Here I would like to present tips, tricks and handicrafts, which have resulted from the practice of astrophotography. Unless otherwise indicated, all contributions by me.
A flatfieldbox for free ?
A UHC-Filter at the zoomlens
Calculation of a dew heater system
A field of view calculator for Cartes du Ciel
Modified Scheiner aperture
A Flatfieldbox for free?
Free certainly there is no Flatfielbox.
But this for the Astro Photographers often indispensable utensil also caused enormous expenditure of time and of course costs.
As many photographers already have a notebook to work with, I had the idea to use this as Flatfieldbox. So download the program "Nokia Monitor Test" (, and install it.
Click the button "colors" and you can turn on the monitor completely white (or other colors).
A UHC filter at the zoomlens
Many astrophotographers have the problem to attach filters to their zoomlens. Adapters are expensive and not always the best solution.
My proposal:
Calculation of a dew heater system
If at a cold and humid astronight the optics starts to steam up, the photo fun (and the observation fun) mostly stops abruptly.
You can prevent this evil with a dew heater. Indeed, such a heating can also pull substantially power from the anyway obsessed astroaccumulator.
So it´s important not to oversize the heating. You need less power than you think. Here is an easy formula to calculate the required power.
 P = 0.07 • A • v ÷ (sqrt (1 + v ²))
P = required power in watt
A = Surface of the front lens or Schmidt plate record in cm ² (as we all still knew from school is A = pi • r ²)
v = r ÷ L where r is the radius of the dew cap and L is the heated length of the dew cap
sqrt = square root
A small example: Refractor with 100 mm optic, dew cap ø 130 mm, length dew cap 100 mm
P = 0.07 · 78.5 · 0.65 ÷ 1.19
P = 3 watts
This is at 12 VDC about 0.25 A power for a 100mm optic. As you already can see, the power consumption remains really low.
This can still be reduced, while you wrap a heat insulation around the heated dew cap.
Modified Scheineraperture (construction plan by Manfred Mrotzek, from "Sternkieker" 02/2007)
The focussing in astrophotography. A topic that has already brought some to despair so well. The Scheineraperture should be a help here. Generally, a cap with two or three holes, which has to be mounted on the front of the lens.
So a star seems to appear two or three times in the eyepiece (or on the screen). Only in the exact focus the 2 or 3 pictures went into a single point. Very often, this arrangement has proven to be a bad compromise.
Adjustment of the polar finder from the EQ6
Adjust the polar finder of the EQ6
In my last Astro excursions it has always disturbed me, that the polar alignment of the mount with the polar scope was very inaccurate.
The alignment procedure has stretched very much in the length.
So I looked at the Polar finder a little bit closer and had ascertained that the front and back part of the seeker did not curse at all.
The critical place is the connecting thread of both main parts of the seeker. (red arrow). Here does no reasonable guidance exists, thus both parts are able to bend some degrees in every direction.

You can examine this very simply during daylight, while the latitude is set to about zero degrees and the Mount rotating with relaxed clamping and without counterbalances in the RA axis. The polar scopes screen should ideally stand still. If the image is dancing wildly up and down, so the polar scope is out of alignment. The problem with this is, there is no chance to adjust the built in polar finder. So I had to produce a help device. It looks like this:
A simple part with a mounting thread M28x1 and 4 drillings for the adjustment.
The whole can be clamped in the lathe, then let it turn slowly and look through the polar finder scope.
Now by the small drillings the front part of the finder scope can be straightened with standing machine rather exactly,
until the image stands still while looking through the finder scope.
After this it has to be fixed. I have just taken some epoxy glue.
Polar finder then has to be screwed back into the mount.
Finally the symbolic disc with 3 small socket screws has to be adjusted, until the crosshair also stands still and concentric while rotating the Mount.
I think, I managed a good alignment of the polar finder
with this approach .
Ready is the "free" Flatfieldbox. This even with brightness control (via the laptop scheme). To eliminate Hotpixel and other disorders, I placed milk glass pane of Plexiglas in the size of the screen in front of the notebook display. With this setup you can reach easily good results.
Of course there is no display with a 100 percent homogeneous brightness distribution, as well as this method is only practical for smaller optics.
If we do not set the standards too high, this arrangement is entirely sufficient.
Here I just inserted a 1,25" filter into the hood of a sigma zoomlens. For the connection a tightly sitting O-ring was pressed in the shaft.
That is ample, and has been proven in practice.
This fits at least with a lot of Sigma lenses certainly. For other manufacturers, it can sometimes be, that the outlet shaft is slightly larger.
In this case you can help yourself with a little bit of tape to attach the filter.
It is important, and here we come to the disadvantages of this idea, that the filter sits straight in the shaft, not tilted.
The reason is the following: if the light cone from the lens passes the filter in a great angel, (the light cone itself is quite unfavorable enough),
the passing frequency is cuted-off or shifted. With narrowband filters it becomes more critically.
Whether it still functions with a H-alpha filter, I am not able to say.
At least with the UHC filter in this arrangement it still seems to work well.
                          out of Focus                                                    nearly in Focus                                               exactly in Focus
This method is very accurate, because already the smallest inaccuracies mean a clear divergence of the ideal cross form in the focus.
Moreover, it is well recognizable whether you are quite wrong intra- or extra-focal.
I hope, with this "Mrotzekaperture" focussing for many star friends is made easier
Here the holes have been replaced by elongated slots. This should face at an angle of 90 °. The distance from each of the slots should be large. The size of the slots is not decisive. However, you should bear in mind, the smaller the holes, the less light passes through, and you can still use only very bright stars for focussing.
It is important that the slots in the middle of intersection are to be located in the middle of the (imaginary) holes of a conventional Scheiner aperture. (here shown in gray). Otherwise, the images of the slots would cross out of focus.

The image of a star at the focussingprocess
                                                     Scheineraperture                                                           or Hartmanaperture
                                                                 This is the modyfied Version by M. Mrotzek
A field of view calculator for Cartes du Ciel
If you want to know the FOV of your eyepiece or the field of your CCD chip in a starmap program like Cartes du Ciel, you have to deal with formulas. To shorten this procedure, my colleague Bernd Mahlcke wrote a small utility for this.


Here you can key in just the size of the CCD chip or the data of the eyepiece as well as the focal length of the optics, and after a click on "calculate" the values are given.
Then you can drag and drop it in the suitable input fields of Cartes du Ciel.
At this point once more a big thankyou to Bernd for the invested time and trouble.
Gesichtsfeld.exe is Freeware and only available in German.
It runs under Windows XP, Vista and 7 without installation.
The programme may be transmitted only free of charge.
File size 400 KB.

Download Gesichtsfeld.exe


Pictures a

nd Texts © Volker Umland 2007-2021
Tips´n Tricks