Secondary advice wanted.

[Astroman]

Hi all,

I have an 8" F/7 dob so basically a 203mm FL1447.8mm Newt. The tube Dia is 254mm. I worked out that the focuser is 70mm from tube to where the ep should focus this includes a 10mm spacer needed to focus at about the same distance as the old focuser. I used the formula Secondary to Eyepiece distance which was 197mm then used the formula mirror diameter divided by the focal length times 197mm, which equals 27.2mm This resultant is the diameter of the short diameter of the secondary (theoretically) I measured the one in the scope and it is 40mm in dia.

Given this I have a secondary which is 1.47 times bigger in the scope that is theoretically needed, and as we know the smaller the obstrution the better the image. What are your thoughts on this, I am tempted to buy a smaller secondary and spider assy, but don't really want to if it wont make much difference.

Has anyone thought about this or had a similar situation?

[janoskiss]

I have no experience but 1.5 times oversized sounds like a lot. If your scope is otherwise near perfect, it probably does make a noticeable difference. Does it look that much bigger? i.e., if you look at the primary through the focuser, how much of the secondary does it cover?

[Astroman]

I can see the whole of the primary in the secondary with lots of room around it.

[Starkler]

A 40mm secondary used with a 200mm primary gives a secondary obstruction of 20%.
This is by no means excessive and I wouldnt bother reducing it unless the primary mirror is super sharp and you want to specialise in planetary observing with it. Advice I have had suggests you need perfect conditions and great optics to see any improvement by reducing the obstruction from 20%.

In my 10 inch f5, the secondary obstruction is 26% !

By having a smaller secondary you are throwing away fully illuminated field size, which matters for wide field observing, but not for planetary where the object is small.

[Astroman]

Thanks Geoff,

I didn't really think that it was bad, just the calculations made me think a little.. With that info, I guess it pays not to look into formulas too much.

[RAJAH235]

FWIW, The 2ndary minor axis on my Meade 10" is 2.37" or 60 mm.
Geoff, What's your's..... Meade reckon around 24% obstruction with mine. :shrug: :D L.
ps. Meade simply 'GLUE' the mirror to the flat holder/base plate. ie; same diam. L.

[Starkler]

Mines a GSO 10 inch dob with a standard 63mm secondary, but the plastic holder adds maybe 3mm extra to the diameter.

[mch62]

Have you used Newt 2.5 a free ware program.
If not get a hold of it and first read the help files in particular the section "notes on scope design".
Punch in you numbers and work from there.
There is a better program around but this one is quite easy to use.

From what you have given your obstruction ratio is about 20% , visualy any thing under 20% you would be hard pressed seeing the difference unless really experienced at seeing details.
The important thing is the 100% fully illuminated field and yours works out at arount 12mm.
You want to have that at the size of a any CCD chip your going to use and or around at least the 75% illumination to cover the size of the lowest power eye piece field lens(the bottom lens).

your 75% is about 27mm.

If your really wanting to go smaller in secondary size you would need to use a low profile focuser to maintain a resonable field size. like 25mm profile.
Also using a smaller tube diameter but this will cause heat currents in the optical path unless you use an open truss tube.

So it's gets to be quite a change if modifing an existing OTA but feasable if starting from scrartch to make a ultra high contrast Newt while maintaining a resonable fully illuminated field.
In your case it's not worth the small change in obstrction ratio for the expense.

Mark