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Old 03-08-2019, 03:54 PM
morls (Stephen)
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Focus

Hi,
I've been reading about Bahtinov masks, and have just had a revelation (or at least, understood something very basic about focus).
If I'm moving from object to object, I re-focus each time. However, some observers using a Bahtinov mask describe reaching focus with the mask, and then beginning to observe/image.
Leaving aside adjustments needed when doing long exposures and precise imaging, is it the case that once focus is achieved on a star then all other objects will be focused?
I can understand the idea that all other stars will be focused, being ostensibly infinite points, but what about planets?
I know this is probably a very elementary question, but it's something I've not considered until now...
Cheers
Stephen
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Old 04-08-2019, 06:50 AM
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Outcast (Carlton)
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Hi Stephen,

I can't answer with any definitive knowledge but, I use a bhatinov mask on a star first when doing planetary imaging & my images seem to come out pretty sharp so, I would think from my experience the answer is theoretically yes.

However, the issue would be whether or not all your eyepieces are par focal. If not, you would need to refocus on change of EP.

Hope that's helpful...
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Old 04-08-2019, 07:46 AM
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Once focused it should stay in focus but it does not necessarily stay focused. A variation in temperature changes your focus so at the pointy end of astrophotography folk spend big to get a focused with a heat sensor so software can adjust the focus. And I expect ones scope may flex in different ways as you change your view which may alter focus..I find with my simple photography rig it helps to check focus regularly on long runs of exposures to be safe and certainly on changing from one filter to another. I expect the elevation of objects plays a part also. I suppose the need to refocus comes with some kind of change in something.
Alex
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Old 04-08-2019, 07:52 AM
RyanJones
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Hi Stephen,

A bahtinov mask is used to achieve true focus for imaging as the camera sensor does not have an adaptive lense like your eye. Yes it will get the focus close but you will still have to make adjustments for your particular eyes. I personally have pretty bad eyes. If I focus with my bahtinov mask for imaging, visually I see nothing but blur. It it true however that a bahtinov is used to achieve infinite focus and as such once you are focused on anything in space, everything is in focus. I hope this clarifies things a little.

Cheers
Ryan
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Old 04-08-2019, 08:19 AM
Startrek (Martin)
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Stephen
Your focus will definitely drift out a bit during long imaging sessions, happened to me last night whilst imaging M22, especially from lower altitudes to higher altitudes towards the zenith
Check your focus every 2 hours or sooner if seeing is poor
Cheers
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Old 04-08-2019, 09:37 AM
morls (Stephen)
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Thanks everyone, really appreciate the explanations. Always something to learn...

Stephen
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Old 04-08-2019, 06:09 PM
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How often you need to focus when imaging can also depend on what scope you are using too, my little ED72mm is stable for ages, but with temperature changes my SCT shifts focus drastically in very short order, it can be from pin sharp to out of focus doughnuts in 15 minutes if the temperature change of the night is large.
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Old 07-08-2019, 04:00 PM
tileys
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I have found that I need to refocus occasionally during an imaging session (esp if imaging across different sections of sky in one session) - I tend to use the FWHM feature in Sharpcap and when happy then I image. I was tending to do that once at the beginning of an evening and then assuming that since I was not touching the focus tube or camera - wouldn't need to do so again that evening BUT I found that occasionally my focus looked a bit soft. My scope is a 8" F/4 though and focus is pretty fiddly at the best of times with those units.

Using the FWHM feature is a quick step to incorporate every target if needed and focus is such a critical factor in being happy with the end result so I have adapted my workflow to suit.
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Old 08-08-2019, 07:40 AM
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With probably 700mm of tube between the primary and secondary mirrors a quick bit of googling suggests that 1 degree of temperature change of an alloy tube will shift your focal plane by a couple of hundred microns, assuming that you are using the ASI294 camera your critical focus zone at F4 is about 40 microns. Given that, it is certainly worth refocusing a couple of times during the night at least.

On my SCT in hotter weather (Using an automated focus motor) I was refocusing after every second 5 minute sub, better to throw away the time I could have used to get half a dozen subs in the night than to end up with heaps of soft subs. That certainly happened a few times before I automated focus.

