the other night I found myself with a couple hours of clear sky near the end of my shoot and decided to try and see if I could produce a decent image using the auto sequence in Maxim DL. I picked a galaxy (M109 I think) and set the exposures to 20 sec for alternating RGB and let it run. I first focused using the blue (I think--cant remember). It shouldn't matter as the Astrodon series E gen II filters are supposed to be parfocal. Well......they weren't. Not be a long shot. Using Sub frame selector in PI to judge focus (I use FWHM) I was very disappointed to see that one filter was decent, while the other 2 were 2 and 3 times the FWHM of the first (I don't recall which). I was shooting at a pixel scale of 2.46 arcsec/pix in poor seeing, so a FWHM of 5 arcsec was about as good as I could get. Not too bad at 2 pixels per star core. But the other two filters came it at 7.7 and 11! I have since learned that this might be expected with refractors, as they focus each color a bit differently....even good ones. Reflectors do not have this problem. But I find it hard to believe that people using refractors can't image this way. I have considered offsets--but I am not sure how tio go about getting the software to implement them, how to determine what they should be, or how consistent they really are. I If someone can point me in the right direction it would be appreciated.
Rodd
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Hi Rodd, you can use NINA or CCDCiel amongst others to image sequences automatically and in both pieces of software there is the ability to autofocus (assuming you have an ASCOM of INDI enabled focuser) then in the setup you can define different focus points for each filter. I have a reflector so haven’t come across such large differences in focus but this would be one solution to your problem.
Rob
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Rob Johnson:
Hi Rodd, you can use NINA or CCDCiel amongst others to image sequences automatically and in both pieces of software there is the ability to autofocus (assuming you have an ASCOM of INDI enabled focuser) then in the setup you can define different focus points for each filter. I have a reflector so haven’t come across such large differences in focus but this would be one solution to your problem.
Rob
Thanks Rob--I was hoping there was a way to do it without going the auto focus route. I haven't learned how to do that yet. I think I will do that next. the problem is I get so few clear nights I just can't lose one, or the better part of one, to getting auto focus to work.
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Rodd,
N.I.N.A. is free and supports relative focus offsets between filters so as to avoid repeated refocusing if you are cycling filters between exposures.
The "Cuiv, The Lazy Geek" Youtube channel offers good N.I.N.A. tutorials (he contributes to the open source code),
https://www.youtube.com/channel/UC65vvpQDX5rymeqrYt-Bb1g
Here is the docs page at N.I.N.A's site that describes autofocus and filter focus offsets,
https://nighttime-imaging.eu/docs/master/site/advanced/autofocus/
You find a time of night when the temperature won't change much over the hour or so while running autofocus on every filter you commonly use and put the focus offset counts into N.I.N.A. You can specify a particular filter to autofocus with and then just apply the learned offsets for the others. Also you can setup per-filter exposure times to use during autofocus runs. Finally, there are ways to tell N.I.N.A. to refocus every so often; on pier flips; if measured focus drifts by x percent, etc.
Most of the time I image at f/4.9 to f/1.4 and don't find that any brand of filter is truly parfocal at fast f-ratios, and some vendors state as such.
Depending on the altitude angle and filter color, relative air mass you are looking through will also disperse blue light more than the red end of the spectrum. For this reason and since I have to change filters manually, and because my skies suffer a brighter greenish Las Vegas light dome to my east, I image first with red, then blue near zenith for minimum dispersion, and then green as the object goes into the darker western sky.
Jim
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Some ASCOM filter wheel drivers, e.g. ZWO, allow you to set a focus offset for each filter. Select the reference filter and focus, each of the other filters will change the focus by the amount programed in the driver.
Focus all of your filters once, decide which is the reference filter and set it to 0, set the other filter offsets appropriately.
I Use APT which has a similar feature.
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Jim Lindelien:
Rodd,
N.I.N.A. is free and supports relative focus offsets between filters so as to avoid repeated refocusing if you are cycling filters between exposures.
