12" REFLECTOR - Work has begun.

[PKay]

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.

[billdan]

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.

[PKay]

Thanks Bill for replying.

Yes I have a telrad and will use it.

Why do you think the OAG system would be better?
Is it to do with the longer focal length?

Will my present guide system (separate scope and camera) not work?
I intend to mount it at the central balance point right next to the dove tale.

[billdan]

Hi Peter,

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.

Cheers
Bill

[PKay]

Thanks Bill

What you say makes sense.
I will give this rig a go and see what happens, mainly out of interest.

And then consider OAG if I have to.

[PKay]

Bill (or anyone who has any idea)

Just a thought.

If I was to mount the guide scope further toward the front of the main scope, would that help?

[billdan]

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.

Cheers

[xelasnave]

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.

[Amaranthus]

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...)

[PKay]

Hi Barry

I was wondering when something like this would come up.
I will give it a go, maybe in the end just for the learning experience.

At that point I may justify spending mega bucks :-)

[Camelopardalis]

If you get the balance right it might be OK. Any big scope is going to act like a sail in any wind.

[doppler]

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.

[Amaranthus]

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.

[doppler]

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.

[PKay]

Interesting perspectives.

It is hard to take it all on board.

So in the style of 'suck it and see' the welder has been running hot for a few days now.

The rib structure design comes from...wait for it...a toilet roll!!

The mirror sits on the left of the image.

The 2 bolts are for the dovetail.

And then...attack with the 60mm holesaw.

[ChrisV]

It's great. I'm enjoying watching this build :thumbsup:

[xelasnave]

I am inspired.
I have an idea for a 12 inch
No secondary just a camera four sruts from the mirror.
Alex

[xelasnave]

Maybe just three stuts.
Alex

[ZeroID]

Isn't that the Hyperstar configuration for Maks ?

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 ...:question: Could then afford the minicamera ... maybe.

I feel another project coming on ... :shrug:

[xelasnave]

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

[xelasnave]

I forget the fl of mine but it would be a little long maybe near six feet.
A design with yet no measurements...
Alex

[RickS]

An image scale of 0.63 arcsec/pixel is a good match for imaging small objects like galaxies. It will require good guiding but I'd argue that it's not heavily oversampled unless the seeing is consistently horrible. Image scale of 0.63 arcsec/pixel is well sampled for seeing around 2.2 arcsec. That might be better than average seeing in a lot of places but it's nice to able to take advantage of good conditions when they arise.

Cheers,
Rick.

[billdan]

If your interested in putting the camera at prime focus, you could keep it simple and do what this guy does with a 14 inch.
No idea how he controls focus though.

EDIT: You wouldn't have to worry about the wind, and at a dark site, stray light shouldn't be an issue.

[xelasnave]

And I thought three struts was simple.
Focuser could be moved with a simple nut and bolt arrangement pushing it up and down...
I wonder how his images turn out?
Alex

[billdan]

Alex, like what you said but with a motor for accuracy, you would need to buy a linear actuator like I've attached that pushes or pulls the camera up and down.

[xelasnave]

Thank you Bill.
Alex

[bojan]

Hmmm...
This design gave me some ideas...

I have 10" f/5.6 Newt built with Parks fiberglass tube (33cm dia) and Coulter Optical mirror (5cm thick, 4.5kg).
The OTA (with everything on) has ~12kg, a bit too much for EQ6 (yes, I know, lot of people will say the carrying capacity of EQ6 is 25kg (with counterweight), but generally I prefer not to load things up to the hilt).

To reduce the load, I built the Serrurier truss frame with 25mm aluminium square tubing, but ended with 10kg OTA (frame itself is 8kg, only 2kg weight reduction).

Perhaps the removing of the middle part of the fiberglass tube and replacing it with struts will be more weight-effective... I need to do some number crunching now.

[PKay]

So...progress on by butcher shop effort.

Lifting it all up onto the mount I thought about Strongman Mike.
Wish he was there to help...

Balance is good. Had to add 3.6kg of lead (half a day forging).

Total weight of rig ended up at 20.5KG

The EQ6-R copes easily with slewing at top speed.

First issue:
I attached the Laser Collimation tool and spun the scope around at all angles.
Not a good result. It varied quite a bit.
May well be a show stopper.

Tonight will be to find focus using the moon and take a few happy snaps.

Tomorrow will attach the guide scope, and well...have no idea of how that will turn out.

[Amaranthus]

However it works out, I love the MacGyver skills on show.

[PKay]

I love that show (or did).

Last night was a disaster.
The rack & pinion focuser seized.

This morning I found a metal shard in the works.
Fixed it.

Moon was still out so tried to get focus.
After an hour of pure frustration I gave up...to find...I had left the cover on the main mirror :screwy:

[ZeroID]

Should keep my big mouth shut, :rolleyes:
I just picked up an 8" f6 for a good price. I think the seller might have aimed it at me after I enquired and told him what I was doing. Also includes a laser collimator. No diagonal. He had it as a project but gave up.

