My trusty ole lightbridge is getting a complete makeover.
It will become a dedicated astro photography platform (I hope).
Am working in conjunction with an engineer for structural design issues.
Having said that, if I have overlooked the bleedingly obvious then please let me know!
1: Removed the large support bearings - not a DOB anymore.
2: Check weight - 19.2kg
3: Total weight must be below 20kg (mount limit)
4: Removed 500 grams from bottom ring beam (30mm holesaw).
5: Removed 500 grams from top tube (60mm holesaw).
6: Installed camera (the camera will face downwards) and re weighed and found balance point .
Still to go:
7: Weld on dovetail mount bracket
8: Weld on structural ribbing on bottom tube.
9: Re weigh and calculate how much weight reduction is needed on bottom tube.
10: Attack the bottom tube with 60mm holesaw.
11: Mount guide camera and finder.
Peter, looks good what you have done so far, very professional.
You'll need a shroud and a dew shield, which you can make out of a yoga mat. These are lightweight and cost about $8 ea at Kmart they are 500mm wide and 1800mm long, 5mm thick.
I think you should employ an OAG and not use the finder for guiding, but get a Telrad or RDF for your first star alignment. Or just blind platesolve and not bother with The Telrad /RDF.
I wasn't thinking of the difference in focal length as much, but that does also come into it. The finder guider is a focal length of 320mm vs (assuming F5 for the 12 inch) 1250mm. This means 1 pixel of movement in the finder/guide cam relates to nearly 4 pixels of movement in the imaging cam.
I was thinking more of flex between the two scopes. The primary is never bolted down but a small degree of movement is allowed ( or you get astigmatism), that's why mirror clips are never supposed to touch the mirror.
So the movement of the scope across the sky means the primary can move by a few microns.
There is also sag in the focuser from the weight of the imaging chain that will change as the scope moves across the sky ( due to gravity).
Now an OAG will pick up all those movements and compensate for it.
With my old EQ6 and the 8in F5 it made a big difference going from finder/guider to OAG. Before I was limited to 3 min exposures max before I got egg shaped stars, with an OAG I could easily do 15 min subs with good round stars.
Having said all that OAG's are a pain to get right, it took me 4 hours to get the Orion thin-OAG working properly.
My finder/guider was at the front of the scope, but I don't think it really matters. Just make sure it is bolted down solid and make sure it is aligned with the main scope.
Centre a star with the main scope and make sure that same star is centred in the finder/guider, or else you could get field rotation if the guider is looking at a different part of the sky.
It all looks good Peter.
I did some photos about 10 years ago with my 12 inch full tube riding on my eq6.
I would line it up manually rather than slew using the motors and made a point to go for stuff somewhat overhead. Also I would balance everything for that specific region...
I could run it unguided with reasonable results.
But when it came to setting up and putting it away I had to give up on the idea.
Maybe place your guide scope as close as possible to the main camera on the basis that any flex will manifest on both.
Perhaps even collminate when the scope is in place to capture ...maybe a cord pulling down one side to flex it out...
Or place the guide scope ahead on the mounting plate...
In any event you will get to a place where you will know safe exposure times so really be happy working in that zone.
For the present I have limited myself to 30 second exposures whereas ten years ago although I would post such short time exposures I never regarded short exposures as useful..
But they work and so if you can only get say ywo minutes I think you will get some great captures.
It is like playing a guitar...you find its limit and play it knowing how far you can push it and be happy ...
You may reach a point where the whole set up you now work upon will not meet your expectations at which point you may find investing big money is the next step.
Anyways I hope you complete this project to your satisfaction.
Peter, not to be a spoilsport, but 20kg is going to be too heavy for the EQ6-R for imaging, especially at that focal length. A good rule of thumb is to try not to go much beyond half of the mount’s rated capacity for astrophotography. Also, your image scale will be 0.63 arcsec per pixel, which is a pathway to pain for guiding (and will be heavily oversampled...)
I always wonder about the payload limits in regard to imaging. I have been using a 10" f5 on a HEQ5 pro at maximum rated payload for over a year now without issue. The scope slews nicely and tracks/guides smoothly (most of the time). The only issues are with balance and wind, but these are related to the physical size of the ota not the weight.
I did a google search to see if anyone had broken a mount by overloading it, only found one example and that one toppled over, the tripod gave way. I have mine on a steel pier next to the shed, only a wheelbarrow of concrete underneath, it hasn't moved yet.
I say go for it, I have come across a few in the past successfully imaging with a 12" on an NEQ6.
Rick, it’s not about whether the rig will hold it, its about whether the imaging will be usable. Also, I agree re: 12” newt, but astrographs, even at this size, are generally much lighter than 20 kg... or else people determine they need a bigger mount!
Peter, you do not need to spend mega bucks. A cheap route is to get an 6” f/6 newt which will have a 900 mm focal length and will weigh only ~5 kg. See for example the GS-530 from Andrews Comm, for just $350. And at f/6 you can get away without a coma corrector.
You’d get a lot of joy and great images with such a setup, and learn a lot, minus the frustration. Your mount and camera are terrific, so no need to further invest in those with such a setup.
Barry, aperture has got me, I have smaller scopes for imaging ( 8" and 6" reflectors) but I keep going back to the 10". It's certainly not easier but everything is much brighter and much better resolved in the bigger scope, so shorter subs required and less time for guiding errors to manifest.
But then again I've come from film imaging and find modern mounts and digital camera's a lot less frustrating or challenging to get some reasonable results.
I nearly built a newt in that configuration but it would have been too long to fit in the Ob cos it was an 8" f8 mirror ( 1600mm + )
If I can find a cheap enough 8"-10" F5 mirror I may have another go with the ASI1600 on the front of it. Or maybe one of the new ASI mini cameras.
Might 'borrow' the mirror out of the Serrurier ... Could then afford the minicamera ... maybe.
Brent...
Same here.
I have been thinking and drawing that 12 inch...three struts so there is nothing to it...
Designed a focuser..
I am thinking three poles from the mirror straight to a point holding the camera but I am not sure how having the poles in the optical path may present problems. ..
But the prospect of having a ultra light rig that works ...well the chalenge of it...I am thinking strimgs under tention to add strength...easy to do...effective to minimise flex.
Alex
Could then afford the minicamera ... maybe.
