Have I bought the right Bahtinov?

[MattB]

Hi all, could use some advice on my Baht mask I'm having trouble with.

From

First Light Optics - StarSharp Bahtinov Focus Masks

I bought the filter for a

Startravel 102 f5

to use on my

Celestron C102 Wideview (4"/102mm Achromatic Refractor - F5, 500mm Focal Length)

But when I tried to use it, the target came out in the shape of the bahtinov mask... which was unexpected! I'd already focused fairly close by eye (using a DSLR, I was snapping the Orion Nebula).

Any suggestions? Have I bought the wrong thing?

Many thanks,

-Matt

[lukebl]

Sounds like focus was way out, as you will get a little image of the mask when it's very out of focus. I have a mask intended for a 200mm scope, and it works perfectly well on my 250mm.

[MattB]

But when I took an image without the mask on the focus seemed pretty good. I focused by eye, shot a few frames, then put the mask on to try it.

Will have to try again, I may be mistaken.

Would it make any difference if the mask was on the end of the dew shield? Does the mask need to be perfectly centred?