Critical Focus Zone

Original Critical Focus Zone Calculation:

CFZ = 1.6 * λ * f^2

λ = 500 nm (green)    f = 10 for Edge11

New Critical Focus Zone

Traditional Definition

CFZ = 4.88 * λ * f^2


Goldman NCFZ

NCFZ = .00225 * θ * sqrt(τ) A f^2

θ = seeing (3 arc seconds)

τ = focus tolerance 15% visual, 5% AP?

A = Aperture



CFZ1 =  1.6 (500) (10)^2 = 80 micron

2 micron/step per spec

CFZ1 = 80 micron (1 step/2 micron) = 40 step

CFZ2 = 122 steps

NCFZ = .00225 (3) sqrt(5) (279.4 mm) (10)^2   =  420 micron (1 st/2 micron) = 210 steps

NCFZ is 5 times bigger than CFZ1, twice CFZ2



CFZ1 = 1.6 (500) (4.5)^2 = 16 micron

6000 steps / 2.1 cm * (100cm/m) * (1/10^-6)  .28 step/micron    or 3.5 microns per step

16 micron (.28 step/micron) = 4.57 step

CFZ2 = 13.9 step


NCFZ = .00225 (3) sqrt(5) (90 mm) (4.5)^2   = 27.4 micron

* (.28 step/1 micron) = 7.67 steps

NCFZ is 1.7 times CFZ; still very small



Mark IV Light Box for Flats

I finally got my Mark IV version of the light box built and operational. Procedurally it works well; ACP can change between two brightness settings to account for the dimmer filters. It is kind of huge (2.5 feet cube) so it takes a lot of room in my small observatory.

Unfortunately, the flats produced are poor. It looks like the Tak can detect lighter/darker zones from the bulbs depending on where the OTA is pointed; I don’t get a nice even flat. Also, I can’t position the box well so the OTA is orthogonal to the lighted panel so I think that also leads to a gradient across the flat (top to bottom).

So, I am back to doing dusk or dawn flats. Tricky doing dusk flats because it is so hot outside (95 degrees at dusk, only going to get hotter here in Arizona) so the camera can’t get cool. I am trying for -10C, but 0 degC may be more in reach. I could water cool the ST2000, I may test that out. The STF8300 does not have water cooling available.



FocusMax Temperature Compensation

As part of my plan to use the Edge as the guider for the Tak, I worry that the Edge will get out of focus as the night goes on. The main imager gets focused regularly through ACP, but the guider does not get focused. The Tak holds focus pretty well, but the Edge has bigger fluctuations, I think. Unfortunately, it is a pain to switch FM and Maxim over to focusing the Edge, do the focus, then switch everything back again.

Solution? Maybe I should finally get the temperature compensation working in the Optec (on the Edge). Maybe this would keep the Edge in better focus over the night. Supposedly slightly out of focus is better for guiding, anyway. I suspect that does not apply to large donuts like I was getting after awhile.

To check this out, I first gathered some data manually using my standard settings (NearHFD 15, 5 steps in 5 samples). I collected data every 30-60 minutes, doing several FM runs at each time to see how the data spreads. The results are shown:

Manually collected data with Edge11

Manually collected data with Edge11

OK, this looks weird. I am expecting a linear response – it Looks like I get this for awhile, then the temperature levels out?

Jeff Dickerman’s Data

Some discussion on FM Users Group provided some very nice data from Jeff Dickerman, who I believe is with Optec. He has looked at a Meade, which shows the expected linear relationship:

Jeff Dickerman's Data

Jeff Dickerman’s Data

Note the blue area, indicating the size of the New Critical Zone.

He also looked at the Edge 11, same setup as I have:

Jeff Dickerman's Data

Jeff Dickerman’s Data

Jeff’s conclusion was not to even use temperature compensation on the Edge. The effect is small (11 steps per degree compared to the Meade 75 steps so he just refocuses every so often. Note that on the Meade, he still refocuses – he just does it every 120 minutes instead of every 60.

