Help with Secondary mirror size

[Astro78]

Can someone please assist with working out secondary mirror size?

The scope is 200mm F5 (1000mm). I believe the current secondary is 55mm and since it needs replacing anyway I figured a smaller secondary would be better for contrast.

One big factor seems to be the size in diameter of the focal plane. This is what's got me stuck.

A 45mm secondary will reduce the obstruction from 27.5% to 22.5%. That's got to be good!

Using a 32mm TV plossl the current 55mm secondary seems to fully illuminate this eyepiece to perfection (using some calculator) but how much smaller can I go with a reasonable loss in magnitude? These calculations are doing my head in.

If i can get better contrast for higher magnification but still use my lower mag eyepieces with little effect then great -

[Starkler (Geoff)]

Hi Trav. Do you use any 2" eyepieces? If so the required focal plane illuminated size would need to be bigger than that needed for a 32mm plossl.

Rule of thumb is to calculate for 70% illumination at edge of field for the field of your longest fl eyepiece. To calculate this you need to know measurements from your primary to the centre of your secondary, focal length etc.
One thing not often considered is that the worst part of any secondary optically is at the edge of it. The chase for planetary contrast could have you being worse off if you downsize too much and rely on the edge being good even for a centred high powered view.

Best thing is to download a cope of newtwin.exe and plug in the numbers.

[Astro78]

Thanks for your reply Geoff, very much appreciated.

I downloaded that software and now am more confused? It's basically saying i'm getting "Vignetting of 75% ray at front aperture". I have no idea what this is? But reducing the secondary size to 45mm (from 55mm) all good. It doesn't seem to say the actual size of the focal plane either?

I don't use 2" eyepieces and can't see it happening for a while due to cost (decent ones anyway). So the 32mm would be the longest focal length (27mm field stop), but what if I mustered up the courage for a XW in a long focal length in the future? Recently got a 2nd hand XL 7mm which has led me on this difficult learning process to try to maximize it's performance

[Starkler (Geoff)]

Originally Posted by Astro78

Thanks for your reply Geoff, very much appreciated.

I downloaded that software and now am more confused? It's basically saying i'm getting "Vignetting of 75% ray at front aperture". I have no idea what this is? But reducing the secondary size to 45mm (from 55mm) all good. It doesn't seem to say the actual size of the focal plane either?

What this means is that the ota tube geometry would be the cause of vignetting given the parameters you have entered. ie the limiting factor in your widest field viewing. I wonder if you have entered the dimensions properly. The focal plane for example should be some distance above the top of the focuser when it is racked fully in by something like 15mm.

What you are trading off is illuminated field size vs contrast improvement by using a smaller secondary. If your scope is not intended to be a dedicated planetary scope and using premium grade optics, i really wouldn't suggest you bother. What 75% illuminated field size does the software tell you in each case?

[Astro78]

Hi Geoff,

Seems the inside tube diameter was off, should be 235-237mm not 230mm and this fixed the problem of "Vignetting of 75% ray at front aperture".

The field marked "spare focuser in travel" was wrong too.

Can i please confirm that when calculating "to front of tube" and "to back of tube", this includes the metal parts that screw into the front (holding spider/secondary) and back (holding mirror) and not the actually tube without these?

Also there is a 35mm high 1.1/4" adapter should this be included in the "focuser height" field? This must be the add 15mm odd you refer to?

75% Illuminated field is around 35-36mm. With little change unless inputting smaller secondary.

100% varies from 6-13mm depending on where i'm calculating focuser height.

Really appreciate your help btw!!!

Not a dedicated planetary scope but it is around 7 years old so thought a new primary mirror would be good, quite sure the current one needs re-coating anyway. While i was at it, figured why not upgrade the secondary.

[Astro78]

anyone?