Recording Focus position During Sequence For Assisting with Flats


#1

Hi All,

I was just wondering if there is anywhere in SGP, that records your focuser position at the the time of running a particular filter during a sequence.

The reason i ask is that i would like to take some flats using the same focus point that each filter used during the run of a sequence, so i dont need to slew back and forth between my target and my light box during an imaging session.

Does this seem like a reasonable questions, or is the focus not critical when taking flats, i.e. it just has to a roughly close to the focus at the time of imaging.

Rich.


#2

Well, SGP already will move the focuser to the default position for that filter as set up in the filter setup if you are using the flats wizard. This is probably not your most recent position but I seriously doubt there would be very much difference in the flats. I have sure not seen any even with programs that do not adjust focus position for flats at all.


#3

Personally I have found that focus for flats isn’t all that critical…but your milage may vary.

The best way to do this is to setup filter offsets, as an added bonus this will also allow you to use a single filter to do your focusing which is practically mandatory with narrowband. Once the filter offsets are setup you can then tell SGP to “Auto Adjust Focus per Filter”. This will cause SGP to move to the offset between the two filters essentially keeping you in focus even when filters change.

So then at the end of your imaging session you take your flats. The focuser will then change position each filter and keep your flats relatively in focus.

Hope that helps,
Jared


#4

If you capture images with SGP, SGP records lots of information including focus positions in FIT header. I use PixInsight to view FIT header. You could take an average of all focuser’s positions for each filter and use this average for taking flats.

Like Jared said, I don’t find focus positions critical when taking flats. For example, if I imaged with Red filter, then Green filter and finish with Blue filter, I use the very last focuser’s position from Blue filter and take flats for all three filters. My filters are Astrodon and they are pretty parfocaled.

Peter


#5

Thanks All,

My filters are cheaper Chinese filters, and don’t think they are parafocal, so i think the best way for me to do it is to follow jared’s advice and setup focus offsets for each filter.

i’ll have a look through the tutorials to see how to set it up.

Many thanks for all the suggestions and advice,

Rich.


#6

Rich,

In the filter list there is a spot for focus point. When I got my filters or if I manually move my focus by accident/purpose, I just run auto-focus on each filter and record it. Then, I put those numbers in the list. SGP will figure out where it should be from there.

Keep in mind it needs to be ‘roughly’ in focus when you start though.


#7

Many thanks all,

I think i am due some clear sky tonight, so i’ll have a play around with the focus.

topboxman, i’ve just recently purchase pixinsight, so i’ll have a look to see if i can see the fits header information you mentioned as i should be able to get the focus info from some images i took last week.

Rich.


#8

It’s called “FITSHeader”. SGP stores many useful information including temperature of focuser if your motorized focuser has built-in temperature probe. Very useful tool. You will love PixInsight.

Peter


#9

Just FYI you can also view the FITS header in SGP:

More details here:
http://www.mainsequencesoftware.com/Content/SGPHelp/ViewingFITSHeaders.html

Thanks,
Jared


www.mainsequencesoftware.com