The "Cuiv, The Lazy Geek" Youtube channel offers good N.I.N.A. tutorials (he contributes to the open source code),
https://www.youtube.com/channel/UC65vvpQDX5rymeqrYt-Bb1g
Here is the docs page at N.I.N.A's site that describes autofocus and filter focus offsets,
https://nighttime-imaging.eu/docs/master/site/advanced/autofocus/
You find a time of night when the temperature won't change much over the hour or so while running autofocus on every filter you commonly use and put the focus offset counts into N.I.N.A. You can specify a particular filter to autofocus with and then just apply the learned offsets for the others. Also you can setup per-filter exposure times to use during autofocus runs. Finally, there are ways to tell N.I.N.A. to refocus every so often; on pier flips; if measured focus drifts by x percent, etc.
Most of the time I image at f/4.9 to f/1.4 and don't find that any brand of filter is truly parfocal at fast f-ratios, and some vendors state as such.
Depending on the altitude angle and filter color, relative air mass you are looking through will also disperse blue light more than the red end of the spectrum. For this reason and since I have to change filters manually, and because my skies suffer a brighter greenish Las Vegas light dome to my east, I image first with red, then blue near zenith for minimum dispersion, and then green as the object goes into the darker western sky.
Jim
*the problem is j gong know how to autofocus. I do want to learn but when?
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Hi Rodd,
You mean you are not actually using autofocus? It is much harder without it... I use Sequence generator pro. Basically, you have to test it before finding the right number of steps and the amplitude of each step. The program will start from the hypothesis that you are starting by being in focus or close to it. Then if you defined a step size of 10 steps per data point and 9 data points it will move the focuser out by 4x10 steps (the other 4 data points are in focus). It measures the star parameter (HFR) for every step by going in (it will gradually decrease) and once it starts to be too much in focus the HFR will increase again. It uses all the data points in the graph to trace a 'best fit' curve which gives the best focus at its apex. The difficulty is finding the good combo of step size and number of steps, it depends on your F ratio. So trial and error at first.
Your FWHM are really different, but right now the focus will shift a lot as the temperatures go down during the night and you have a significant difference in temperature between the start of the session and the end. So FWHM could be increasing even when using only one filter. In order to verify if they are parfocal you should have verified the focus for each RGB filter in the beginning of the session. My SHO Astrodons are not completely parfocal for ex.
Hope this helps. CS,
Bogdan
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Bogdan Borz:
Hi Rodd,
You mean you are not actually using autofocus? It is much harder without it... I use Sequence generator pro. Basically, you have to test it before finding the right number of steps and the amplitude of each step. The program will start from the hypothesis that you are starting by being in focus or close to it. Then if you defined a step size of 10 steps per data point and 9 data points it will move the focuser out by 4x10 steps (the other 4 data points are in focus). It measures the star parameter (HFR) for every step by going in (it will gradually decrease) and once it starts to be too much in focus the HFR will increase again. It uses all the data points in the graph to trace a 'best fit' curve which gives the best focus at its apex. The difficulty is finding the good combo of step size and number of steps, it depends on your F ratio. So trial and error at first.
Your FWHM are really different, but right now the focus will shift a lot as the temperatures go down during the night and you have a significant difference in temperature between the start of the session and the end. So FWHM could be increasing even when using only one filter. In order to verify if they are parfocal you should have verified the focus for each RGB filter in the beginning of the session. My SHO Astrodons are not completely parfocal for ex.
Hope this helps. CS,
Bogdan
Well, focus can’t shift due to temperature in 20 sec. I took 30 20 sec subs of each and the fwhm values of all the red were the same, the green were the same and the blue were the same (within .01-.3 arcsec). So it can’t be temperature. When a red sub is 5 arcsec and 20sec later a green sub is 7.5 arcsec then 20sec later a blue is 11 arcsec, and finally another red 30 sec later is agin 5 arcsec, I think that is proof it is the filters
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Rodd Dryfoos:
Bogdan Borz:
Hi Rodd,
You mean you are not actually using autofocus? It is much harder without it... I use Sequence generator pro. Basically, you have to test it before finding the right number of steps and the amplitude of each step. The program will start from the hypothesis that you are starting by being in focus or close to it. Then if you defined a step size of 10 steps per data point and 9 data points it will move the focuser out by 4x10 steps (the other 4 data points are in focus). It measures the star parameter (HFR) for every step by going in (it will gradually decrease) and once it starts to be too much in focus the HFR will increase again. It uses all the data points in the graph to trace a 'best fit' curve which gives the best focus at its apex. The difficulty is finding the good combo of step size and number of steps, it depends on your F ratio. So trial and error at first.
