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Old 24-11-2013, 11:36 AM
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Peter.M
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operation fastscope

Hi guys, I have been thinking about the possibility of creating a superfast scope for doing narrowband imaging from my house. I have a GSO 8 inch f4 in pieces and thought that would be a good starting point. What I am thinking of doing is creating a carbon fiber truss tube to hold the mirrors. The main thing that I want to do is eliminate every point of movement in the system, which I feel is the main weakness of the scopes. This would involve making my own spider as the stock one is too weak and redesigning the mirror cell which I also think is a major weakness.

My eventual plans would be to reduce the scope to f2.8 with one of the ASA reducers, but I am a little worried that the level of correction will not be up to scratch. The only images I can find online have weird stars, and I don't know if that is because of the operator not getting collimation perfect or their scope didn't hold collimation well enough for f2.8 imaging. It would be an expensive exercise to get one and then decide that it wasn't corrected enough for me. If anyone has experience with these reducers, specifically with the kaf 8300 that would be greatly appreciated. Alternatively if anyone has suggestions on any part of the build that would be helpful too.
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Old 24-11-2013, 07:50 PM
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bkm2304 (Richard Brown)
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Hi Peter,

I would suggest you get an off the shelf Celestron Fastar or Starizona Hyperstar modification for those scopes. This converts an F10 to F2 and gives about 25x the field of view.

Have a look here.

Richard.
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Old 24-11-2013, 07:53 PM
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Sounds like a good idea Peter, would be great for widefield shots. I think you'd have to possible alter the secondary (Depending on your CCD size). Not sure if you are aware but Teleskop Express sells off-the-shelf CF upgrades for GSO scopes. They dont come cheap though
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Old 24-11-2013, 08:58 PM
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Can I ask why the scope is in bits?, an 8"f4 should be fine for Adelaide's skies I would think.. although at f2.8 wow thats cutting down the exposure times... Would it be cheaper just getting an ASA scope, or even a SCT with the Hyperstar from Starizona.. Much less hassle to if the reducer doesn't work with the GSO optics.
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Old 24-11-2013, 09:07 PM
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AstroJunk (Jonathan)
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Just a little word of warning, I love my f2 Hyperstar - simply stunning performance, but I have heard that there are issues with narrowband filters at that speed. Have a good read of the filter manufacturers sites before committing to anything less than f3!
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Old 24-11-2013, 10:05 PM
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Why not just use a bigger chip at f4 and bin the pixels?
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Old 26-11-2013, 12:31 PM
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this sounds like an awesome project. will be following with interest!
good luck peter!

Russ
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Old 26-11-2013, 06:43 PM
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I've got a friend who has been really struggling to get his Hyperstar producing decent images. It can collect a lot of data really quickly but keeping decent collimation and focus is major PITA.

Unless everything is locked down really hard and you have a great focuser I'd not recommend going much under F/4.0.

Good luck,
Cam
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Old 26-11-2013, 07:31 PM
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Quote:
Originally Posted by bkm2304 View Post
Hi Peter,

I would suggest you get an off the shelf Celestron Fastar or Starizona Hyperstar modification for those scopes. This converts an F10 to F2 and gives about 25x the field of view.

Have a look here.

Richard.
I am thinking of creating a newtonian not a SCT.

Quote:
Originally Posted by Meru View Post
Sounds like a good idea Peter, would be great for widefield shots. I think you'd have to possible alter the secondary (Depending on your CCD size). Not sure if you are aware but Teleskop Express sells off-the-shelf CF upgrades for GSO scopes. They dont come cheap though
Part of the allure is creating something myself, so I wouldn't buy the tube off the shelf.

Quote:
Originally Posted by Astroman View Post
Can I ask why the scope is in bits?, an 8"f4 should be fine for Adelaide's skies I would think.. although at f2.8 wow thats cutting down the exposure times... Would it be cheaper just getting an ASA scope, or even a SCT with the Hyperstar from Starizona.. Much less hassle to if the reducer doesn't work with the GSO optics.
The scope is in bits because I was trying to track down sources of collimation wandering, and then I ended up building my current scope. The corrector itself is around 1200 an ASA scope while I would love to own one would set me back over 10 grand landed.

