SCT collimation questions

[kingkong]

hi all SCT masters, i need some advice on an sct newbie problem.

the setup in question is: c8 with 2" liuming star diagonal and 1.25" 20mm generic ep

i noticed some serious problems with the image:
- bright star appeared like double star near focus!
- saturn showed up as double image near focus
- out of focus stars suggested the sct was out of collimation - the center black dot was slight to a side, not too badly
- it is impossible to get pin-point stars

i tried collimating the out-of-focus donut, painfully, i managed to get what appeared to be reasonably centered black dot. the donut appears round enough at this stage.

but as i got nearer to focus, the donut got smaller and slowly turning into an egg with a black spot - it became oval! as it got nearer to focus, the egg turned into multiple stars!

could someone please suggest what's wrong with my sct?
or is it just tube current?

thanks in advance.

[Meng Lee]

hi all SCT masters, i need some advice on an sct newbie problem.

the setup in question is: c8 with 2" liuming star diagonal and 1.25" 20mm generic ep

i noticed some serious problems with the image:
- bright star appeared like double star near focus!
- saturn showed up as double image near focus
- out of focus stars suggested the sct was out of collimation - the center black dot was slight to a side, not too badly
- it is impossible to get pin-point stars

i tried collimating the out-of-focus donut, painfully, i managed to get what appeared to be reasonably centered black dot. the donut appears round enough at this stage.

but as i got nearer to focus, the donut got smaller and slowly turning into an egg with a black spot - it became oval! as it got nearer to focus, the egg turned into multiple stars!

could someone please suggest what's wrong with my sct?
or is it just tube current?

thanks in advance.

Hi, I am not fully sure, but you can try this.

Remove diagonal to make sure its not due to diagonal

try different eyepieces of focal length around 15mm to make sure its not due to the eyepiece.

SCT is a closed tube so should have small tube current problems. So make sure the SCT is outside for around 1 hr then do the star test.

If despite all above and the problem persists, then do the star test carefully, rack in and out of focus slowly and msg here again what are the intra and extra focus ring pattern.

I do feel that its due to the eyepiece, since you mentioned the "double image" effect which probably due to insufficient coating on the lenses.

[weixing]

Hi,
First, try the diagonal and eyepiece you used on a working scope. If the image is ok, then your eyepiece and diagonal is ok.

but as i got nearer to focus, the donut got smaller and slowly turning into an egg with a black spot - it became oval! as it got nearer to focus,

Does the major axis of the "egg" change direction as you change from one side of focus to the other?? If yes, then I think it is astigmatism.

the egg turned into multiple stars!

If it is cross, then I think it is still astigmatism. Err... is it possible that the star you do the star test on is a binary star or multiple star system?? If it really a single star, but turn into multiple star when focus, I've no idea what could cause this?? Hmm... just wonder will this happen if the corrector plate orientation is wrong??? :roll:

Have a nice day.

[Canopus Lim]

Do note that you have to star collimate at high powers as low power collimation is not as accurate.

Double image could be due to some reflections or ghosting. Perhaps you block stray light with the SCT dew shield and cover the eyepiece when looking through. If it still has that double image, it could mean other things that are not aligned properly or maybe you are looking at a double star?

If you are seeing the egg shape AT THE CENTRE OF THE FIELD, it most probably mean astigmatism. It could be the eyepiece astigmatism, the scope's astigmatism or your eye's astigmatism. Therefore, to make it less possible is your eye astigmatism, use higher power (exit pupil about 1mm or less). If the egg shape is still seen, despite at the higher power, rotate the eyepiece on the diagonal. Does the egg shape rotate with the eyepiece? If so it is the eyepiece problem. If it is not your eye and eyepiece problem, then it has to be your scope's problem. But firstly, take away the diagonal and check if the egg shape is there.

[acc]

But firstly, take away the diagonal and check if the egg shape is there.

Yup I think that is something you should try out first. Remove the diagonal or substitute another. Hopefully that's the cause of your problem..

[VinSnr]

hi all SCT masters, i need some advice on an sct newbie problem.

the setup in question is: c8 with 2" liuming star diagonal and 1.25" 20mm generic ep

i noticed some serious problems with the image:
- bright star appeared like double star near focus!
- saturn showed up as double image near focus
- out of focus stars suggested the sct was out of collimation - the center black dot was slight to a side, not too badly
- it is impossible to get pin-point stars

i tried collimating the out-of-focus donut, painfully, i managed to get what appeared to be reasonably centered black dot. the donut appears round enough at this stage.

but as i got nearer to focus, the donut got smaller and slowly turning into an egg with a black spot - it became oval! as it got nearer to focus, the egg turned into multiple stars!

could someone please suggest what's wrong with my sct?
or is it just tube current?

thanks in advance.

1) Make sure the magnification is at least 200X for a C8. You are only using 100X with 20mm

2) make sure you are not looking at a double star. Which star did you use?

3) Don't ever use planets! You need a point source...not planets!

4) Is the collimation of the diagonal ok?

Check the above first!

[kingkong]

ok. a brief test with other eyepieces after dinner showed that it's no longer double-staring. might be the cheap skywatcher plossil... btw, i was using sirius... then acturus...
then the clouds rolled in fierce and lightning getting closer.
will test again the next time the sky is clear again, which is rarely this few weeks.
this is my 1st collimation of the sct... i still haven't got to the high power airy disk part yet. will need to do that before i try webcaming saturn... the whole purpose of getting the c8 in the first place is to image planets. after that someone can have the scope. will anyone be interested in an old c8 with bob nobs and gso 2-speed crayford focuser? cheap cheap. may be s$800 may even throw in an meade lpi .
i think the scope was pre-owned by another singastrorian.

[Meng Lee]

btw, sometimes ghost images appear when looking at bright objects could be due to the trashy, so-called "Skylight Filter". I find that it deteroriates images seen in the scope, causing scattering, sometimes ghosting.

[kingkong]

oh.... thanks Meng Lee for that suggestion. i'll try with and without the skylight/dust filter also.

btw, did i say s$800? it will more likely be s$600 but will have to be picked up from my place . and also, it was pre-owned by an ex-singastrorian. i was clearly way past my bed time last night....

[VinSnr]

oh.... thanks Meng Lee for that suggestion. i'll try with and without the skylight/dust filter also.

btw, did i say s$800? it will more likely be s$600 but will have to be picked up from my place . and also, it was pre-owned by an ex-singastrorian. i was clearly way past my bed time last night....

you didn't mention you have a skylight filter. I suggest you remove that. That item sometimes give reflections on bright objects