I'll modify the 8F8 OTA first, replace the front struts with 400 mm shorter ones which should make the scope much more manageable in the Ob. See how that works in a Newt configuration. Then I'll think about other options on prime focus... or I might just do that first .. hmmm. :question:
ASI 120mc ?
!600mm with manual filter changes ?
Options .... :confused2:

[xelasnave]

Bojan that looks magnificant.
If you want to loose weight have you looked into drilling out stuff.
Even at current weight it will work on eq6 if on a pier and if you balance for the region you shoot.
But you want remote but you can still balance for a particular region even remote.
I have been thinking but I dont know if the 12 inch is still there as the house has been empty and in the bush etc.
I have been thinking three carbon fibre struts but you would loose via primary mirror coverage..need to work out viable pole diameter and what it might take away and how to manage difraction which I think I could do...maybe.
Anyways I would like to rescue that mirror even if I have to build a new scope around it..
I probably will need a block and tackle the mount is the least of my worries.
I have been slack the cube obsrvatory has not progressed I have to get it up a floght of stairs and I keep putting it off.
Still have to decide on which roof or lid design to go with.
Alex

[xelasnave]

I am thinking round poles will cause difraction problems forgeting area loss...so I am thinking a like ban the bomb templete at the top could be cardboard really that may fix it ..
Alwx

[xelasnave]

Even with 1inch rods you would lose 15 %
I expect you would have to brace them with wires maybe fill them with foam back each russ up with a series of smaller poles...thats it...
Say a 1 " pole secured to a 3/4 inch to a half to a quarter and thst may do it and if so it would little weight...and maybe hold the mirror in a little different using carbon fiber foam sandwich. ..and maybe wrap the poles in carbon fibre ...
Alex

[bojan]

Polyurethane foam was definitely planned.
Holes ø7mm in centre of the each rod, from where the rod was filled from can.

[PKay]

Barry (& Rick), I think you were right.

After 8 hours of testing it sort of worked.
I had to re balance & collimate at the same attitude as the target.
And then re calibrate PHD as well.

Soon as I moved the scope it was all over.

Too much hassle.

I did get a result however:
http://www.iceinspace.com.au/forum/attachment_browse.php?a=223972

[raymo]

Bojan,
if you're not prepared to image with 12kg on an EQ6R, you might as
well get an EQ8, and be done with it. The 6 is rated for a payload[not total
load] of 20kgs; 12kg is only 60% of that, which is just fine, the 6 will cruise along. The 6 is not rated to 25kg including weight[s], it is rated to 20kg
carrying the necessary weights to balance that payload, which will be at least three 5kg weights, plus an extension shaft. I ran just over 10kg on my
HEQ5 for years with no problems.
raymo

[bojan]

EQ6 (I have two, one donated by Avandonk couple of year ago, I am using it when in Ballarat) may just work for my 10" Newt, especially because I plan to house it under the dome...
It is just my feeling the rating (20kg) is a bit optimistic (for imaging), I tried to load it once with my Newt (12kg, mount was on tripod) and, well, it was OK, it didn't break down but it was swaying a bit when pushed.
Maybe the situation will be better when on pier.

EQ8 is still too expensive... however if something good comes my way, I will take it :-)

[raymo]

Yes, 20kg is too much for imaging, but over the years I have used several
sixes for imaging with payloads of 12-15kg with no problems. I even put
12.5kg on my HEQ5 once, and whilst it was obviously at its limit, it did the
job without complaining. Its not really the weight that limits these mounts,
they are pretty solid, its the windage, its amazing the load they will carry when on a pier.
raymo

[Amaranthus]

Sorry it didn't work out Peter. But definitely don't give up on imaging with a fast newt - they are great. And remember, image scale rules, so get that right. It means matching your focal length with your camera specs.

[PKay]

Hi Barry (or if any one else knows)

I don't understand 'image scale', so i have supplied a few facts about my equipment:

I have imaged with both scopes, and both seem to work fine.

Camera: 13.4 X 17.7mm sensor. 3.8um pixel size.

Scope 1:
85mm refractor.
f/6.6
FOV 82 X 108 arcmin.
Resolving power 1.36 arcsec.
Image scale 1.4 arcsec/pixel

Scope 2:
304mm reflector.
f/5
FOV 30 X 40 arcmin.
Resolving power 0.45 arcsec
Image scale 0.51 arcsec/pixel

[bojan]

??
Image scale tells you how many arcsec on the sky are covered by single pixel.

[PKay]

thanks bojan

Simple when you look at it like that.

I suppose my question should be about how image scale and camera resolution are related.

I was told that the big scope would result in oversampling??

[bojan]

There is no direct relationship between image scale and camera resolution, scale is related to geometry of the optical system (focal length and physical size of the pixel
But smaller scale (less arcsec per pixel) means higher resolution, if not limited by optical quality of the telescope.
Oversampling could happen if the physical pixel size is much smaller than Airy disk... and generally it happens if aperture is small and FL is long.

[Amaranthus]

The image scale is good for your refractor and oversampled for your reflector (though not heavily). You could try 2x2 software binning for the latter setup. But also relevant is your typical seeing at your imaging site. A general rule of thumb is to try and get the image scale at somewhere between half and a third of your average seeing. If it is way 2 arcsec then aim for an image scale in the range of 0.7 to 1 arcsec/px (roughly). Most sites are a bit worse, however, so something in the range of 0.9 to 1.5 is usually great.