On cloudy nights, I...

Wanna make a scope? Or better still, grind a mirror yourself. Or, you have some good tips in making a really useful accessory? This is the place to show what your hands can do...
Post Reply
User avatar
neoterryjoe
Posts: 289
Joined: Sun Nov 14, 2004 12:15 pm
Favourite scope: c5
Location: je ( 1.34, 103.73 )

On cloudy nights, I...

Post by neoterryjoe »

...do some diy to try improving my scope.

Recently, I flocked my c8 after reading Barry's article on cooling the c11 which include how to take out the sct corrector properly( dont be mistaken, articles on how to dismantle a sct are actually aplently, even in singastro, Im just lazy ). Reasons for flocking is obvious, though theoretically (as some say) it should not make a difference, some swear that there are visual differences. Anyhow, I wish to spend more time with my scope..

Material is easily gotten from art friend (though very expensive at 13 bucks per half meter, but still less than if you get from scopestuff-(they say the material is more durable) ). I attached the material in sections of width 10cm to make things easier for me. Just take note to mark the orientation of the corrector and the position of the cork spacers. (I note that for my c8, even with the spacers in place, the corrector can still move about quite freely, hence, you might want to take more precise note of its positions(b4 taking out) by, maybe adding more spacers to make it secure. Then again, such minute radial changes might not have much difference optically, though it affects collimation). One of the "difficulties" I met was that the corrector seems to be "stuck" in its position when I tried to grab the whole corrector out by its secondary mirror assembly(after taking out the retaining ring). It came out at the second try though.

A night with Jupiter after flocking with the c8 is great. I cant say for sure how much improvement is made in the image as I didnt have an eye for little details before, but several details such as the NTB and separation of the SEB into N and S components were obvious. It was satisfying for me. Do note that performances of scope varies, you might have seen more details with, maybe a c8 with XLT; I have not seen many c8's other than my own. In any case, I was satisfied and am enjoying my scope fully.

The cat folks at Cloudynights have been emphasizing about collimation, esp with scts, saying that the lack of good collimation is one of the main reasons why people have impressions that scts are poor performers as compared to other optical designs.

http://www.cloudynights.com/ubbthreads/ ... ll/fpart/1

This is a good article on collimation, effects and also central obstruction of sct:

http://legault.club.fr/collim.html

Now the main thing about collimation is that you have to use a bright star at high powers. It is more convenient with a tracking mount, but what about those without one? One can either, be very patient and skilled in tracking+collimation at high powers using an alt-azi mount, or one can use an artificial star. For those who are more diy-inclined, this is a good site:

http://tanveerg.googlepages.com/buildin ... intsource)

Its a little technical at first glance, but anyone can follow it. I tried to make one, but being VERY lazy, I didnt bother to use an ep to enlarge the point source to the precise size as I will have to centralise the ep to the point source; I merely used a bicycle back light(which uses very bright red led), using a needle to make a pinhole in a paper and tape onto one of the 4 leds. However, I realised during collimation that the light seem to be very dim; it might be best to enlarge the light source (brighter) and then reduce it to the appropriate size as according to the calculations.

The main thing about collimation is the last stage which most people would have left out due to it being tedious -- in-focus collimation(see legault's page). However, it is really easy, contrary to what I thought initially; I did it in minutes using the artificial star. You just need to have patience and an eye for details. After (rough)out-of-focus collimation, one has to focus the star and collimate the scope such that first diffraction ring circles the central disk fully and completely without any break(see 3rd step). My c8 has a collimation of fig d) initially and around fig a-b) after the procedure. One thing I will do is to note the position of the skew, defocus the star, stick my hand in front of the tube to know the position of the skew and the screw to adjust, refocus the star and do the collimation as per normal. I realised that even after rough collimation, I need to turn a substantial amount to achieve in-focus collimation! This is enough to realise how important the last step is.

Now its time to wait for the skies to clear! Cant wait to see jup again with such precise collimation!

tj
User avatar
Meng Lee
Posts: 1227
Joined: Wed Mar 28, 2007 1:36 pm
Location: NTU, Woodlands

Post by Meng Lee »

Hi Teng Jie,

Its great to see you writing this to make SCT users more aware of how much they can push their SCT to the limits.

Many SCT owners, especially school clubs do not collimate their scope. They fail to recognise that the secondary mirror has a 5x effect. A slight miscollimation will render their scope working like a 3~4 inch with 60% obstruction and glaring spherical aberration.

Just do the collimation and it will perform much like an 8 inch. As Tengjie said, only when you have achieved the in-focus high power star collimation, then you are ready to use it as a good scope. But do note that the final stage of collimation requires very small turn of the secondary , about 1/20 turn only.

Use a high contrast eyepiece in order to do it. Good plossls, orthos and any high end eyepiece will show u clearly the diffraction rings to do the collimation.

Hope all SCT users collimate and enjoy your SCTs.

Meng Lee.
User avatar
neoterryjoe
Posts: 289
Joined: Sun Nov 14, 2004 12:15 pm
Favourite scope: c5
Location: je ( 1.34, 103.73 )

Post by neoterryjoe »

hey ml!
yes! Collimate it well, and it(c8) will perform with an equivalent contrast of a 5" unobstructed and light grasp of a 7.++" scope. haha..

Well, I guess part of the fun in bonding with your scope is to tinker with and understand more about your scope. I guess the c8 will be my most used scope and will last long for me till I see the need to upgrade again..

Hope to view thru your c11 in the near future!

tj
Post Reply