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INDI Library v1.8.5 Released (19 Apr 2020)

April 2020 release of INDI Library v1.8.5 introduces new drivers while providing fixes and improvements to existing devices and core framework.

Where to set focus offset for different filters?

11 months 2 weeks ago
mlarsson
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Where to set focus offset for different filters? #42603
Hi!

I just have got my first filter wheel. And now I'm beginning to play with it. So I am wondering about focusing - should there not be a place to set a "focus offset" between filters, so I can use the L filter to focus and then capture with, say, Ha? If so, where is this setting?

Magnus


C8, Losmandy G11, Canon 1000D astromodified.

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11 months 2 weeks ago
rlancaste
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Where to set focus offset for different filters? #42613
Hopefully this helps:

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1 month 1 week ago
airscott
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Where to set focus offset for different filters? #56421
I realize this answer is almost a year old, but I have a closely related question:

Since filter offsets are set inside the Sequence Queue, does this mean they are "local" to a specific sequence?

In other words, do I need to set them separately for *each* sequence? Our do they apply to *every* sequence that uses the filter wheel?

Thank you for any advice on this.

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1 month 1 week ago
mlarsson
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Where to set focus offset for different filters? #56428
Hi!

I'm not sure what you mean with setting it in the sequence queue. You can access this offset window from several places, if I am not mistaken, for instance from the Capture module. And they are global settings. When changing filter, these offsets are used, regardless of what is in the Scheduler sequence. So: global. You set them once.

Please correct me if I misinterpreted.

Magnus


C8, Losmandy G11, Canon 1000D astromodified.

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1 month 1 week ago
porkyhttp
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Where to set focus offset for different filters? #56438
But OFFSET value are related to what main value???? The value of the first declared filter??? I have filters in these sequence L-R-G-B-5-6-7, if I want configure offset what i have to enter? Example 0 for L, 20 for R, 15 for G and 25 for B????

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1 month 1 week ago
rlancaste
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Where to set focus offset for different filters? #56447
The offsets are relative to each other, so if you have a bunch of filters that are parfocal, call those 0 and do the other offsets relative to that, that would be easiest. If they are all different, then it’s up to you which one you call zero.
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1 month 1 week ago
porkyhttp
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Where to set focus offset for different filters? #56463
Baader filter shuould be parafocal, but I see a little difference from a filter to another. So as you wrote I should set 0 for the first filter and then add +20 for R, -5 for G and +30 for B refered to my example "0 for L, 20 for R, 15 for G and 25 for B".
Correct?
Obviously automatically Ekos have to change the focus in order to respect the offset values

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1 month 1 week ago
rlancaste
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Where to set focus offset for different filters? #56465
I don't believe you understand what I am saying. The filter offsets are just simply relating how the focus distance of each filter compares to the other, NOT how far to move it when going from one filter to the next in any particular sequence. So your filter offsets could be 0 L, +20 R, +15 G and +25 B, or they could be -20 L, 0 R, -5 G, +5 B and it would have exactly the same effect. Also it doesn't matter what numbers you use, as long as the relative distance between the offsets remains the same. For your example, +1000 L, +1020 R, +1015 G, and +1025 B would work exactly the same as well. Ekos will calculate how far to move it between each filter based upon difference between the offsets. So it doesn't matter how you order them in your imaging sequence or even if you switch filters in a different operation. The offsets are global. If you set the filter offsets relative to each other, then any operation Ekos does, when it changes the filter for any reason, it takes the filter offsets into account and adjusts the focus by the difference between the two offsets of the filters it switches between.
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1 month 6 days ago
airscott
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Where to set focus offset for different filters? #56516
Thank you Rob!

"The offsets are global" is precisely the answer I was hoping for.

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1 month 5 days ago
jiberjaber
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Where to set focus offset for different filters? #56546
I noticed my filter offsets are now zero for some reason - so a good time to see this thread!

So to be clear, is the method below of establishing the offsets correct for what you have explained? :

Assuming L,R,G,B,Ha,OII,SII in the filter wheel (L = position 0)

Autofocus all filters, note absolute value of focus position for each filter, for example Lf = 1000, Rf=1025, Gf=950 Bf=1001 etc...

Keep offset value for L as 0 so all other offsets are relative to that, i.e. L filter is the reference filter, thus:

R_offset = Rf-Lf = 1025-1000 = +25
G_offset = Gf-Lf = 950-1000 = -50
B_offset = Bf-Lf = 1001 - 1000 = +1
etc...

Then these are the values to put in the offset fields for the filters.
After that only use the reference filter for autofocus at the beginning of each session (also only have L filter with the autofocus box ticked?)

Is that a correct understanding of what you've explained?

thanks :)

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1 month 5 days ago
rlancaste
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Where to set focus offset for different filters? #56555
Yes jiberjaber, your offsets should work just fine, or you can also use the "absolute" values you said as well. Since the focus offsets are relative to one another and focus changes are differences between the offsets, then it wouldn't matter to have all the offsets shifted by the same value.

As for which filter you use for focusing, that should not matter as long as the offsets are all entered correctly for each filter and as long as your focuser moves correctly based on the offset when switching filters. I say this because some focusers don't reliably have the same numbers for the same focus positions when focusing in or out and sometimes the focus drifts a little. So really the focus offsets can only be used as long as your focuser can reliably move by the same amount each time.
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1 month 5 days ago
ChrisRowland
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Where to set focus offset for different filters? #56579
What I'd do is focus in a sequence like this: L, R, L G, L, B, L, Ha, L, OIII, L, Sii, L and repeat until bored.

This should help to allow for focus change with temperature, you may also get a focus temperature coefficient out of this.

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