Your FWHM are really different, but right now the focus will shift a lot as the temperatures go down during the night and you have a significant difference in temperature between the start of the session and the end. So FWHM could be increasing even when using only one filter. In order to verify if they are parfocal you should have verified the focus for each RGB filter in the beginning of the session. My SHO Astrodons are not completely parfocal for ex.
Hope this helps. CS,
Bogdan
Well, focus can’t shift due to temperature in 20 sec. I took 30 20 sec subs of each and the fwhm values of all the red were the same, the green were the same and the blue were the same (within .01-.3 arcsec). So it can’t be temperature. When a red sub is 5 arcsec and 20sec later a green sub is 7.5 arcsec then 20sec later a blue is 11 arcsec, and finally another red 30 sec later is agin 5 arcsec, I think that is proof it is the filters
Right, I thought you were cyclying through each filter. It's clearly the filters. But parfocal filters does not mean that they will produce the same HFR for each filter, it means that for a given focuser position, each filter will be in focus. If you leave the Bahtinov mask in place after reaching focus for G and then you change for the R and B, they should be in focus. The HFR will be different between filters (blue bloats more than g for ex.).
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Bogdan Borz:
Rodd Dryfoos:
Bogdan Borz:
Hi Rodd,
You mean you are not actually using autofocus? It is much harder without it... I use Sequence generator pro. Basically, you have to test it before finding the right number of steps and the amplitude of each step. The program will start from the hypothesis that you are starting by being in focus or close to it. Then if you defined a step size of 10 steps per data point and 9 data points it will move the focuser out by 4x10 steps (the other 4 data points are in focus). It measures the star parameter (HFR) for every step by going in (it will gradually decrease) and once it starts to be too much in focus the HFR will increase again. It uses all the data points in the graph to trace a 'best fit' curve which gives the best focus at its apex. The difficulty is finding the good combo of step size and number of steps, it depends on your F ratio. So trial and error at first.
Your FWHM are really different, but right now the focus will shift a lot as the temperatures go down during the night and you have a significant difference in temperature between the start of the session and the end. So FWHM could be increasing even when using only one filter. In order to verify if they are parfocal you should have verified the focus for each RGB filter in the beginning of the session. My SHO Astrodons are not completely parfocal for ex.
Hope this helps. CS,
Bogdan
Well, focus can’t shift due to temperature in 20 sec. I took 30 20 sec subs of each and the fwhm values of all the red were the same, the green were the same and the blue were the same (within .01-.3 arcsec). So it can’t be temperature. When a red sub is 5 arcsec and 20sec later a green sub is 7.5 arcsec then 20sec later a blue is 11 arcsec, and finally another red 30 sec later is agin 5 arcsec, I think that is proof it is the filters
Right, I thought you were cyclying through each filter. It's clearly the filters. But parfocal filters does not mean that they will produce the same HFR for each filter, it means that for a given focuser position, each filter will be in focus. If you leave the Bahtinov mask in place after reaching focus for G and then you change for the R and B, they should be in focus. The HFR will be different between filters (blue bloats more than g for ex.).
That’s true. But when I focus carefully while imaging I get decent fwhm for all filters. Blue is not typically as good, as you say, but usually I can get them within .5 arcsec of red and green. Or 1 on a poor night. Not double of triple
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One common approach to dealing with focus shift in refractors is to create an offset for each filter's focus position. You'll need to determine the offset values empirically. To do this, you can use the following procedure:
- Start by focusing one filter (e.g., blue or luminance) as your reference filter.
- Once you achieve good focus with the reference filter, note the exact focuser position.
- Swap to another filter (e.g., red or green) and defocus it slightly.
- Then, use this offset value when focusing with that filter in the future.
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