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Focusmax Settings for Narrowband

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#1 Swanny

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Posted 17 September 2019 - 11:52 PM

I am having a bit of trouble with the focus on Focusmax and narrowband filters (Chroma 5nm). I have it set to a max of 30 seconds because 15 seconds wasn’t enough per the program. So 30 has it exceed the 15mn to focus. Any ideas on settings I should change to make it work would be very helpful. Thanks!

#2 Jon Rista

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Posted 18 September 2019 - 12:54 AM

So...just to make sure I understand...you are acquiring 30 focus samples? If so...why so many? Can you use fewer than 30 samples? Would 10 samples be enough for FocusMax to find the point of focus?


Edited by Jon Rista, 18 September 2019 - 12:54 AM.


#3 Swanny

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Posted 18 September 2019 - 01:05 AM

I am acquiring up to 30 seconds per image for focus. In order to focus on the star otherwise it cannot find star. It went through the whole process at 15 second exposures then, when focusing at the end, said couldn’t find the star.

#4 Jon Rista

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Posted 18 September 2019 - 01:28 AM

I am acquiring up to 30 seconds per image for focus. In order to focus on the star otherwise it cannot find star. It went through the whole process at 15 second exposures then, when focusing at the end, said couldn’t find the star.

You said it took 15 minutes though. For the process to take that long, I assume it would have to take 30 exposures at 30 seconds each... Is that actually the case? If you are taking 30 exposures, why?

 

As for not finding the star, that could also be due to taking too many exposures. Depending on the focus step size you have set, moving out 15 samples from proper focus may very well put you so far out of focus that the signal from the star is spread out so far it cannot be picked up by the sensor anymore. In which case, you definitely wouldn't find any stars to focus on.

 

Anyway, more information is needed. How many samples? How large a focus step size? When you start focus, what does the first frame look like? Are the stars HUGE defocused discs? If so, are they ultra faint? If so...then that's a problem. :p



#5 Chucke

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Posted 18 September 2019 - 01:33 AM

Have you correctly determined the near focus position as described in the documentation?  If so you shouldn't be so far from focus as to make the out of focus star too huge.


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#6 Swanny

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Posted 18 September 2019 - 01:44 AM

I don’t think the star is out of focus too much.  The near focus takes just two steps to get there.  That is not a long time.  It is when it runs all the close focus exposures where it eats up the time.  Probably 15 or so.  Let’s say I am focusing with my IR cut filter....it takes 2 min and 20 seconds.  I have no issue with Focusmax other than on the narrow band filters.  I must need to change a setting to not exceed the 15m.  I had to increase the exposure time to 30 seconds to get past the near focus run.

 

The image from focus has dim stars that are already tight.  I was doing the inner part of the Iris first with the IR filter for a later run once the moon is not an issue.  So I was already pretty close.



#7 Jon Rista

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Posted 18 September 2019 - 03:08 AM

I don’t think the star is out of focus too much.  The near focus takes just two steps to get there.  That is not a long time.  It is when it runs all the close focus exposures where it eats up the time.  Probably 15 or so.  Let’s say I am focusing with my IR cut filter....it takes 2 min and 20 seconds.  I have no issue with Focusmax other than on the narrow band filters.  I must need to change a setting to not exceed the 15m.  I had to increase the exposure time to 30 seconds to get past the near focus run.

 

The image from focus has dim stars that are already tight.  I was doing the inner part of the Iris first with the IR filter for a later run once the moon is not an issue.  So I was already pretty close.

So it sounds like you ARE taking lots of 30 second exposures? Why so many? I haven't used FocusMax in years, and am mostly familiar with SGP. But I am able to complete an entire focus run, with 15 second subs using 3nm filters, with just 9 exposures. Taking 20-30 exposures seems like overkill... Can you configure that, reduce how many exposures you take for the near focus run? IIRC, FocusMax does a parabolic or hyperbolic fit, right? You shouldn't need all that many samples within near focus to get a good fit...



#8 Swanny

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Posted 18 September 2019 - 03:27 AM

Yeah.  I have run all the V curves for the library and all.  That is what I am trying to find.....how to reduce all the exposures itnia taking.  Tomorrow I will give it a deeper dive and see what I can do:  



#9 Swanny

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Posted 18 September 2019 - 04:12 AM

For fun I tried the 10second focus on M45 using my Ha and it worked fine.  I think the issue is that around the Crescent there is not a bright enough star to focus on in 10 seconds.  I don’t mind stating the night off focusing on a brighter star to get focus.  But the temp drop tonight from start was a little over 4F.  With my scope I like to refocus every 1 degree or so.  And don’t want to have to move around in between.



#10 rgsalinger

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Posted 18 September 2019 - 10:53 AM

First of all, Focusmax V4 allows you to slew to a bright enough star to avoid this problem. It's called Acquire Star. I think that you must be using a free older version. You have the SKYX, if you have Image Link then you can use this feature. I slew to the nearest 5th magnitude star and that allows me to use just 10 second at most exposures. Second, you can bin the exposure 2x2 which gives you a 4x better signal. I have found - test with your equipment - that best focus is obtained at the same place regardless of binning. You will never get the exact same number, of course, but my results are always within the CFZ for my scopes. 

Rgrds-Ross


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#11 Swanny

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Posted 18 September 2019 - 12:26 PM

I have the purchased version 4.  I use acquire star when commencing a focus.  I must have a setting in there under camera I think at 1x1 binning.  I was looking for where to change binning last night.  At 2am or so I thought hey....I gotta work tomorrow.  I did get focus and a few hours on the object.  I just want to make it work 'better' which is on the user not the item.  The only time in this hobby that wasn't the case was with my camera.  

 

I do have SkyX and will try to synch them up tonight.  I had plans to start narrow band imaging this spring when I got my filters.  But a camera sent to the manufacturer....as well as monsoon season....has put me behind 3-4 months.  I have been doing visual again in the interim until last night.  

 

Then I will work on the temperature compensation Focusmax has in there..  



#12 rgsalinger

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Posted 18 September 2019 - 01:08 PM

If you are using Acquire Star then you must be using the SKYX Image Link. Within the Acquire Star configuration panel there are settings per filter for how bright/dim a star you want to focus on. With my LRGB setup I tend to use stars around mag 7 with my F8 telescope. With a faster scope you will need dimmer stars. With narrow band (if memory serves) I'm using 5-6 mag stars and I've see the focus time to at least 2 seconds. 

 

Within the focus setup there are two binnng parameters one for "pointing" images and one for focusing images. I keep both at 2 these days.

 

Rgrds-Ross 


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#13 Chucke

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Posted 18 September 2019 - 01:54 PM

Possibly your focus convergence parameters are set too tight for your seeing?




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