Confession time

[Carole]

I have finally succumbed to an electronic filter wheel.  

It's been getting more and more a of a chore to keep going outside to change the filter and the last straw was when I had done a meridian flip and it had missed the target afterwards.  As I was doing NB it was easier to re-find the target using my empty filter position and then move back to Oiii once found.  
Well being at the meridian the only way I could check that I had then re-selected the right filter was to lay on my back to see the number underneath the FW    (I was using my long ED120 scope).

Also if I can get it all working it will be nice to be able to set a sequence instead of getting all HA one night, all Oiii another night etc.

The main problem had been trying to find a FW that could take 7 positions for 2" filters.  

Anyway I found out that the Atik EFW2 will take 36mm unmounted filters which will work with the Atik383L, so I took the plunge.  

So I will eventually have 2 x 4 position 2" manual filterwheels and a set of 2" LRGB and NB Baader filters for sale, though some-one has already put a tentative bid in for the NB ones.  

I am not selling them until after Kelling though as I have a friend who wants to try out mono imaging and I said he could borrow them.  

Hopefully the weather will recover in time for next weekend.  

Carole

[RobertM]

But you should still have to go out side and focus after a filter change

[The Thing]

Digital focuser next then.

[Carole]

But you should still have to go out side and focus after a filter change

I theory yes, but I have parfocal filters and every time I have checked them over the last couple of years I haven't had to change the focus, so I have given up bothering.

Carole

[RobertM]

Parfocal filters - if only !  Believe the hype at your peril; they might appear parfocal at f/8 but they are far from being exactly so.

I think refocusing is your next challenge Carole, certainly some of your NB images appear to have different size stars for Ha and OIII and leaving the focus as is might be part of it.  Refocusing between filters is something I've been doing for a long time.  Duncan is right, automation removes the guesswork since sometimes the only way 'out of focus' is visible is after a 10 min sub and then you're cursing...

Robert

[Carole]

Actually I do myself an injustice and I don't know why I said that as I have been doing only 1 filter per night and checking the focus beforehand.

Sorry, had a heavy few days with one thing and another.

Carole

[JohnP]

remember having this discussion with Carole a few months back...

[Carole]

I always use the Bahtinov mask with the central spoke dead centre. 

Carole

[RobertM]

I always use the Bahtinov mask with the central spoke dead centre. 

Carole

Yes but that can still mean you are out by enough on longer subs if you do it manually.  I've found it difficult to get accurate focus at shorted focal ratios - it looks good with a 1 min sub but not 10 mins where everything is more critical.

Do you use Bahtinov grabber to ensure you get within the CFZ ?

Robert

[Carole]

Stopped using Bahtinov grabber as it was giving me false readings.

Sometimes use FWHM

Carole

[The Thing]

Bahtinov grabber is always right. it uses physics to get around problems calculating from a star image that's varying with setting etc.

[Mike]

I use the version of BG that's built into APT.

[Carole]

Bahtinov grabber is always right.

Sorry but it's not always right.  It was telling me I was in good focus when I was miles out.  that's why I stopped using it.

Carole

[Mike]

Try the version that's in APT.

[RobertM]

Bahtinov grabber is always right.

Sorry but it's not always right.  It was telling me I was in good focus when I was miles out.  that's why I stopped using it.

Carole

Then you must have something wrong with your imaging train.  The star should be in the centre of the ccd when you use it otherwise any misalignment in your optics will mess focus up.  Now it could be that misalignment shows in a differing focus across the image but that's no fault of the mask.

I would suggest focusing on a centred star (luminance filter) using the BM then leave it on and take a longer sub to ensure the mask lines appear on stars over the frame.  Pick a good star field for this.  That should show any optical/alignment issues.

Robert