Where's the achromatic aberration coming from?

[clafann1]

Noob here, forgive with my ignorance

On a reflector telescope, the achromatic aberration can only come from the eyepieces, correct?

Since all eyepieces are made of lenses, they all have achromatic aberration, so, which eyepiece types have less (or correct it more)?

There's a lot of information about the telescopes lenses and mirrors, but very little about the eyepieces...

 

There's also something curious I found out on high power eyepieces: it seems that on a 4mm eyepiece you can see exactly the same "viewing area" as the 25mm eyepiece, still, since the hole of the eyepiece is so tiny, you can only see the center, the remaining image can only be viewed by "peeking" at an angle. that totally surprised me lol

Edited February 9, 2022 by clafann1

[Zermelo]

Yes, a reflector shouldn't be introducing CA, though certain designs (Jones-Bird) also include lenses within the telescope.

There are several kinds of optical distortion that may be introduced by an eyepiece. A specific design will usually trade off one against another. In a decent modern eyepiece, you shouldn't be seeing CA so bad that it's a problem.
These are some introductions to eyepiece distortions here:

http://uncle-rods.blogspot.com/2015/09/are-my-eyepieces-good.html

http://websites.umich.edu/~lowbrows/reflections/2007/dscobel.27.html

https://www.cloudynights.com/topic/795988-how-to-evaluate-or-compare-eyepieces/?p=11463451

 

1 hour ago, clafann1 said:

There's also something curious I found out on high power eyepieces: it seems that on a 4mm eyepiece you can see exactly the same "viewing area" as the 25mm eyepiece, still, since the hole of the eyepiece is so tiny, you can only see the center, the remaining image can only be viewed by "peeking" at an angle.

Shorter focal length eyepieces of certain designs (e.g. Abbe orthoscopic) tend to have small eye lenses, little eye relief (you need to put your eye very close to the eyepiece to see the view) and narrower apparent fields of view. This can result in smaller true fields of view, i.e. the amount of sky that you can actually see.
You can experiment with how the apparent field of view affects the true field of view in a specific telescope with this tool:

http://astronomy.tools/calculators/field_of_view/

 

[Zermelo]

I would add that some beginner telescopes include a very short focal length eyepiece like a 4mm so that they can advertise a higher magnification. My first budget scope included a 4mm EP that was not just uncomfortable to use, but in practice unusable in that scope because the magnification was too high for the size of the telescope.

If you can post the make and model of your scope, we might be able to say more.

[mikeDnight]

Yes you're right. All eyepieces have potential to create false colour. Many modern eyepieces use special glasses in their design which controls it. Wide angle eyepieces show lateral colour and you'll often find a colour fringe around the edge of the field of view. It often amuses me when people critique apochromatic refractors and complain about colour, as its usually the eyepiece that's the problem. Personally I find simpler designed narrower field eyepieces to show less, or even no perceivable false colour. Orthoscopic's and quality Plossl's can be excellent and give superb on axis sharpness that has the edge over wide angle designs, but they are usually limited to a 42° to 52° apparent field of view. Of course when observing deep sky any lateral colour present isn't really noticeable. It usually only becomes evident when observing the Moon. It's a case of horses for courses really, with wide field for deep sky and high definition minimal glass narrower field for Moon and planets. 

[vlaiv]

2 hours ago, clafann1 said:

On a reflector telescope, the achromatic aberration can only come from the eyepieces, correct?

It can also come from atmosphere - at least what people often perceive as chromatic aberration.

If you see image like this:

where there is vertical separation of red and blue parts of spectrum - it is atmospheric dispersion. It happens when bright object is viewed at low altitude - close to horizon.

[Louis D]

7 hours ago, clafann1 said:

There's also something curious I found out on high power eyepieces: it seems that on a 4mm eyepiece you can see exactly the same "viewing area" as the 25mm eyepiece, still, since the hole of the eyepiece is so tiny, you can only see the center, the remaining image can only be viewed by "peeking" at an angle. that totally surprised me lol

Perhaps with short eye relief 4mm eyepieces this is the case; however, long eye relief eyepieces like Pentax XW, TV Delos, and Baader Morpheus don't have these issues at any focal length.