Auto Focus, Off Sets, Astrodon, Step Size

Sequence Generator Auto Focus
1

Okay let me see if I can ask this and get to the point quickly and not reinvent past posts.

I’m trying to figure out how to get my auto focus to work with offsets and my astrodon filters.

What I think I understand about Auto Focus…

  1. I know you designate your “Focus Point” in your filter setup menu
  2. I understand all the boxes and what needs to go in them in the “Auto focus Settings Dialog” box
  3. I understand how to start looping an exposure to get the HFR 4 to 5 times higher than the original number and then use the formula of subtracting your in-focus numbers to get the step size for the particular filter that you are using. All done through your “Focus Control”
  4. I understand that when you have “Auto adjust focus per filter” checked that it takes into consideration that the focus point will change from night to night based on variables like the temp outside etc.

What I am not understanding is how to pull all this together. I think when I am reading what I wrote above that I might be combining a few different scenarios that don’t necessarily go together.

I think I understand that using the formula listed above will give me the step size for each of my astrodon filters but after reading various posts it seems that using that formula does not necessarily give me the actual step size that will be used all the time in my auto focus sequence.

How do I determine what that step size should be?

For Example:

If I get the focus number for my Lum filter, and then figure out the focus number for my Ha filter, I’m guessing that I just subtract/add it to the Lum’s number to get the number of steps between the two filters, and then use that number to tell SGP to add/subtract from the Lum’s focus point to get the focus point for the Ha? And then do the same for the other filters. Would that number then be considered the Ha’s “Focus Point?” And is that the number I should enter into the “Focus Point” line item in the “Define Filters” box?

If that’s the case then it stands to reason that no mater what the focus number ends up being on Lum for any given night it will then add/subtract that predetermined number for Ha, set it there, and then run an auto focus to get it dialed in exactly? Correct?

All this seems to make sense when you look at using that formula to get the step size if you were using auto focus for each filter individually but I’m missing something here on the final step size.

In the end will there be one number for step size in the “Auto Focus Settings Dialog” box and if so how do I determine what that number will be.

Sorry if this was convoluted but I’m trying to figure out what to say as I write it.

Thanks Sean

2

I think you are confusing a couple of different things. You basically have it right but I think you’re overthinking it.

Your step size has to do with running autofocus (not “step size for each of my astrodon filters”).
The “Auto adjust focus per filter” has to do with determining the filter offsets.

When you run autofocus you want your outermost points of the V curve to be about 4x larger than your in-focus point. This is not critical so don’t worry about it too much. For example, if your in-focus HFR is 1.0, set your step size in the autofocus dialogue so that when the autofocus first runs and moves the focuser to the outer position, the resulting HFR is about 4.0

Here’s the simple version of what you need to do.

  1. Use autofocus to focus your LUM filter - write down the focus step position (let’s say it is 20,000)
  2. Use autofocus to focus your RED filter - write down the focus step position (let’s say it is 20,100)
  3. repeat 1 and 2 and find the focus points between the LUM and whatever filter you’re on

As the temp drops the focus step position will change, so that is why you should focus the LUM before each autofocus run on the other filters. It also helps to do this when the temperature outside is very stable.

Let’s say you end up with these number (as the temp is dropping a bit):
LUM - 20,000
RED - 20,100 (+100)
LUM - 19,900
GRN - 19,800 (-100)
LUM - 19,800
BLU - 19,750 (-50)
LUM - 19,700
HA - 19,900 (+200)
LUM - 19,700
OIII - 19,800 (+100)
LUM - 19,800
SII - 19,700 (-100)

So then you take these numbers into the Filter setup dialogue to enter the offsets. These are relative offsets, meaning that no matter where the focuser step is at when focused on the LUM, “auto adjust focus per filter” will adjust for the difference between the LUM filter and the filter you are on.
So you enter in the filter setup dialogue:
Lum - 20,000
RED - 20,100 (+100)
GRN - 19,900 (-100)
BLU - 19,950 (-50)
HA - 20,200 (+200)
OIII - 20,100 (+100)
SII - 19,900 (-100)

Now let’s say when you actually focus the LUM filter later in the night and the focus stops at 18,000. When you move to the Red filter SGP will automatically move the focuser out 100 steps to 18,100 since the relative difference between the LUM and RED filters was +100 as set up in the filter dialogue.