I have a Celestron focus motor on my SCT and the ZWO one on my ED72, I am a bit tempted to sell the Celestron one and get another ZWO unit to fit to it if I keep imaging with that scope as the Celestron focus motor lacks a temperature sensor, hence refocusing every couple of subs.
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Old 08-08-2019, 08:43 AM
tileys
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Wow - 40 microns - really - it's a wonder it ever achieves and keeps any kind of focus.

I intend to get a ZWO EAF unit when they can supply a bracket that fits the GSO 10:1 focusser.
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Old 08-08-2019, 09:03 AM
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That "last little bit" can sometimes give a notecably better result. I have taken to looking at the image stats in SGP as the sequence progresses, the plan for future sessions is to take more subs than I intend to stack, then dump any with a satellite trail and then figure out an arbritrary value for average HFD or FWHM (Focus) and median pixel avlue (Background brightness) and toss any subs that fail either of those. Better to integrate 40 really good subs than 60 "OK" ones.
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Old 08-08-2019, 03:18 PM
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multiweb (Marc)
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I think focus is the most important thing when you image and the most overlooked. It depends on your scope and how big your CFZ is. With my old Q if temperature drops 0.5 degree I'll be out. With my C11 it's a lot more forgiving. With my CN-212 it's sharp enough to show small changes even at long FL.

So focus goes out quickly at the beginning of the evening when you start your imaging session then stays somewhat linear throughout the night provided there aren't rapid temperature shifts.

I do the initial focus with a Bahtinov. It's quick and easy and good enough. Keeping focus while imaging is the challenge.

Practically when some scope cool they tend to contract or expand so you'd need to rack the focuser out or in to compensate. If you use an OAG it's very easy to see your guide star and other faint stars in your guider fov. You know you're close to focus when you can see other fainter stars in the same field. So that gives you a good indication of where you sit. You focus and more stars appear or disappear.

Another way to check where you're at is to view the FWHM of your subs with a tool such as CCD Inspector as they are saved to your HD. If there's a trend towards growing values then unless the seeing or your guiding has turned bad you know you're slowly going out of focus and you can compensate accordingly while you're imaging before it gets too bad.

I know there are tools such as Focusmax or others that will use a V curve to find perfect focus but what's the difference doing it with a Bahtinov mask if you're going to be out within the next 30min or hour? You'll have to spend time doing another V curve and waste imaging time. Unless you're remote doing it manually is quicker.

I'm now looking into a system called Lacerta done by the bloke who did the ONAG. That might be a good all round solution to non stop focusing if it works as advertised.
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Old 08-08-2019, 03:42 PM
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The_bluester (Paul)
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Quote:
I think focus is the most important thing when you image and the most overlooked. It depends on your scope and how big your CFZ is.
I agree there, I keep being tempted by a new 8" RASA to do my stuff at F2, images are finally turning up on Astrobin to get some sort of handle on what they are like, but many of them look out of focus to me. A combination of not quite getting it right on a scope with a very small CFZ and/or leaving it alone for extended periods.

My ED72 is a lot more forgiving, but the C925 would be wasting subs after half an hour if it was reasonably early in the night. That said, before I got it a focus motor I did see telltales that even the ED72 shifted somewhat over time as it cooled down in the evening.

I automated focus for both my scopes as I am aiming to automate as much as I can. I believe the less times you physically touch the scope or mount in the night the better. Cost, among other things means that is not an option for a lot of people but I can manage it so automated I go. If you can afford it, IMO you will never look back once you set it up!
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Old 08-08-2019, 03:45 PM
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multiweb (Marc)
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Another thing I do is wrap the aluminium tubes in insulated silver bubblewrap and velcro them. It stops the tube getting too cold too quickly. It made a big difference for me especially with the oldQ. Not perfect but definitely helped as well with the CN-212. My C11 I replaced with a CF tube so no issues there.
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Old 08-08-2019, 04:26 PM
morls (Stephen)
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Quote:
Originally Posted by multiweb View Post
Another thing I do is wrap the aluminium tubes in insulated silver bubblewrap and velcro them.
I've got this type of thing happening with my 180Mak, and while only visual up till now I've been able to begin observing straight away, without waiting for the scope to cool. It'll be interesting to see what happens when I start imaging.
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