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GordonH

"Tadpoles" - first light with Takahashi BRC-250

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Hi all

I have been dying to get out and try my new scope and finally lastnight there was some clear sky to give it a try. Focusing was very difficult as the BRC-250 uses a helical focuser and when you turn it the camera assembly moves as well and as the scope was a long way from near focus this meant alternating between the focuser and the instrument rotator in order to stop the cables from twisting up. The focuser is very smooth but at the same time very firm so it takes a bit of turning. It took about half an hour just to get focus. Because of the camera rotating I didn't use my usual method of using one star to focus on in Maxim DL and using the inspect tool, instead I did it visually using the difraction spikes on a bright star as a guide (when out of focus the difraction spikes were split) so please excuse the image if it appears slightly out of focus. I am waiting for an adapter to fit my FLI PDF focuser in the system and when that is in things will be much easier as I can get close to critical focus with the helical focuser and then lock it it and leave everything to the PDF focuser, so at the moment the camera is not the correct distance from the built in field flattener. Anyway, enough of the excuses. I managed 8x10 minute unguded subframes of IC 410 of which 4 were ruined by cloud and after the 8th subframe that was it for the night as the clouds stayed. So this is a 4x10 minute image in Ha only. Only simple auto dark subtraction was done and no flats were applied. I have included the full widefield and an extreme close up of the Tadpoles themselves

Thanks for looking

Best wishes

Gordon

:(

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post-13589-133877353565_thumb.jpg

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Glad you got to play at last Gordon - you must have been itching to get that out!

That focuser sounds like a bit of a pig - whats the benefit of having a twisting arrangement? stability? I've not heard of a helical focuser before, I must admit!

(google break)

Ahhh - that makes sense - it keeps the mirror in the same plane and stops it shifting :(

Looks like this is going to be a great scope though - us mere mortals would be happy with those - nice one!

Cheers,

Richie

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Well done Gordon....glad you've managed to catch some photons :(

Apart from 1 x 15 minute sub, I've not managed anything with my new rig yet !

I used to have a helical focusser on my old Charles Frank 6 inch newt....it was pretty good actually, but in those days I was purely a visual observer so the rotation wasn't a problem. PITA for imaging though....what on earth were Takahashi thinking of :)

Cheers

Rob

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nice first light gordon...

the focusser does seem a tad overinvolved.

but a consistant, accurate focus is required for imaging. especially long subs.

the FLI PDF will hopefully clear things up, then focus will be a doddle, and take 5mins.

great image

paul

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Looking pretty good Gordon the stars look pretty tight. I spent 10 hours on this a couple of weeks ago and I'm still not satisfied with it. I will give it another go when conditions are good.

Regards

Kevin

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Only the cheapest Borg focusers rotate - the more costly ones are very good for accutate focus. Nice start though considering all!

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Nice Gordon, the quality of the tak shows through there for sure.

As for focussing, get yourself a decent Bahtinov mask. (Have a word with psychobilly!) You could have got it to perfect focus in about 60 seconds. They really are that good. I used to waste literally hours of precious imaging time focussing, but since getting the bahtinov that time is spent imaging. Once you see how accurate the mask is, you know you can absolutely trust it, more than maxim or your eye.

I have a 1:10 focusser on my GSO. Using the star image from the mask I can SEE the difference to focus that 1/30th of a turn, (or maybe less) will make. Also, if tightening the lock screw should happen to nudge focus slightly out, you can see exactly which way it nudged it, and by how much, so that you know how much to compensate by. (Shouldnt think a Tak has that problem tho!)

Looking forward to some more from this setup, thats for sure!

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Hi TJ

Once I've got the PDF focuser in position it shouldn't be a problem as each increment with that is 1/14000 of an inch. I always find the inspect routine in Maxim DL gives very accurate results using the FWHM measurement on a particular star and getting that figure as low as possible. With the H36 focusing can be a bit slow because even when using small subframes in the focusing routine the read and download time mounts up to about 10 seconds or so, so I think 60 seconds is a bit optimistic for me and have you ever tried using a BRC-250 focuser? It is so solid and absolutely no play whatsoever in the threads so it takes about 10 seconds to do one complete turn of the mechanism which in turn moves the focuser in or out 1mm. Thanks very much for the tip though, see you at Astrofest

Best wishes

Gordon

:cheers:

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It all looks very promising Gordon, and so it should be!!! It's going to be very exciting seeing what you come up with. So many great targets for you with the galaxy season on the way!

