Advice please - Need help choosing secondary.

[richardda1st (Richard)]

Hi ATM champions.

I have just installed a new primary (thanks Lars) and I'm now looking to upgrade the secondary.

The new primary has a longer fl as compared to the original primary by approx 60mm. I have extended the truss poles by adding an extension plate to each(with a set of holes to allow for inaccuracies in my measuring) refer pic. showing these preliminary plates. Focus is now okay.

As it stands now, the distance from the primary to the center of the secondary is 1055mm, plus 230mm to the focal plane, so fl is now 1285mm. Mirror diameter is 250mm.

The Mead light bridge is badged as 254mm fl1270, f5, I didn't check the original fl. but mirror dia is also 250mm. The original secondary is 63mm (2.48")

So f/ratio is 5.14.
So nominal secondary size would be 230mm/5.14 = 44.75 (1.76")
If I have correctly understood the procedure from "Sizing Up the Newtonian Secondary" (www.garyseronik.com/?q=node/8)

The nearest standard sizes (from Antares Optics) are 1.52", 1.83" & 2.14".

My observing includes anything that's above me in the night sky, so no preferences to dictate the choice of secondary.

My thoughts would be the 1.83", but maybe that's to close to 1.76" and the 2.14" is a safer bet as this will give plenty of area to insure full use of primary mirror at the outer edges. I think that a bit bigger obstruction at the middle is not as bed as loosing light from the outer area of the primary due to a small secondary.

Also, can anyone give an opinion as to the wave (PV) rating of the original secondary, I would like to get a better quality one without going overboard. I have no numbers for the new primary to gauge quality.

Hope this all makes sense.

Thanks, any advice would be appreciated.
Richard

[Satchmo]

Richard

I ran your numbers and the 1.83" will give a 6mm diameter fully illuminated field and the 2.14" a 15mm. The light drop off ( vignetting ) is very gradual in an F5 scope so either size will work fine. Edges of diagonals can sometimes have issues so I would go for the larger diagonal.

Compared to mirror figure quality and surface smoothness, central obstruction is one of the most overrated aspects one can tweak. Anything below 25% obstruction will perform very well. I would recommend at least the 1/12 surface diagonal. Spherical errors on the secondary do not bring down the Strehl ratio nearly as much as errors like spherical aberration on a primary.

[astro744]

I recommend the following site for sizing your secondary:

http://www.bbastrodesigns.com/diagonal.htm

The last parameter is critical and depends on your tube radius + focuser height. Note aim for 70% illumination to match the field stop diameter (radius) of your lowest power eyepiece that you intend on using frequently.

eg. (mm)

250
1270
54
210

A 54mm secondary with 210mm to focal plane gives approx. 70% at 21mm off axis radius. A 31mm Nagler has a field stop of 42mm and is a perfect match as your lowest power eyepiece giving an exit pupil of approx 6mm. Your 100% illumination circle is 12mm. You would great greater illumination if you had a lower profile focuser or larger diagonal provided there was no vignetting anywhere in the path by tube or focuser. See also NEWT at http://www.dalekeller.net/ATM/newton...t/newtsoft.htm

[richardda1st (Richard)]

Hi Mark.

As the original secondary is 63mm (2.48") I'd better check my dimensions again. If my numbers are correct I will choose the 2.14” 1/18 PV.

I do not fully understand the illuminated field. I would have thought that if the secondary is large enough to receive the full cone of light, allowing for alignment issues and secondary imperfections near the edge, the illuminated field stays the same. I can only see the illuminated field reducing when the secondary is too small to receive the full cone of light. What I’m trying to understand is why does the 1.83” give a 6mm diameter fully illuminated field and the 2.14" give a 15mm. More to learn.

By the way, did you notice that my original concern with this new primary’s fl was in the opposite direction, being longer and not shorter? You were right the Meads stated specs is just nominal.

I have not had much observing at the moment due to the conditions, but what I have done so far is good.

Richard
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ps

Thanks astro744
I knew I should have checked for a new post before I submitted mine.
I will check them out tomorrow. As I said above I will check my dimensions.

[astro744]

Here's some light reading:

http://www.skyandtelescope.com/howto...tml?page=4&c=y

http://www.garyseronik.com/?q=node/8

Note depending on your sec. holder you may lose some width around the edge of your diagonal. Err on the larger side as the impact on contrast is minimal.

Old school says that a 0.5 degree illuminated field is ample to cover the full lunar disk. This equates to approx 11mm with a telescope of focal length = 1270mm (11= 0.5x1270/57.3). If you are a variable star observer you would want a greater 100% fully illuminated field to ensure reliable magnitude comparisons unless you were to limit your observations to narrow field eyepieces.

[richardda1st (Richard)]

Not much change to my measurements. A more careful measuring is 1055mm from primary to centre of secondary, 235mm (not 230) to focal plan and 250mm primary.

Should I concern myself with the secondary offset? There seems to be a few differing view points regarding offsetting the secondary.

Any comments as to why Meade would fit a 63mm secondary.
LB 10” actual measurements - 1000mm from primary to centre of secondary, 245mm to focal plan and 250mm primary. Minimum secondary would be 49.2mm; 63mm seems way too large. Is there a good reason.

The illuminated field.
I have read all those sites and others, but I don’t understand the illuminated field and how it’s affected by the secondary. I need drawings/pictures. I'll keep looking and reading.

[ausastronomer (John Bambury)]

Because they use all the same components in the tubed newtonians which are often used for imaging, thus requiring a larger fully illuminated field than a visual scope.

Cheers,
John B

[richardda1st (Richard)]

Thanks John, if that's right, and I have no reason to doubt you, I can ignore the size of the original as I can't see me getting into astro imaging. I'm satisfied to admire other people's work.

I would appreciate some comments regarding offsetting the secondary before I purchase the 2.14". If offsetting is worth while, would that affect the size of the secondary?

Thanks
Richard