getting it together... optics

[m37]

Well, I'm a couple of months in and having the time of my life. Scope and setup nicely tuned, imaging is getting there. Great stuff.

Just wanted to consolidate some of the theory I've picked up, if someone could give this the once over and let me know if I'm in the right area I would be really grateful. Sorry for the long post but there's a lot to learn!

1. Light rays from a point at infinity entering a telescope tube can be considered parallel to the tube as the curvature of concentric wavefronts becomes effectively flat enough to be considered flat at such large distances. e.g. point a below.

2. The refractive index of a lens is its influence on light compared to light in another medium, and is measured against light in a vacuum. Higher refractive index of objective lens means light is travelling slower through the lens compared to a lower RI medium such as air. This determines ultimately the angle light leaves the lens and therefore changes the distance between the objective and the point of focus (focal length). Different lens shapes, arrangements and materials therefore give scopes different focal lengths as in this diagram.

3. The focal length of an objective does not change as you move the focuser in/out. The focuser is to make the distance between the objective and the rearmost surface of the eyepiece the same as the sum of the objective and EP focal lengths e.g. 500mm scope + 10 mm EP = 510mm total focal length therefore 510mm from center of objective to rearmost surface of eyepiece must be 510mm to achieve focus.

4. The reason different objects come into focus at different points of focuser in/out travel is that the focal point (fp) is located at 1/fp which = 1/ep - 1/ob (1/focal point = 1/ep focal length - 1/objective focal length). So a more distant object requires more inward focuser travel than a closer one.

5. The effective focal length of a ccd is its diagonal chip measurement

6. For an object to focus at a total focal length of 506mm (500mm scope + ccd of 6mm) the distance between the objective and the chip must be 506mm.

7. Adding a barlow effectively doubles the focal length of a scope so the same object and setup but with a barlow means to focus the same point needs 500 x 2 + 6 = 1006 mm between objective and ccd. For a 500mm f4.9 refractor this would theoretically require a 500mm extension tube (!).

8. (last one I promise) If the minimum focal length achievable with said refractor (due to focuser travel range) is 390mm then our 506mm focal length with a 0.6x reducer would be 306mm. Therefore this scope could not achieve focus due to lack of inward travel (back focus). A 0.8 x reducer would work as it would require 406mm total focal length which is within the scope's range.

9. (I lied, there's one more) Cheap achro doublet refractors give chromatic aberration because R,G and B light all have different wavelengths. This means the wavefronts are at very slightly different angles and so come to focus at slightly different points. Fringe killers or sem-apo filters etc. are not actually improving focus by changing the focal points of different light wavelengths, they are just reducing the out of focus light to a point where it is less noticeable.

Really sorry, that's loads. I won't be offended if nobody replies!!!

Cheers!

[Jiggy 67]

Wow..... You've lost me!! Lol

Sent from my iPhone using Tapatalk - now Free

[ronin]

1 and 2 seem OKish, the diagram of (1) does not show incoming light so if the small circle is the source (object) then the distance from the mirror to it would be 2xF. What is drawn and written is ambiguous.

3: First bit correct focal length of the primary does not alter. The focuser is to get the image plane of the main objective and the object plane of the eyepiece coincident. What happens is the image formed by the primary becomes the object to be viewed by the eyepiece. Different eyepieces have their object plane at different places, some are inside the eyepiece and can be considered a virtual image.

4: Forget EP focal length. The imaged formed by a primaty follows 1/f=1/u+1/v that is where the primary forms an image and there is no eyepiece or eyepiece focal length to consider.

5: Uh ? It is a chip, flat what focal length ?

6: If the focal length is 500mm then the chip has to be at 500mm otherwise the image is out of focus.

7: No otherwise you would have to add an extension tube the same length as the scope whenever using a barlow. Leave you to consider a 5x barlow in a Tal.

8: Gave up even thinking about it.

9: Wavefronts are not at different angles incident to the lens, the refractive index is wavelength dependant so for different wavelengths the focal plane is different.

[umadog]

Ronin's hit a lot of the points. I'll just add:

3. The eyepiece focal point is where the field stop is located. Often that's inside the eyepiece somewhere.

5. I think you mean the effective FOV

7. The barlow does effectively alter the focal length, but it does so but altering the angle of the light cone arriving at the image plane. So it doesn't increase the overall length of scope in any significant way. SCTs work this way: that's why they're compact yet still f/10 (or whatever).

8. Again, you're confused with what a reducer does. It alters angles not lengths.

[m37]

Cheers guys, hitting the books again!

[cantab]

2. Refractive index depends just on the composition of the glass. The focal length of the lens, or equivalently the optical power (which is 1/focal length), depends on the shape of the lens as well as the composition of the glass. So two lenses of the same glass but different shapes will have different focal lengths, as will two lenses of the same shape but of different glasses.

3. This only strictly applies for a single-lens eyepiece I think, not for the multi-element designs always used nowadays.

5. Sort of. For a camera chip, the field of view depends on the focal length of the lens and the physical size of the chip. To define an "effective focal length" if you want to make an analogy to eyepiece views, you first need to decide what your eyepiece is for comparison, since two eyepieces of the same focal length but different designs themselves have different fields of view.