Quote:
Originally Posted by clive milne View Post
Why not just use a bigger chip at f4 and bin the pixels?
The attraction of sticking with the current camera is partly due to cost, but also due to the larger chip requiring a larger secondary mirror and corrector.

Quote:
Originally Posted by LightningNZ View Post
I've got a friend who has been really struggling to get his Hyperstar producing decent images. It can collect a lot of data really quickly but keeping decent collimation and focus is major PITA.

Unless everything is locked down really hard and you have a great focuser I'd not recommend going much under F/4.0.

Good luck,
Cam
I don't think I have seen a decent image produced with a hyper star. The focuser I would be using would be able to use focusmax so I would not have to bugger around doing it, and using carbon fiber should hold it in focus well.

I am less worried about the tight requirements at F3ish and more worried that even in the goldylocks zone the corrector will still not perform as I want it too.
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Old 26-11-2013, 08:06 PM
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Quote:
Originally Posted by Peter.M View Post
I don't think I have seen a decent image produced with a hyper star.
http://www.flickr.com/photos/ewwwtop...27231/lightbox

Either your standards are incredibly high, or you haven't looked very hard
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Old 27-11-2013, 06:04 AM
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multiweb (Marc)
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Quote:
Originally Posted by Peter.M View Post
Hi guys, I have been thinking about the possibility of creating a superfast scope for doing narrowband imaging from my house. I have a GSO 8 inch f4 in pieces and thought that would be a good starting point. What I am thinking of doing is creating a carbon fiber truss tube to hold the mirrors. The main thing that I want to do is eliminate every point of movement in the system, which I feel is the main weakness of the scopes. This would involve making my own spider as the stock one is too weak and redesigning the mirror cell which I also think is a major weakness.

My eventual plans would be to reduce the scope to f2.8 with one of the ASA reducers, but I am a little worried that the level of correction will not be up to scratch. The only images I can find online have weird stars, and I don't know if that is because of the operator not getting collimation perfect or their scope didn't hold collimation well enough for f2.8 imaging. It would be an expensive exercise to get one and then decide that it wasn't corrected enough for me. If anyone has experience with these reducers, specifically with the kaf 8300 that would be greatly appreciated. Alternatively if anyone has suggestions on any part of the build that would be helpful too.
Been down this path and as you said changed the spider which helped tremendously. I've also modified the primary cell mounting and that made it rock solid and hold collimation. So no problem stiffening it. I have reservations bringing it down to F/2.8 though. I'm not sure if you'll manage to correct the whole imaging circle with shorter FL.
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Old 27-11-2013, 06:41 AM
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I think that GSO/ASA combo is the basis of the Boren-Simon scopes. Lots of images from them out there and they look OK to me.

the ASA corrector specs show it covering the full 8300 sensor.

Worth checking how much sensitivity you lose due to waveband shift in narrowband filters at the light cone edges at f2.8. Might limit how narrow the filters can be or reduce the extra gain you get from the faster scope.

Crikey you are brave to want to go to f2.8 though - wasn't getting/keeping things aligned and focused at f4 enough of a challenge? I would be particularly concerned about image plane tilt with that fast a scope. Carbon fibre will help with temperature stability, but there will be other bits in the image train that are aluminium, so you will still have focus drift with temperature (enough to be an issue at f2.8). Good luck though - sounds like a fun project.

Last edited by Shiraz; 27-11-2013 at 06:52 AM.
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Old 27-11-2013, 07:09 AM
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Quote:
Originally Posted by multiweb View Post
Been down this path and as you said changed the spider which helped tremendously. I've also modified the primary cell mounting and that made it rock solid and hold collimation. So no problem stiffening it. I have reservations bringing it down to F/2.8 though. I'm not sure if you'll manage to correct the whole imaging circle with shorter FL.
I too have reservations about the level of correction at such a fast focal ratio which is why I asked if anyone was using one. I have read about your modifications and they look great.
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Old 27-11-2013, 07:14 AM
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Quote:
Originally Posted by Shiraz View Post
I think that GSO/ASA combo is the basis of the Boren-Simon scopes. Lots of images from them out there and they look OK to me.

the ASA corrector specs show it covering the full 8300 sensor.

