Achro Refrac contrast question.

[GasGiant]

Hi folks

So Im loving the contrast I get with the Seestar images.  I believe this is similar to traditional achro refracts, in that they produce nice contrasting images/viewing ?

Im trying to hone in on a scope type for my next purchase !  

Carl 

 

 

[Elp]

According to ZWO the Seestar is a triplet apochromatic optical design.

 

[GasGiant]

Oh, whats the diff ? 🤣

[bosun21]

Just now, GasGiant said:

Oh, whats the diff ? 🤣

The quality and glass type of apochromatic refractors focus the three colours to almost the same point with a doublet and more or less spot on with a triplet. An achromat doesn't get them as close to the same focal point.

[Elp]

Acromats typically leave a blue or yellow fringe around a bright target like planets or the moon due to as explained above, you'll see it a lot in cheap refractors. More expensive or longer focal ratio acromats control the chromatic abberation better, similar with camera lenses when stopping down the aperture (and increasing the focal ratio). Doublets you'll see an improvement, triplets even more though the effect and "issue" is down to the observer doing the viewing. You see the issue more starkly in images.

Both my Z61 and Starfield 102 are doublets and give similar views to the above image, they are FPL53 glass doublets, both apochromatic but being doublet one wavelength of light if considering RGB will be slightly off focus, I think it's the blue with these scopes. Never had an issue visually with them. A Newtonian won't have this CA issue as it uses mirrors rather than refracting light through glass lenses.

[Mr Spock]

Difference between the two, single shot for each:
I've processed to monochrome to loose the colour fringing.

120mm achro:

100mm apo:

[Elp]

Contrast is also another subject. Usually for a scope to have good contrast requires good internal baffling either with a material or paint to reduce internal reflections of light getting through to the eyepiece or camera sensor, this can also be helped with the use of dew shields to prevent stray light from entering the front objective. Some telescopes have physical baffles inside the scope (raised surfaces perpendicular to the length of the scope) to act as physical barriers to the light, the SF102 I've mentioned above has this. Doing so improves the perceived contrast or difference between light and dark areas.

[LondonNeil]

Lovely images Mr Spock, particularly the apo, but both good. 

[Louis D]

Here's a composite image of the image through various telescopes of mine taken on the same night:

They are all shown at the same resolution from my smartphone camera.  I didn't both to 2x the ST152 achromat because the 1x image was so mushy.  Ignore the brown coloration.  It's an artifact of the eyepiece I was imaging with.

Here's a composite showing various filters applied ahead of the camera when using the ST152:

You can improve sharpness by filtering out more and more poorly focused wavelengths of light.  Green is the sweet spot for achromats.

Longer f-ratio achromats will have fewer issues, but then you'll have to deal with a really long tube.  Think f/15 to f/25 to get closer to ED and APO performance out of an achromat.

[Elp]

It's an interesting comparison. But you can't do an objective comparison via an eyepiece as it'll introduce its own distortion of the light, this will also vary depending on where in the eyepiece the image was taken from, either way your comparison is a visual guide to variation you can see via different optical setups.

This is one of my earliest images via my Z61, note the lack of any fringing, and it hasn't been edited to hide any abberation:

[vlaiv]

It is very easy for an achromatic refractor to reproduce images obtained with Seestar50 - even cheap achromatic refractor.

With achromatic refractors - there is something called CA index - or index of chromatic aberration - which is a number obtained by dividing F/ratio of the lens with aperture in inches.

50mm is 2" and telescope needs to be F/10 or slower to show virtually no CA. In fact - if we take very cheap and common 80/900 refractor and put 50mm aperture mask on it - we will get F/18 instrument with 2" of aperture. I think that sharpness of such arrangement will beat seestar in terms of performance (if we speak of sharpness and contrast).

Even very fast ST80 will come very close to be color free and very sharp without any filters if we put 2" aperture mask. Most people with SkyWatcher version (not sure if other versions have this feature) can actually try this by removing small central cap of the lens cover:

[GasGiant]

So much confusion.  lol.

Thank you for the replies, folks.  Lots to learn !