Sanity check - 14" f/3.6 Newtonian Astrograph - Secondary mirror placement inside "light cone"

[Astrofail]

Hello,

When placing the secondary in the light cone, should you go inside it with the secondary edges pruning off "useless" edge rays to minimize secondary size as much as possible or aim to capture the entire cone as much as possible?

In the below schematic illustration the scope is being built around the 3" TeleVue VIP3010 Paracorr using a FTF3215B-A focuser for a final image circle of 44 mm to accomodate 36x24 sensors. The secondary shown in the illustration is a 5" placed to capture the entire cone.

I am also curious if the secondary offset looks proper. Please reference the mesurements in the attached illustration.

Any input/feedback welcome

 

[Oldfort]

I wouldn't know the answer, but you illustration is very difficult to read.  It's practically monochrome, the rays are rather faint, and the numbers too small.  Perhaps a brighter background would help.

Edited September 3, 2022 by Oldfort

[Astrofail]

21 hours ago, Oldfort said:

... you illustration is very difficult to read.

Oh yes, it s actually a 9999x9999 screengrab straight out of Fusion360 so there is enough resolution in there, you just have to click the image and wait for the higher resolution one to load and then it becomes redable. Sorry.

[Captain Scarlet]

I can't really see the diagram very well, but hopefully I can partly answer some of your questions.

When you suggest placing the secondary "inside the light cone" I guess you're talking about the on-axis light cone, i.e. the cone formed by light coming from an object at the centre of the field. While it's true an undersized secondary will indeed "miss out" rays coming from the outside edge of the mirror, the same is not necessarily true for off-axis rays. Beyond a certain angle off-axis, light from the edge of the primary will strike the secondary and get reflected into your eyepiece or sensor. So your image plane will be increasingly "contaminated" by edge-of-the-primary rays the further off-axis you get. If you want to mask off the primary's edge, you will need to do it at the primary mirror itself.

Sizing of the secondary is basically a choice between contrast and illumination: either using a small secondary (but still >= size of the on-axis light cone) to minimize CO and hence maximize contrast; or using a larger secondary to increase the illuminated field at the image plane at the expense of contrast. A visual observer would tend towards the former, AP-ers the latter. Either way there is no real argument to size the secondary smaller than that covering the on-axis light-cone. Meaning that you will always be dealing with a more or less oversized secondary.

As for the correct placement of an oversized secondary,  there is no real absolutey correct answer. Conventional wisdom suggests that a correctly-sized secondary will appear perfectly circular in perspective from the focal point. However, this is only true if the secondary is exactly the size of the on-axis light cone (i.e. not oversized or undersized). If either of those conditions is not met (eye not at focal point, or an oversized secondary), then the secondary should not necessarily appear circular, depending on your definition of "correctly-placed".

There are two intuitively obvious approaches when trying to decide a "correct" method for placing the secondary. One (the easiest) is to place the secondary such that it, the outer edge of the primary, and the inner edge of the OTA (if visible) all appear concentric from a collimation cap at the focal plane. This intuitively seems the right thing to do, but in fact it yields a non-circular fully-illuminated field at the focal plane. The other approach is to start off with the requirement that the fully-illuminated field be symmetrical and circular at the focal plane, and sizing the outside edge of the secondary to coincide with the outer edges of the light cones that produce this circular field. The problem here is that the shape of the secondary will then not be the standard elliptical 1.414:1, nor should it appear circular from the central focal point.

Note that I haven't yet mentioned "offset" here. Offset up and down the OTA sorts itself out when using the optical tricks ("make circular") to get the desired perspective view of the secondary from a collimation cap, say. (i.e. the focal point). Transverse offset does need to be calculated and built in to the transverse position of the secondary, however. The trouble is, each of the methods described above yields a different value for the offset.

However, the differences between these alternatives are small and, in my opinion, negligible. So as long as the secondary appears _roughly_ circular from the focal point, and the outer edge of the primary and the tube edge also appear roughly concentric, you should be fine.

Draws breath.

Cheers, Magnus

 

 

Edited September 6, 2022 by Captain Scarlet