View Full Version here: : Ditching the secondary
25-05-2012, 12:26 PM
I read a recent thread somewhere, and an AP chap said it has been 5 years since he looked through an eyepiece. It got me to thinking why not ditch the secondary and use a slimline camera at prime focus.
The Atik 420 and QHY8L are around 60mm diam, smaller than most secondarys on a 10in plus scope. So with an appropriate spider and focuser, should'nt you get brighter images.
Other advantages I see are No Vignetting and simpler collimation.
Disadvantages would be
a) limted to OSC as a filter wheel would create a larger shadow.
b) Could'nt use OAG, would have use a guidescope with no flexure.
c) Have to use skinny cables running along the top of the spider
I did search on other forums and I could not find anyone using this technique so may be there are other pitfalls I haven't envisaged.
I was going to refurbish this old beast and give it a new life.
I have seen it done, was on a pretty large scope too. The owner had glued ribbon cable along one of the spider vane's.
I have seen seen this done on a commercial 18" scope but the name alludes me, I also considered it for my own 18" Newt. The idea should produce very sharp images, the only downside is the scope might be slightly longer and the images would be mirror reversed, no deal with software. The scope would be easier to balance also.
Clear skies Ken
25-05-2012, 05:24 PM
A chap called Dan Price has done this by replacing the secondary cage of his Obsession (20 or 25") and fitting a de-rotator under the camera. As of a year or two ago he was producing some nice images. Of course there was an obscure fellow by the name of Herschel who thought along similar lines a while back who still managed to be fairly productive with it!
25-05-2012, 05:45 PM
I haven't put a camera in place of the secondary mirror but I have built and used a automatic Secondary Mirror Dew heater with computer ribbon running along a spider leg with no visual degradation.;)
Hyperstar is effectively the same principle on SCTs. I guess your camera diameter needs to be about same size (or less) as secondary obstruction for it to work reasonably well. Would need decent mounting - not nice if camera falls down on primary..... :eyepop:
25-05-2012, 07:34 PM
What about focusing? Isn't that also something that is needed? The Hyperstar and Fastar arrangements will move the primary mirror, but somehow I don't think the dobs are set up for this.
25-05-2012, 08:27 PM
The only photo I have seen was from Europe and it was a commercial One Metre wide, F2 Az/El rocker box style of Goto scope. It had 4 truss rods going from the rocker box to the camera holder ( no spider), and all the cables fed down the inside of the truss rods.
I guess with 1 metre wide aperture you ain't going to track for very long, so field rotation would not be an issue.
If you got your measurements from primary to CCD chip correct, you would only need a fine focuser. A simple method would be similar to the middle bolt on the secondary. By using a 1 inch fine threaded bolt and nuts, you could lift the camera holder up or down, only need +/- 5mm.
You would also need some safety chains in case things go kaput.
For cabling I have a dead Epson A3 dot matrix printer, the carriage motor uses a long thin ribbon cable with heaps of tracks on it, so everything could use the one cable, USB, power, focuser and dew heater.
Unfortunatly if I were to go ahead with this type of project, I have heaps of other issues with this scope to solve first, to make it viable.
Hmm. Yes, that could be a minor technical issue John......!
26-05-2012, 12:37 PM
There are commercially available telescope setup for this.
Ken the 18" you mentioned perhaps its this one Centurion 18.
Also this chap Johanes has done some amazing work with his scope.
http://panther-observatory.com/ see under Observatories - Telescopes. He has a 16" Cassegrain that can be used at Primary focus and Secondary focus. Its made by Philipp Keller, link below to some info on his scopes range from 400mm to 600mm with prime and secondary focus options.
27-05-2012, 11:47 AM
Another thing, you will need to lengthen the distance between your primary and spider/focuser/imagining plane by an equivalent distance as from the secondary across to your imaging plane, possibly somewhere around 6-12" depending on the size of your primary and the focal plane point.
I suppose it all comes down to, what are you going to gain by doing it?
Seems like a lot of work and cost, for the gains you "might" achieve.
30-05-2012, 01:09 PM
Many thanks for those links Netwolf, images off that Centurion are great. I wouldn't like to wheel the beast in and out every night though.
Looks like prime imaging is mainly used by big mirrors and low F ratios.
I agree Tigerdes it wouldn't be worhwhile modifying a fully functual scope, but if starting from scratch with a big mirror, would be OK.
The trade off from buying a large secondary and 3 inch focuser would be in trial and error getting the concept to work properly.
If it does work ( smooth focus, easy collimation, no vignetting), you gain extra light (10%) than conventional.
Here are a few more examples:
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