Filter offsets w/ Astrodon filters


#1

Hi all,

I recently got a QSI683 with Astrodon filter set (narrowband + LRGB). From my understanding, whether or not they are truly parfocal depends on the optical train. In testing with my 11" Edge SCT + focal reducer, they seem to be parfocal. I used 1 minute exposures on Ha for AF, and the difference in focuser position was negligible.

Just curious what others have experienced. Also, is the best way to determine offsets simply to run AF with high exposure times on the narrowband filters to determine offsets?

Thanks!
Brian


#2

I have Astrodon 3nm filters as well and they do not appear to be parfocal in my refractor.

However with lower f/ratios the light enters the filter at a steeper angle and is refracted more. So faster scopes are harder on filters and will generally require more focuser movement between filter changes than slower scopes. My scope is F/6.3.

Jared


#3

Thanks Jared – I don’t have the right adapter yet for the camera (coming any day now) but also have a Tak FSQ106 f/5, so I’ll be sure to watch that one more carefully.

When determining the offsets, would you recommend just running AF with long exposure times to get optimal focus points? Your system is faster than my f/7, but roughly 60s exposures seemed to produce reasonable results…


#4

I have astrodon filters and still prefer to use offsets as a general rule. With my 152mm f/8 refractor I can get away with just using the LUM for focus for all filters. I also use SLR lenses (135mm f/4, 55mm f/4) and with those lenses the focus point is dramatically different. When I’m in focus with Blue for example, Red stars look like little donuts.

I would suggest 30-60sec exposures for narrowband focus/offset calculation, but that will also depend on your system.


#5

I have both an FSQ106 (f5) and TOA-130(f7), both with Astrodon filters. I have found there are differences in focus, although not huge, so tend to use offsets but only if I am either doing narrowband or a combination night of narrowband and LRGB. If I am only doing LRGB, it is almost as quick to just focus thru the imaging filter. Therefore I maintain four profiles, two for each scope. One set for each scope with offsets and focus through luminance and one set for each scope w/o offsets, with focus through the current filter.

I think that most of the differences are not the filters but rather due to refractive elements that are not truly apochromatic, these could be objectives, or flatteners, or reducers, or whatever. I suspect that with a scope that has only mirrors, they would be close to perfectly parfocal but probably 90% of the imaging systems have some sort of refractive part.


#6

@CCDman - I’d never thought of setting up two profiles, with and without offsets for narrowband/lrgb. I was wondering how to get around this as I only wanted offsets for NB, thanks !
@Bhwolf - yes , I’d suggest long exposures on af runs but for narrowband only. I ran 25s exps for narrowband for a while (with similar gear to yours) but have found that 45s gives much better v shapes. But I wouldn’t go anywhere near that length with lrgb…


#7

I run my FSQ at F3,6 and each filter is noticably different


#8

I’m able to use 20 second exposures for focussing with Baader Ha and OIII filters.
80mm Oynx F/6.25 Atik 383L+ binned 2x2. Astrodon filters are narrower so may need longer exposures.
Focus positions (Moonlite Focuser)
OIII 2913
Ha 2929

IIRC it’s 4.5 microns per step so that’s 72 microns different.

Do other people have numbers? They seem to be recorded in the image header.

Chris


#9

For offset determination, I prefer to use a bahtinov mask and
bahtinovgrabber (http://www.njnoordhoek.com/?p=660). Then I know that I’m
pretty much in the middle of my critical focus zone and not somewhere
towards the ends.

Also, I evaluated the temperature coefficient beforehand and included it as
the temperature was slightly different.

 MarkS

#10

For the LRGB, I set up a sequence with “focus on filter change” with very short exposures and no tracking (since the actual light frames are throw-away) and ran thru a half dozen (L-R-L-G-L-B) noting the focus point for each filter on each image. I used an average of these for the focus positions in setup. I adjusted the focus points for temperature before calculating the averages since I already had that data.

For the Narrowband, one could use something with very bright stars, maybe M45. In my case I used FocusMax and MaxIm for that as I had that set up already because that is what I used to use.


#11

I do the same as Mark. For narrowband I find a very bright star and setup the offsets. Using sgp to determine them with the 3nm filters was requiring 60 second exposures for auto focus runs binned 3x3. Some things are just faster with a mask…

Jared


#12

What a great idea! “focus on filter change” lets the equipment do most of the work. Thanks.


#13

I have often thought that it would be nice to have a feature of the program or maybe a script that would do the exposures and automagically calculate and (optionally) insert the offsets. This is not my original idea since ACP has such a script for ACP users that does the same thing.

Not too hard to do semi-manually, however.


#14

We have discussed something similar that would also categorize the focus system (steps etc) . However as you’ve mentioned this is not hard to do manually. Thus it gets pushed down on the todo list. But it is there!

Thanks
Jared


#15

Makes sense. There are other things I would put higher on a wish list as well. Multiple cameras would probably be number one on my list, but that is a pretty small user base.


#16

Multi cam (really coordinated dither) almost made it into 2.4 but there were a couple of issues that were too big to address during this beta cycle. It will likely come out in a 2.5 release or as an update to 2.4. Also it will most likely be an add-on.

Thanks,
Jared


#17

Woot! I await this with bated breath! Hope it is available to use in the summer when we get clear skies. Add-on is just fine with me, if you can afford two cameras, you can probably afford the add-on! :wink:


#18

@earlackbar
I’d be really interested to know how you get on with AF on your FSQ @ F3.6 - this is what I’m imaging with (with astrodons and a lakeside focuser) and I’ve been finding it a bit hit and miss. Do you get consistently good results and v curves?And what step size do you use?

Thank you


#19

Hi Kit

It has been a nightmare with autofocus and its still hit and miss some nights, i can watch the auto focus take itself out of focus and say its better when its WAY out, I see this with individual points where it caculates worse focus, where in reality i can see with eyes mk1 that the stars are smaller.

I have a step size of 35 at present, out backlash of 127 steps is taken care of in the ascom driver for the Lightphase focus controller. I use a Generic robofocus compatible stepper on the scope.


#20

What is the physical step size (mm or microns)? I used to use a Robofocus on my FSQ and it was not great even at f5. Since I replaced the focuser with a Starlight with the HSM motor (1 micron step size) it is bulletproof. I can see how too big a physical step size could be a nightmare at f 3.6.

Going to a real engineered focuser instead of the add-on Robofocus was one of the best purchases I have made for my system, it really reduced the frustration levels. As soon as I tried the Starlight system, I immediately sold both my Robofoci on Astromart and put a second Starlight on the other scope.


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