Mystery End Cap Hole??

skygawker (Allan) · #1 08-06-2008, 09:01 PM

Can anybody tell me the purpose of the smaller capped aperture in my 10" dob's main OTA endcap? There's nothing in the scope's manual about it and I'm naturally curious. Something to do with solar observation maybe?

iceman (Mike) · #2 08-06-2008, 09:12 PM

Yes - it's for solar observation. BUT only with a solar filter fitted onto it.

Either baader solar film or a glass solar filter.

Honestly though, you're better off with a full-aperture filter rather than the small one. Get some baader film and make your own white light filter.

http://www.iceinspace.com.au/index.p...63,136,0,0,1,0

erick (Eric) · #3 09-06-2008, 11:37 AM

I'm sure I read somewhere that there is an argument for having a smaller aperture for solar observation. I recall it goes something like - you have buckets of light, so there is no need for a larger aperture to collect light. When observing the sun, it is normally in a heated atmosphere (ie daytime

) and hence stability of the air column between scope and upper atmosphere is important for resolution. The maths apparently suggests that a smaller aperture will provide better resolution that a larger aperture under such conditions. Well I think that is what I read.

Hence the many offaxis small aperture solar filters available - it's not just for lowering the cost.

iceman (Mike) · #4 09-06-2008, 11:48 AM

yeh I've read that too Eric - that you can potentially get a more stable image because you're going from an f/5 to f/15 or something.

I don't buy it though - you're losing resolution. A 50mm aperture versus 250mm aperture - there's no question what's going to deliver the best resolution. Just keep observing and waiting for the best conditions.

Terry B · #5 09-06-2008, 11:49 AM

Quote:

Originally Posted by erick

I'm sure I read somewhere that there is an argument for having a smaller aperture for solar observation. I recall it goes something like - you have buckets of light, so there is no need for a larger aperture to collect light. When observing the sun, it is normally in a heated atmosphere (ie daytime

) and hence stability of the air column between scope and upper atmosphere is important for resolution. The maths apparently suggests that a smaller aperture will provide better resolution that a larger aperture under such conditions. Well I think that is what I read.

Hence the many offaxis small aperture solar filters available - it's not just for lowering the cost.

Yres but the small holes a probably too small. Angular resolution is proportional to the diameter of the mirror and the resolution gets pretty low with a 40mm hole like on my endcap.
The solar telescope at the ATCA has about a 120mm mirror.
See http://www.ips.gov.au/Main.php?CatID...cName=Overview

astro_nutt · #6 09-06-2008, 04:28 PM

Hi Skygawker,
You can also use the smaller aperature for observing the Moon too!!
Cheers!

skygawker (Allan) · #7 09-06-2008, 10:06 PM

thanks guys - but I'm a bit worried about pointing that big mirror at the sun even thru' that relatively small aperture (but hey it's as big as the primary on my first tasco refractor all those years ago - I had a special solar filter for that). My Mum said never look at.....well you know the rest. I also read somewhere that some scopes' bits will actually melt if you don't use the correct methods to use them for solar observation. I don't see much plastic in my scope but I don't know about the secondary mirror/mounting adhesive.

What about the the 'projection' method? I remember this from my early days. Can I do this with a reflector thru' the reduced aperture without OTA component meltdown? I'm curious to observe our daystar for sunspots again - it's the only star we can see that shows a disk and the signs of rotation. But SAFETY FIRST!

Terry B · #8 09-06-2008, 10:35 PM

Quote:

Originally Posted by skygawker

thanks guys - but I'm a bit worried about pointing that big mirror at the sun even thru' that relatively small aperture (but hey it's as big as the primary on my first tasco refractor all those years ago - I had a special solar filter for that). My Mum said never look at.....well you know the rest. I also read somewhere that some scopes' bits will actually melt if you don't use the correct methods to use them for solar observation. I don't see much plastic in my scope but I don't know about the secondary mirror/mounting adhesive.

What about the the 'projection' method? I remember this from my early days. Can I do this with a reflector thru' the reduced aperture without OTA component meltdown? I'm curious to observe our daystar for sunspots again - it's the only star we can see that shows a disk and the signs of rotation. But SAFETY FIRST!

The filter MUST go on the front of the scope. This prevents the mirror cooking. The solar filters that go on the eyepiece are illegal and very dangerous.

skwinty (Steve) · #9 09-06-2008, 11:12 PM

I often use my 12" Newtonian to observe the sun.
I use 2 layers of mylar as a filter.
When I used the cap hole with the mylar the result was less than satisfactory. So I made another cover with a 4" inch hole covered with the mylar and found this to produce excellent results.

I have observed the sun for at least an hour with no heating or problems with any components.

Hope this helps.

JimmyH155 · #10 10-06-2008, 10:28 AM

I have just bought a new grab 'n go - namely a Celestron XLT 120 refractor. It is a marvellous scope. It too came with a smaller cap in the large endcap, and in the instruction book, Celestron said that because of the chromatic aberation seen in refractors (not with the fancy glass) , you can avoid the effect by looking at bright objects like Moon and planets, by keeping the large endcap on and just uncovering the small cap. (ie using the centre of the lens only)
Well I thought I would test it, and about a million years ago when we had a clear sky

, I lined up Sirius, and sure enough, with the full aperture, there was this purplish-blue haze all around it, but when I tried the little cap, hey presto, no blue light and Sirius looked just as bright...

So it works fine on bright objects, and on the fainter objects, you dont get chromatic aberation anyway, so just use it full open.
Why do people buy those vastly more expensive apo or triplets or whatever they are called? I understand you can now buy an anti-aberation filter now, so goodbye blue or purple fringing???

RB (Andrew) · #11 10-06-2008, 12:02 PM

Quote:

Originally Posted by JimmyH155

Why do people buy those vastly more expensive apo or triplets or whatever they are called? I understand you can now buy an anti-aberation filter now, so goodbye blue or purple fringing???

Ego probably?

WadeH (Wade) · #12 10-06-2008, 08:55 PM

Welcome Skygawker, another from our side of the Island, great!

As for solar viewing I use a full apeture JMB glass filter and it works a treat. You can see the setup on my scope on the picture below. Dont forget that these filters are not just for sunspots but some great viewing of transits (Mercury and Venus) can be had too!

Just remember to put the filter on the scope before aiming at the sun or you will have melted eyepiece covers before you can say oh crap!

skygawker (Allan) · #13 11-06-2008, 08:03 PM

Thanks WadeH. The JMB filter looks nice but a little out of my price range (saving for some better EP's). Think I'll have a go at making one. Our good friends at MyAstroShop are currently selling Thousand Oaks R-G Solar Film (30cmX30cm) for $45. This apparently gives an orange disk and dark background. Howse the skies in Bullsbrook? Probably a bit crappy looking south but I'd think not so bad to the north.