3

Okay!!!

Now that makes sense. I thought I was confusing two things together.

Thanks for the detailed response. I have already copied and sent it to a few others that were waiting to hear this response.

The last question I have is about the step size i input in the “Auto Focus Options Box” Sounds like I need to run tests on the Astrodon’s and see what size will give me the HFR of 4 on the outer focus point.correct?

Last night I was using all three NB filters and saw that the SII and the Ha are pretty much in the same focus point. However the OIII required me to hit my coarse focus “In” for a 150 steps to get it into the same HFR category as the SII and Ha.

I’ll have to find a step size that sits somewhere in the middle of that to deal with both focus points.

I think I have it now. Thanks again so much.

Such a pain this is!! I switched scopes and all this started. I went from an ED 102 to the ED 127 and my auto focus fell apart. Prior to that the ED 102 was spot on. Perfect V every time.

I’ll get it.

Sean

4

Just use your lum filter to determine this. I assume you’ll be focusing with lum, using the “auto adjust focus per filter” option to keep the filters in focus. Your “auto focus step size” has little to do with the filter that is in your imaging train and more to do with your F/Ratio, PIxel size, and how much your focuser moves for each step. The filter that is in the light path has almost 0 affect on this value.

If you’re referring to the “Auto Focus Step Size” then no, you won’t. Those values are the same across all filters.

There are 2 concepts at play here:

  1. Focus Points Per Filter. These are setup in the Filter Tab and allow you to create offsets for your filters. Since most filters aren’t parfocal you need to adjust the focus when you change filters. Like you mentioned that your OIII filter needs an a focus change of 150 steps compared to your SII and HA. That is what you’re referring to here.
  2. Auto focus steps. This is the amount of steps that auto focus will move when it samples to determine if you’re in focus. You want to run some tests (generally with your lum filter) and setup your steps such that your high HFR is 2-3X your low HFR. So if you have a low HFR of 1.6 you want to shoot for a high HFR of 3.2 - 4.8. Also make sure you do these tests running using the same binning that you will use for auto focus (Generally 2x2 or 3x3).

First you need to determine where each filter is in focus. It sounds like you’re currently doing this. After that is complete and you have your focus points setup you can then use “Auto Adjust Focus Per Filter” and set your lum filter to be your focus filter. This will use the focus points as offsets when you change filters. For instance if you’re on your Lum and you’re at 1500 steps, when you switch to OIII the focuser will automatically move 150 steps in (or whatever you have setup as the difference between Lum and OIII. So when your lum is in focus, switching to other filters will keep those filters in focus as well.

Hope that helps. The two things are pretty separate. You can always not use the “Auto Adjust Focus Per Filter” and focus with the actual filter itself. However you’ll need to have considerably higher exposure times for your narrowband filters.

Thanks,
Jared

5

Thanks Jared this really helps. It’s taken me a lot of time in the field using the software to finally be able to tackle this part of focusing and to be able to understand your responses. I think it has finally sunk in. I just applied what you told me and my ML hit focus the first try. My spread was no more than 4 and I ended up with a perfect V and .73 HFR for my Ha.

I have had great luck focusing through my Ha with 10 seconds set as my exposure. I would have thought it would be longer but it works so Im not looking a gift horse in the mouth.

The SII is much denser than the Ha and takes 12 to 15 seconds.

Again thank you. I’ll try to work with what you have given me for a while before I come back with anymore issues.

Have a great evening.

Sean