I am having trouble getting good focus on my SBIG, I am not used to what I am seeing and it is different from the DSLR. Firstly how far off is the focus on these two images?
M42, 3 by 240, 3 by 60, 3 by 30, 3 by 10 + darks
SBIG 2K @ - 15c, TV85
M50, 1 by 60 + darks
SBIG 2K @ - 15c, TV85
Secondly here is my procedure
With the SBIG in continuous focus mode and watching the sharpness graph
(1) Select a subrame with a bright star and adjust till maximum sharpness and star looks OK.
(2) Select a subrame with a bright star, put on hartman mask and adjust till all spikes converge.
(3) Remove mask and set to full frame. Adjust till dim stars 'pop' into view.
Paul, now that youve spent money on a good cam, buy yourself an autofocuser and spare your self the grief, a TCF-S springs to mind. It will make a huge difference.
The focus still looks a fair way out Paul. This is a shot from an ST 7XME attached to my Orion ED80 using a WO 10:1 focuser. I use the subframe to get as close as I can using the graph, then check the full frame for faint stars and zoom in to see how many pixels they are covering. It can take some time.
(I'm looking forward to getting my SCT on a permanent pier with the motorized focuser )
Try your technique on fainter stars.
I use a medium to faint star, from the center of the field, in a subframe and focus until it has the smallest pixel size over a few downloads. It takes time at first but you become efficient. Then flexure stuffs everything, hehehe.
Keep trying
Paul, now that youve spent money on a good cam, buy yourself an autofocuser and spare your self the grief, a TCF-S springs to mind. It will make a huge difference.
Hi Fred,
No problem, I am looking at autofocusers but i already spent money on a 6 in 1 for the TV85 and that was 270 AUD.
To automate it
Plus motor 270 AUD
plus micrometer 270 AUD
plus cable 200 AUD
I will definately look at the TC-F but will try a bit longer manually.
The focus still looks a fair way out Paul. This is a shot from an ST 7XME attached to my Orion ED80 using a WO 10:1 focuser. I use the subframe to get as close as I can using the graph, then check the full frame for faint stars and zoom in to see how many pixels they are covering. It can take some time.
(I'm looking forward to getting my SCT on a permanent pier with the motorized focuser )
Hi P1,
Thats exactly what I wanted to know, as i was not very happy with the stars in the image so it is good to know I can improve it. I will try your technique and count the pixels.
Try your technique on fainter stars.
I use a medium to faint star, from the center of the field, in a subframe and focus until it has the smallest pixel size over a few downloads. It takes time at first but you become efficient. Then flexure stuffs everything, hehehe.
Keep trying
Thanks Jeff,
I will try this. Hopefully with the SBIG I wont have to worry about flexure
I have a Moonlite motorised focuser (manual controller) and SBIG ST7E CCD. I use the CCDSoft focus graph and have found it to be very accurate and reliable.
I get rough focus just by racking the focuser in/out whilst eyeballing the notebook display until a mag 2 or 3 star blooms.
Then I slew to a mag 4 to 6 star, expose for 2 secs continuously in x2 binned mode and touch up the focus using the ‘scope focus knob.
Then using the same star, I revert back to x1 binning (i.e. no binning) and draw a sub-frame around the star for fast downloads (I’ve got the old Parallel model).
Then watch the on-screen focus graph whilst fine tuning focus with the Moonlite controller.
Making very quick presses of the in/out buttons usually finds perfect focus.
This routine generally takes me around 15 to 20 minutes.
Ah, the joys of astrophotography.
They are both soft to me Paul.
I use the Feather Touch, and FocusMax, so it is automated, but you can achieve the same thing manually of course. OK, a good focuser will help, but the trick is to read what the figures are telling you.
Use a star that doesn't overexpose the pixel count, and I have found that about 40,000 is a good starting point. In my case if it gets to about 65000 it is useless. Draw the sub frame and use that in either continuous made, or expose each time manually. I also use the "Large View" and follow the numbers, mostly pixel count, but also flux and FWHM. Bear in mind that the readings will fluctuate with seeing and all manner of reasons. Get the pixel count as high as you can, but if it gets to that 65000 mark, try another and dimmer star.
Above all, keep asking for advice, it is the only way to learn.
Gary
Years ago when I to to play with a SBIG for the night on my system we found the best way is to make a Hartman Mask (disc with 2 holes in it on front of scope) put SBIG in focus mode choose a medium star. From what i can remember focus mode updates very fast, almost like Live View, the Mask makes stars appear double if not focused. Adjust focus till double star comes together. It took us moments to do and gave near perfect focus each time.
Scott
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