Collimation and Focus

[Enimus]

Hi Guys!

Ive had my B&L SCT for about a month now and i'm having a fantastic time viewing the sky's (Mainly Saturn)

I am a tinkerer and like to work out exactly how things work, I also like to get the best out of anything i own and my telescope is no exception.

I have read up a lot on collimation and understand the concept and why to do it, I need to get some of Bobs Knobs as the little allen screws are a pain in the butt. I just wanted to know does the collimation have a direct effect on the scopes abillity to focus at higher mag, since I collimated I have had sharper views but I havent manged to get the collimation 100% there yet (hence why i need Bobs Knobs) and my 9mm and 4mm EP's, although better doesn't quite get there focus wise.

my scope is 6" with 1500mm FL and and F10, should I be able to see the cassini division with a 9mm or 4mm EP? I have used the 9mm with a 2x Barlow and its there but not crisp.

seeing has been pretty good BTW, once or twice at least

Thanks guys!

[StevieDvd]

How are you checking the collimation?

On an SCT it's not as 'straightforward' as on a newtonian - on a video I saw on checking collimation it basically said you collimate your scope to a default setting using a star test and then save that on a laser collimation tool so you can reset it back if it goes out.

The implication was that you can't just line everything up in your garage shed with a laser tool without knowing what the optimim setup of your SCT is.

Not that I've done my SCT yet, so I'm not stating from personal knowledge. But if yoiu have found a good method I'd be happy to take that on board too.

Hopefully any expert SCT collimators will add their comments here as well.

[Ad Astra]

FWIW, I've done 'star tests' during the day by focusing on the bright reflection off the chrome on the corner of a car. As long as you hit a point reflection, it will give you a very reasonable 'star' image. You use it on both sides of focus anyway, and the nice circular pattern pops right out.

Much easier to fiddle during the day, get things 95% accurate then tweak them into perfection at night using Polaris which doesn't move much, so much easier to lock on and adjust without it moving out of the field!

Dan

[Enimus]

Thanks for the advice guys. I have been collimating using the star test working from a 30mm going down to a 4mm

I've read that the laser method is not great for sct's and I'd rather put that money towards a new EP or something.

So I'm assuming the more accurately it's collimated, the crisper the images will be at higher mag?

I will def try out your daytime test. Fiddling about with an Allen wrench at night is a pain. Not to mention a waste of the sky

[rowan46]

I use this hartman variant mask for collimating it works well I made mine out of that craft foam you get from works for 1.99 for 10 sheets made a dew shield for my rdf out of it as well here's the link for the mask and instructions for using it Duncan's Decorating and Repair Services

[rowan46]

FWIW, I've done 'star tests' during the day by focusing on the bright reflection off the chrome on the corner of a car. As long as you hit a point reflection, it will give you a very reasonable 'star' image. You use it on both sides of focus anyway, and the nice circular pattern pops right out.

Much easier to fiddle during the day, get things 95% accurate then tweak them into perfection at night using Polaris which doesn't move much, so much easier to lock on and adjust without it moving out of the field!

Dan

that sounds a great idea for using the reflection off brightwork the only problem is this is the uk not california we don't just get cloud at night. I thought I would have a go and immediately the sun disappeared

[Ad Astra]

that sounds a great idea for using the reflection off brightwork the only problem is this is the uk not california we don't just get cloud at night. I thought I would have a go and immediately the sun disappeared

Sorry to hear that, Rowan - for what it's worth, it was 40F this morning and misty, not much better in the afternoon, and cloudy (again!) tonight. I used to think I lived in "sunny" California... I'm beginning to think hanging out on SGL has affected my local weather!

I know it has affected my idiom - I told a student the other day that they had done a 'Cracking job' on a lab and got a very strange and worried look back.

"Is that good, Prof?"

"Yo, dog, you dropped it like it's hot!"

[big smile now] "Thanks, Doc!"

Hoo boy!

Dan

[ollypenrice]

One detail of importance; do your collimating with the star very carefully centred in the EP. SCTs have a lot of coma as you move off axis and you can end up vainly trying to collimate this out. I was collimating our 10 inch with the help of a professional recently and he demonstrated this to me. I was surprized by how soon off axis the effect was perceptible.

In all this tinkering, don't back off your screws so far that you drop the secondary down the tube!!!

Cassini's is still tight at the moment with the ring orientation as it is.

An alternative to Dan's car bumper reflections would be a small ballbearing stuck to a black card and illuminated by a good fine beam. Works a treat. I got that idea from TeleVue.

Olly

[Enimus]

Thanks everyone for you advice, I have lots to be getting on with now. the ballbearing is a good idea, at least i can get it nearly there without wasting sky time.

Jay

[Moonshane]

An alternative to Dan's car bumper reflections would be a small ballbearing stuck to a black card and illuminated by a good fine beam. Works a treat. I got that idea from TeleVue.

Olly

this is a fab idea. how far away does it have to be?

[Ad Astra]

When I've done it with the chrome bumper trick, I've had to have something at about 10 meters or so away. Don't know if you could do it, say in your living room or garage at 5m or less, but I rather doubt it.

The ball bearing & black card trick sounds neat - as long as the bearing was bright and shiny!

Give it a go and let us know how it works for you.

Dan