So is the H36 chip a full frame job like my Kodak 3200? A real pain when it comes to focusing. Apparently if you place the focus star at the top left and make a small selection this can make things a tad quicker with full frame CCDs. Can't remember how that works but it helps a bit.

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That's the great thing about those masks. The focus is every bit as accurate as maxims fwhm figure, but you arrive at it in a fraction of the time. Mind you, i'm sure that if a Tak ever comes my way, I'll probably spend more time fiddling with it than imaging :(

Will you be on TWC stand? I'll look out for you!

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Hi Guys

Martin, the H36 is a Kodak KAI1600M interline 35mm format chip and I wasn't aware of what you are saying about choosing a star at top left. What I normally do is select a medium magnitude star and use a small selection box around it for focusing, this speeds up focusing a bit but you still need 20 second exposures when using a Ha filter to focus through (I only have narrowband filters loaded at the moment)

Tim, I am not sure if I agree with you totally about the mask being as acurate as the FWHM number using the inspection tool in the focus routine. My reasoning is this:-When I am focusing with the Maxim routine using FWHM I adjust focus while keeping an eye on the FWHM number, the lower it goes the closer I am to critical focus, if the number goes the other way then I know I have gone past critical focus and I readjust to get back to the lower number. I have been using this method with my TMB using the manual 2 speed focuser with a lot of success. When focusing like this I can also see the star on the screen and to be honest with you I cannot tell any difference in the focus of the star by looking at it when it is a bit either side of perfect focus and I suspect it will be the same with the mask in that when you are a tiny bit either side of perfect focus you may not be able to detect any difference in the position of the central spike. Using the PDF focuser will give even more acurate focusing with the Maxim routine. Why is this? The PDF focuser has a maximum focusing range of 0.5 inches but it does this in 7000 steps so each step is 1/14000 of an inch, you can adjust how many steps per mouse click so as you get closer to critical focus you use less steps per click. This becomes more important when using short focal ratios as the critical focus zone of an f5 system is typically about 7 microns. Don't get me wrong, I think you can get very good focus with the mask in a shorter space of time but I think you can only get absolutely critical focus using routines such as maxim and focusers such as the PDF even if it takes a while longer.

I will be at Astrofest on the Widescreen Stand on both days, if all goes well I will be running a beginners guide to narrowband imaging powerpoint slide show.

See you both there

Best wishes

Gordon

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Ah I see, it's the exposure that takes the time rather than reading the chip and download. My CCD has frame transfer architecture which is supposed to be more sensitive than interline but can't just read a small subframe so it is reading the chip that takes the time. Interline CCDs are much much easier to focus. Picking a star in the top left only applies to frame transfer CCDs.

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Hi Gordon,

Thanks for the discussion on focus. I wonder if you could give a little advice :(

I use the Bahtinov mask as well, and find that it does provide a very simply way to get quick focus. However, I also finish off by checking the FWHM values on a single star. What I don't get is how you deal with the fluctuations in the FWHM value during your focusing run. At the moment, I focus at 0.5 seconds but find the FWHM values bounce terribly. Does focusing at 20 seconds fix the atmospherics somehow?

Thanks,

Richie

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Hi Richie

No, unfortunately longer exposures don't cancel out the fluctuations and this is probably the main disadvantage of focusing this way. Sometimes you may have to do several exposures for each focusing step especially when there is atmospheric turbulance. You just have to try and judge when you have the lowest number. Using an electronic focuser where you can record the steps is better as if you go to far you can count how many steps to go back. Some nights are better than others in this respect. At the end of the day there are no short cuts to getting exact critical focus, you can get very near with the mask but if you want to be spot on then this is where software such as Maxim DL comes in but you have to devote a bit more time depending on conditions

Best wishes

Gordon

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