Worth checking how much sensitivity you lose due to waveband shift in narrowband filters at the light cone edges at f2.8. Might limit how narrow the filters can be or reduce the extra gain you get from the faster scope.

Crikey you are brave to want to go to f2.8 though - wasn't getting/keeping things aligned and focused at f4 enough of a challenge? I would be particularly concerned about image plane tilt with that fast a scope. Carbon fibre will help with temperature stability, but there will be other bits in the image train that are aluminium, so you will still have focus drift with temperature (enough to be an issue at f2.8). Good luck though - sounds like a fun project.
I did think about the bandpass shift at f3 but using 7nm filters I'm not sure if it will be an issue. If it was I could potentially screw the filter to the front of the corrector and filter it at f4? Not sure if anyone has tried that but it seems to logically be possible.
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Old 27-11-2013, 07:28 AM
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Originally Posted by Peter.M View Post
I too have reservations about the level of correction at such a fast focal ratio which is why I asked if anyone was using one.
At shorter FL even if you manage to correct your imaging circle I doubt the tube and focuser mounting will be stiff enough and you might get flexure issues and collimation shifts unless you use a set of rings and brace it in position. Also do you think the optics quality is good enough. Just a thought. If you're set on building a F/3 system you might as well want to consider buying a quality primary/secondary pair then build from scratch rather than modding a GSO. Mechanically it's going to be a lot of work. Maybe get a good primary cell from Protostar or other and start from there.
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Old 27-11-2013, 07:43 AM
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Originally Posted by multiweb View Post
At shorter FL even if you manage to correct your imaging circle I doubt the tube and focuser mounting will be stiff enough and you might get flexure issues and collimation shifts unless you use a set of rings and brace it in position. Also do you think the optics quality is good enough. Just a thought. If you're set on building a F/3 system you might as well want to consider buying a quality primary/secondary pair then build from scratch rather than modding a GSO. Mechanically it's going to be a lot of work. Maybe get a good primary cell from Protostar or other and start from there.
The idea would be that the gso mirrors are the only thing from them in the scope, the rest would be custom made from carbon fiber. If the system worked then it would be trivial to replace the mirrors with better quality ones.
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Old 29-11-2013, 02:48 PM
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The idea would be that the gso mirrors are the only thing from them in the scope, the rest would be custom made from carbon fiber. If the system worked then it would be trivial to replace the mirrors with better quality ones.
I'd get a conical mirror with a central bolt from royce or other and high quality secondary then a proper primary cell and build it from there onwards, truss or tube. I tweaked my GSO to a point that I'm happy with how it performs for a 500 bucks scope but that's it. It's still a 500 bucks system with sh|tty mechanics, mildly rough mirror but 'good enough' for what I do. If you're going to invest your time and money in a fast system start with quality components I reckon.
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Old 30-11-2013, 09:21 AM
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Originally Posted by Peter.M View Post
The idea would be that the gso mirrors are the only thing from them in the scope, the rest would be custom made from carbon fiber. If the system worked then it would be trivial to replace the mirrors with better quality ones.
I would fabricate a rigid truss out of (welded) steel and stick with the gso mirrors which are more than good enough. The money you save would go a long way towards buying an atlas focuser.
There is a reason why professional ground based telescopes are still to this day engineered with steel trusses, this being that carbon fibre and all the associated ball end connectors do not offer sufficient practical advantage to warrant their expense. I would also go so far as to say that a steel truss can be made with intrinsically better geometry compared to an assembled carbon fibre truss (as distinct from a monolithic cf truss) because you don't have to make do with trapezoids which only approximate triangles.

c
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