Firstly, Rally, thanks for providing this information. “Note: real world values for the CFZ are approximately 10%-30% greater than the theoretical values...” I have pondered on this. Need more time, but I believe I can understand the reasoning. Assuming that the CFZ shifts based on wavelength (blue short, red long). If you use the calculator you provided, the CFZ does shift in size.
So if we plug some values into the formula - cfz = 4.88 x f-ratio^2 x wavelength
For example if I use the blue (~470nm wavelength), f-ration of F/5 (FSQ) and Pixel size of 9 microns for STL11k (no binning so 1x1) – the CFZ is 57 microns. If I use the same parameters, but change the wavelength to red (~656nm Ha) – the CFZ changes to 80 microns. So based on wavelength alone the CFZ will shift, but I not sure if it is justified as high as 30%.
Perhaps, to circumvent this ever shifting CFZ, calculators/formulas only base the measurement on the start of the visible wavelength being 450nm (blue). Using the above formula with a 450nm value, the FSQ hits the 55 micron CFZ I previously stated. I also plugged in the values for the TOA-130 @ F/7.7 and obtain a CFZ of 130 microns. So in a way, this confirms that the default value for establishing the CFZ is at the 450nm wavelength. Makes sense to an extent, i.e. you wouldn’t try focusing on a star in the NIR or IR wavelengths though it is possible considering they emit over a broad spectrum. On reflection, this would be an interesting test with an achromat. Considering they are not well corrected instruments, focusing all wavelengths simultaneously – compared to an APO that is.
Will give the airy disk some further thought - The CFZ remains the same, but then the CCD Focus Zone (CCDFZ) changes based on pixel bin values. There would have to be more to it than simply the significant smaller size of the Airy disk.
Thanks for formula page too. I’ve been after something like this for sometime. The CCD internal reflections calculator is great. Will be sure to use it in the further to determine what is happening in some of my images.
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Originally Posted by Zuts
Hi Guys,
Still havnt managed to use it yet as it's raining. When I do, do I need to do two seperate sets of V curves, one with a reducer and one without?
Thanks
Paul
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Yes Paul, you’ll need two v-curve profiles – one native (no reducer) and the other with the reducer. This is because the reducer will shift the focal plane. Make sure you perform more than one v-curve run. The more runs you do, the greater the accuracy (each successful run is automatically added to the model). I usually aim for 8 or more. You can have as many v-curve profiles as you need.
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Originally Posted by montewilson
I agree with you that FocusMax seems to have the runs on the board. I am not using DL so getting it work means I will have to enter into the dark art of ASCOM of which I am rather afraid.
CCDWare have CCDAutoPilot which claims it can run Focus Max as well as Bisque imaging programs. It kind of acts as a front desk for all these programs. I will ask on their forum if I need to have DL installed to use FMax. If not, COOL!
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Hi Monte. You don’t need MaximDL to run FocusMax. It can integrate with CCDSoft if that is your acquisition tool of choice. Indeed CCDAutoPilot supports FocusMax. As you mention, CCDAutoPilot is the “front desk” for backend applications. It’s the glue to brings everything together to automate an imaging session. I use ACP which does an identical job – the output I provided above is a snapshot of an ACP log. I have ACP doing everything, plate solving, automatic focusing, filter offsets, automatic guide star selection, telescope point model, automatic acquisition of skyflats and other calibration frames and more. I simply upload my imaging plan that I’ve created from TheSky and watch it do it thing. Both CCDAutoPilot and ACP are very powerful packages. I went for ACP as I’m an ASCOM advocate and perhaps more importantly it has a web user interface (some CCDAutoPilot doesn’t have yet). A thin web interface was a must considering I’m uploading and running sessions over a satellite link to the remote observatory. Satellite is the only form of “broadband” due to the remoteness. Anyway, a slight diversion from your questions…
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Originally Posted by montewilson
What is the minimum linear movement of the tube that RoboFocus can do on the FSQ? You mentioned some values but I am not sure what they relate to. Should I be able to get within the CFZ with with say a couple of pulses to spare or will I be jumping right over it?
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I have not measured the linear movement of the FSQ draw tube. Each pulse is very small and not easily perceivable. I don’t have any precision tools to make a measurement, but when I’m down at the observatory, I’ll see what I can come up with. As previously mentioned, typically a microstep is 0.0254mm (depends on the gearing used in the particular motor).
The focuser should be able to sit in the CFZ for at least a couple of steps before you notice changes to FWHM/HFD figures. Your microsteps per step are too large if you are jumping right over it. You can test this manually. Simply select a star in CCDSoft and start grabbing subframes for it (so you get rapid updates). Monitor the FWHM/HFD values while you manually push the in or out button on the Robofocus controller. Make sure you push and release, don’t hold the button in. One beep noise is one step (not a microstep). Keep moving through the focus range (from out of focus, in-focus, then out of focus). Make the initial first pass and take note of the focus position reported by the RFCP (software). Then make a second pass, but take a closer look at the FWHM/HFD measurements as you proceed. A previously mentioned I usually get 4 steps before values change considerably. Make sure you sit in the CFZ for a while to monitor the FWHM/HFD measurements as they will fluctuate a little due to seeing. They shouldn’t be varying too much as the 3.5 pixel/arc delivered with the STL11k’s 9 micron pixel size is far from high resolution. Once you know how to reach the CFZ manually, the progress to automation will be natural.
You may have already done this but make sure you tell the Robofocus controller the focuser travel (i.e. the draw tube all the way out to all the way in). The process is called calibration. It is important to set this up correctly as you’ll lose your absolute positioning info if not.
To calibrate the RoboFocus, use the OUT button to run the focuser all the way “out” (or at least as close to all the way as you want). Release the OUT button, and turn off the RoboFocus. After ten seconds, turn on the RoboFocus while pressing the IN button. The RoboFocus will read the IN button, will know that it is in the training mode (it will beep five times). Let up on the button during the beeping. After the beeping ends, the focus will begin running in without holding the button. When the focuser has moved all the way in (or as far as you want it to go which may take several minutes), you may press either button to stop the training run. When the motion stops, the RoboFocus records its position as =2 and will record the total focuser travel as the number of steps run, and calibration is now complete. Thus, a count of 2 is the innermost position. As the RoboFocus moves outward, the position increments by one for each step of the motor. You can read the MaxTravel number of steps from the Configuration screen in the RFCP (you will need to refresh the screen after calibration). Something to be aware of is that you don’t need to let the focuser go through the full travel length of the draw tube. I stopped mine around 50 steps before the focuser was all the way in. It doesn’t make a huge difference. What is important that the CFZ position should reside roughly equal on either side of the focus travel.
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Originally Posted by montewilson
As I have the venerable "106" could I use that focal reducer on my scope? That is such an exciting alternative to buying a pricey Canon or Nikon telephoto lens. Would that mean my FSQ will be a 400mm f3.75?
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Sorry, no can do. The FSQ-106N does not support the new Q reducer (TKA36580 Reducer-QE 0.73x (FSQ-106ED)). It drops the FSQ down to 382mm @ F/3.6. There simply isn’t enough backfocus on the FSQ-106N to support it.