field, with colour only really noticeable (to me) on the fuill moon, Venus and Jupiter and the brightest couple of stars. The TAL125r is also an achromat and will resolve more, with brilliantly sharp and contrasty images, with a similar amount of colour to the TAL100rs. Both will give nice wide field views with suitable wide angle eps. I much prefer the views through the achromats to the 80mm apo - warmer, more contrasty, and sharper across the whole field.

Would a 80mm Apo give some super crisp contrasty wide field views that maybe a 120mm achromatic may miss and while the latter will have more light gather it won't be as good quality visually - but will I notice?!

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In practical use a modest amount of SA does not spoil the performance too much - if it did around 80% of chinese refractors would have been returned to their sellers !

As Earl points out there is always a bit a blue fringing which needs processing out. The build quality can't be faulted- has a better focusser than my Equinox 120ED. Good beginers scope- but a triplet should be considered for serious imaging.

Olly Penrice is testing one of these currently and so far has good things to say, also have look through this thread http://stargazerslou...atic-refractor/

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Anyone seriously bothered by CA for visual use might like to consider a Newtonian reflector, these are true APO's and cost a fraction of the price of an APO refractor.

For example: Diameter of the field of view (mm) = 20 / 40 = 0.50, where 20 is the field number of eyepiece, and 40 = objective mag. Eyepiece. Objective. (Mag.).

I have an 80mm Apo f7, and 100mm f10 and 125mm Achromat f9. I only really notice colour on the brightest objects, and even then not too much. The achromats I own are both TAL, and are noted for good quality optics (especially for their price). To a considerable extent longer focal length will reduce any chromatic aberation and if the optics are good enough and the focal length long enough, CA will be pretty much undetectable. Obviously the trade off is a much longer scope, and something much less useful for ccd/other imaging.

The focal ratio needed to make CA unnoticeable increases with aperture. A 4" F/15 achromat will look more or less CA free much of the time but a 5" needs to be slower to get the same effect - more like F/17.

Maybe you could get a Skywatcher 80mm ED or something and fit a better focusser? Like a moonlite (do they do those for 80mm?).

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Good advice would be to look through examples of each, and make a personal assessment of whether the substantial increase in cost is worth eliminating the colour fringing around bright objects. It can be very expensive to eliminate though, both my 4" class Apos cost in excess of £3,000 each.

Maybe you could get a Skywatcher 80mm ED or something and fit a better focusser? Like a moonlite (do they do those for 80mm?).

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the price looks big vs the Skywatcher, but when you replace the focuser and get the flattener to bring it up to match this scope, I think this will still be a better choice.

As already mentioned above, colour fringing (the main issue with Achromatic Refractors) is really only an issue with bright objects such as the Moon and planets. And whether that fringing is a problem or not (for visual use) is very much a personal thing. I know many observers who have no issue with it all, where as others find it very objectionable. I fall in the second category which is why both my refractors are Apo, despite being a visual only observer.

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I think you will be hard pushed to get a better scope for its price than one of these with reducer and gubbins to connect your camera,

For imaging you need a flattener / reducer, in my experience you have to get the matched one to save lots of hassle with spacing.

Sep 1, 2008 — A prism is an optical component that serves one of two major functions: it disperses light, or it modifies the direction (and sometimes ...