Second Try

Reading a Steve Brady presentation at AIC, it looks like I need to tighten up some of my focusing parameters. For example, I know the NearHFD setting needs to be on the flat part of the VCurve. I had mine set at 15; that already “seemed” high, because the Tak has smaller HFD values in general. After some testing, it looks like 25 is a better value. I also changed the convergence criteria to be 3 steps in 5 samples; Steve had shown using 2 steps, which I worry might be too tight? Using the value of 3 steps, serial focus runs are doing a better job being consistent. I need to look at the Tak settings as well:)

Note: my New Critical Focus Zone is 36.7 steps on the Optec, according to something I read (Wodanski, I think).

CFZ = 1.6 lambda f^2

= 1.6 * 500 * 10^2  = 80 microns

For Optec, 7000 steps/0.6 inch?  ==> 36.7 steps

I assume this means that my “blue zone” on a graph like Jeff’s would be about 20 steps above and below the trend line.

OK, now to collect some better data! FocusMax has a nice Wizard function that is supposed to help gather this data automatically, rather than doing everything manually. Of course, the Wizard doesn’t seem to get enabled in FM! Started a thread on FM Users group, and Steve kindly made an updated version ( which now works.

Here is my second cut at data, collected using the FM Temperature Compensation Wizard. Data points are collected every 10 minutes. I am using my tighter convergence settings.

Tighter convergence criteria: 3 steps in 5 samples, NearHFD 25.

Tighter convergence criteria: 3 steps in 5 samples, NearHFD 25.

Well, I again see a weird shape. The shape is not at all the same as what I collected manually! In the beginning the slope is negative, as seen in Jeff’s data. My first try had a positive slope! Both nights started out fairly warm, around 28 degrees C.

The initial slope shows 99 steps per degree, while the total data is 41 steps per degree. The flat portion might be 20? I didn’t fit that section. During the linear portion the focus results fit within my CFZ; clearly the overall data does not.

So, what is going on? I wonder if this has to do with the sealed baffle tube with the Edge’s corrector optics. Maybe the flat section has to do with the two tubes becoming closer in temperature? At first the main tube is cooling (like the Meade), then eventually the sealed nature of the Edge leads to the flatter section.

I would like to rerun this using some kind of fans blowing into (out of?) the Edge vents. Deep Space Products produces these fans to help equilibrate the tube. I would like to try some simply approach before spending the $165. If fans lead to a linear result, maybe compensation makes sense. Otherwise, Jeff is right – forget it, just do regular focusing.


Celestron OAG failure

I spent quite a bit of time trying to get the Celestron OAG to work with the Tak Sky90. In the end, I cannot get it to work – I can’t get the guide camera to come to focus.

I am using the Starfish guide camera. It attaches to the nice helical focuser, but is too far out to come to focus. It needs to get closer by 10-20mm.

The main camera focuses fine. I have tried various adaptor spacings with no luck. I think without the helical focuser it might get there, but then of course I have no way to adjust focus.

I have subsequently played around with using the Edge11 as my guide camera. It looks like I can get the Edge to see out of the slit everywhere except small regions near the horizon at the North and South, so this may work fine.

Unfortunately, when I tried guiding for 30 minutes, I seem to be getting a small amount of drift. It looks like flexure somehow? I will have to play with tightening up everything in hopes of getting that to work.The guiding within Maxim looks fine – the graph is nice and steady. It looks like Maxim is guiding correctly, but flexure (or something) is causing the Tak to drift.

HAlpha 30 minute exposure

HAlpha 30 minute exposure


Another thought – if I use the Edge as the guider, maybe I can set up Maxim to focus the guider (Optec) independently of FocusMax focusing the imager (Sky90/EasyFocus). Would not be able to automatically focus in ACP (well, maybe with some coding) but at least maybe I could easily focus manually. It is awkward to load the Optec focuser into FocusMax, switch Maxim to the other camera, focus guider, switch everything back.