Focus issues...

[biondi]

Hi,

I have a C8 SCT and I attached my Canon EOS 1100d via t-adapter to the ‘scope. I centre on a bright-ish star and attempt to focus. The star starts off as a big doughnut getting smaller and smaller. Simple I thought, get it down to a little pinpoint and job done. Hmm, before the star gets down to a small pin though it starts to get bigger again as if I’ve gone through the in-focus zone. I tried this a few times and I always have the same result, no problem with focus when viewing. I also have a focal-reducer so screwed that into the scope first but I still have the same result.

Does anyone know what’s going on here? I would understand it more perhaps if the focus went all the way but couldn’t quite reach the focus point but I can’t get my head round why it would go through it as such without being able to achieve it?

Is anyone able to explain what might be happening here?

Many thanks,

Matt

[Freddie]

It sounds like you are getting to focus (and beyond) but the star is just showing a little bigger on screen than you are maybe expecting.

[Steve Ward]

You need to get a Bahtinov mask , almost essential for imaging and takes out all the guesswork.

Either from Keith at StarSharp ( http://stores.ebay.co.uk/morris-engraving/Bahtinov-masks-/_i.html?_fsub=420715119 ) or FLO .

[biondi]

Thanks, I've just ordered a Bahtinov mask so will see how I get on with that

Out tonight again so will see if I have better luck though!

cheers,

Matt

[ollypenrice]

You are encountering the unsatisfactory nature of moving mirror focus systems. They are lousy. For imaging it is far better to add a traditional focuser (even a horrible Crayford which I'm famous for hating!!) because these give finer control. One thing that can help is to ensure that, as you try to find focus, you always push the mirror UP the tube. If you don't you'll find it carries on drifting DOWN the tube after you've stopped turning the knob. I can't remember whether that means going clockwise or anticlockwise with your SCT focuser knob but you can easily find out.

Unreasonable as it might seem, trying to focus by looking at a zoomed-in star is almost a total waste of time. It isn't accurate enough. Many capture programmes offer FWHM as a way of measuring stellar diameters but at longer focal lengths and in poor seeing it works badly. Bahtinov!

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233

[biondi]

Thanks Olly, can I ask what you do then? I've just seen your images, they are stunning.

cheers,

Matt

[ollypenrice]

Thanks Olly, can I ask what you do then? I've just seen your images, they are stunning.

cheers,

Matt

Glad you like 'em. On the 14 inch we now use a Bahtinov because FWHM measurements bounced around too much to give a coherent reading. This is because the seeing affects the long focal length images drastically. On the smaller refractors we use Bahtinovs to get very close then finish with FWHM.

A VERY good trick (thanks to Peter AKA Psychobilly) is to put your focus star onto the intersection of the 1/3 lines of your chip when you focus. This gives the best compromise in terms of critical focus, which you cannot assume to be perfect right across the chip. I can't recommend that too highly yet it is not widely known about, it seems.

Olly