Collimation- RC 6"

DamienB (Damien) · #1 11-02-2023, 11:08 PM

Hi all,

So I purchased a GSO RC 6" scope. F/L 1350 f:9. Oddly enough, it doesn't come with top mounting screws/inserts for a guide scope. No mind, I 3D printed one.

I've used it once to image the comet and it was ok. I am yet to check collimation and having watched a few of the videos online, a number of people used a laser insert which I grabbed off ebay. It's arrived at my post box, will collect mid week. I also purchased a tilt adapter when I ordered the scope as again some videos used one and said it was important.

But it did raise a question that a discord I am on didn't answer.

Is there a website where you can upload a photo/exposure and it'll analyse it to say "check collimation"?

Whilst I do understand if you have oval stars or discoloured then yes, collimate. But what if it seems ok? How would I be able to tell if it's slightly off as someone who's never had a scope with collimation before?

PaulSthcoast (Paul) · #2 12-02-2023, 01:47 AM

Damien,

The best way to check your colimnation is with a star test.

Find a star field and bring it in and out of focus.

You will find the stars 'bloat' and form..
Doughnuts.

This is the shadow from your secondary mirror.

If the hole is in the center of the star doughnut..
the scope is in, or close to colmination.

See attached images for reference.

Some images are from a newtonian but the
principle is the same.

Note how the holes in the doughnuts are off center

This is out of collimination.

Paul.

DamienB (Damien) · #3 12-02-2023, 10:43 AM

Thanks. I plan to do this in the next few days. I am just surprised someone has created a simple site where it analyses the image (even if the rules are to upload a photo of an out of focus star). It can then assess the circles and report: collimation 90% etc.

doppler (Rick) · #4 12-02-2023, 01:29 PM

Hi Damien,

With an RC scope you need to use kid gloves, very very small adjustments go a long way. A newtonian has most of its collimation done on the primary whereas a RC you do most of the collimation on the secondary (I've had my 6"RC for a couple of years now and have not touched the primary yet). These are push pull adjustments tighten one screw loosen the opposite one. I just use a plain visual Cheshire collimator to fine tune the secondary. Hope this helps.
Cheers Rick

DamienB (Damien) · #5 14-02-2023, 08:28 AM

Thanks. I had a go and used the laser. Collimation of the laser was ok (followed videos about rolling it while watching it on a wall /X marks the spot).

Collimation of scope seemed to go ok, laser adjusted in the middle of the dot on the mirror and was then in the middle of the laser eyepiece.

One thing I am unsure on, and if it's related is I can't seem to get good focus. I have good focus range; aka can focus right in until i see the spider/circle in the star and also focus right out. But that middle spot never seems crisp. I even tried with a bahtinov mask. With my refractor I could get stars as pinpoints and a bahtinov would give nice sharp spikes.

I took a couple of exposures and the stars seem odd. I'll post a picture tonight when I'm home from work. Could poor collimation cause this?

*edit. Managed to grab a crop of one test image I took. 300s exposure, iso 1250. Uploaded.

doppler (Rick) · #6 14-02-2023, 09:05 AM

If the the star shapes are the same (off center in the same direction) across the whole image it might be camera/focuser tilt. Hard to tell with a cropped section.
Rick

DamienB (Damien) · #7 14-02-2023, 07:24 PM

Uploaded two uncropped/edited shots. One is the zoomed way out whilst pointed at jupiter to reveal the spider. The next is the standard 1250 iso, 300s exposure i took. Tracking was ok, not brilliant. But when you zoom in, all the stars have black spots in them.

So unsure what my next steps are.

doppler (Rick) · #8 14-02-2023, 09:05 PM

Hi, did you get a user manual with the scope? If not here's a link to one, it should give you an idea of what you should be looking at. A high power eyepiece out of focus on a reasonably bright star will give you a better idea on the scopes collimation.

http://www.ioptron.com/v/Manuals/611X_RC_Manual.pdf

DamienB (Damien) · #9 15-02-2023, 11:07 PM

Well i think im not far off. Spent more time in the dark with it on a target and also using the laser collimator.

This is simply a 300s exposure and in jpg as i was about to pack up. Ignore Orion being off centre, i just needed an easy target and to see how the stars turn out.

Do you think i need to keep tweaking it?

doppler (Rick) · #10 16-02-2023, 10:13 PM

looking pretty good, star spikes are nice and thin a good indication that focus is sharp. There's a bit of elongation of the stars but all in the same direction, so just a bit of tweaking on the tracking. But yes very close.
Rick

DamienB (Damien) · #11 17-02-2023, 08:00 AM

One thing I am not sure on are the larger stars seem to have this light burst? It's a nice effect and I don't mind so am pulling at hairs. Just curious as to what would cause it as it's only on half of the larger stars.
Uploaded a cropped image.

Stefan Buda · #12 17-02-2023, 10:14 AM

Those diffraction "rays" are most likely produced by the edge zone of your primary or secondary mirrors. This can be a rolled edge or even dirty optics that are dirtier near the edge. The one-sided distribution is due to vignetting.