The spidervane modification has its roots in the results of the   startests I performed on the scope. In the diffraction patterns the   original 4-vane sipder produced very distinct diffractions. 
This is of   course partly due to the 4-vane configuration, which produces 4 strong   diffraction spikes around bright stars. This  is in some cases visually  very pleasing and is some what expected of  astrophotos. The original  spidervanes also had an other issue which is  visible in the startest  image below.
The vanes produce a somewhat "blocky" star image most likely due to   the 3mm thickness of the individual vanes. It is understandable that on a   "budget" scope some compromises are made to lower the production  costs.
An other issue I wanted to adress with the modification was to   minimize the central obstruction size. In the original configuration the   central obstruction is actually slightly more than the secondary  mirror  size requires. Secondly as I managed to chip the original  secondary  while trying to remove it from the mount, I purchased a  slightly smaller  secondary that still works on the scope without larger  modifications to  the tube . The original central obstruction is 44mm  in diameter, which  amounts to ~34% obstruction. The original secondary  minor axis is 40mm.  If mounted differently would already reduce the  obstruction to ~31%. The  new secondary has a minor axis of 37mm, which,  if mounted correctly  would give an obstruction of ~28%.  I'm hoping to  get a little bit more  contrast. Visually probably not notable, but  when photographing though  the scope then the situation might might be  different.
I  wanted to go for a rather simple design without any "gnarly" parts  and  easily upgradeable. Basically I want to be able to take the whole   thing apart and make changes when necessary. The design basically   consists of three parts; the central hub, detachable vanes and the   secondary mount.
Unfortunately all the texts are in Finnish, but I think you get idea.   As you can see I went for the 3-vane design in order to get rid of the  4  very bright diffraction spikes.
The main problem with the making of the vanes was to find thin and   stiff enough material to do the job. It turned out that a metal ruler   was more or less a perfect solution for it. Being only 1mm thick and   very torsion and bending resistant, especially in the short lengths   needed for this.
The central hub is made of POM (Polyoxymethylene) or simply a type of   plastic which works well for precision parts and can handle   temperatures down to -40C.  The secondary is mounted with three drops of   standard silicone adhesive.
The final spider before painting matt black and installation of the  secondary.  As you can see the difference between the original and the  modified vane  is quite distinct. Of course the whole original plastic  assembly needed  to come off before the new vanes could be installed.  The good thing is  that the plastic tube end was attached to the tube  with three screws  with 120 degree separation, so no drilling was  needed.
Below is the painted spider with the secondary installed (and the   original for comparison) and the spider installed in to the scope.
Ever  since the modification was completed the weather has been quite   miserable, so no star tests have been performed with the new sipder   installed. But when the skies clear and the temperatures rise to a   manageable level I will perform the tests.
ICARUS, a two frame vertical panorama
4 days ago
 




 

 
 
 
hello,
ReplyDeleteMy name is Mitja and Im coming from Serbia. Im bought astromaster 130eq and i wish to do same modifications of spider vane. so i need some more informations about this. links or some plans how to do its ok. my idea its to get 3 vanes 1mm thickness. secondary will stay the same.
Hi Mitja, I'll try to help you as much as I can. My suggestion is that you get rid of the original secondary holder. You can of course keep the mirror. You can contact me directly through my email mtoastro(enter the funny a here)gmail.com.
ReplyDeleteHow did you make the Central hub? Or Is is available for purchase
ReplyDeleteHi Bala, the central hub is my own design and manufacture. It's not available for purchase, but you can use the drawing on this page to make one. All you need is access to a lathe to make the hub and a steel ruler for the vanes. Some of the text is in Finnish but if you want I can provide a translation.
ReplyDelete