Camera advice

[xilman]

Anyone prepared to pontificate on what kind of camera I should consider plugging into my 250cm F/4 telescope? The only hard constraint is that it produce machine-readable images without any tedious mucking about with chemicals. Although that technically that allows for Polaroid™ devices, suggestions along those lines will be quietly ignored.

As I'm primarily interested in photometry, a wide field of view is desirable, which to me suggests a relatively large detector, physically if not necessarily in terms of pixels.

Right now there are a few soft constraints. I already have a buggered-about-with webcam mounted in a home-made 1.25" eyepiece adapter which has a tiny detector with 640x480 pixels and a 8-bit (?) dynamic range. It works but I want something significantly better.

Another soft constraint is that it be as cheap as possible. I want it for a learning exercise and will consider something more capable when I know what I'm doing. If it is going to be disposable, I don't want to dispose of too much money. Being able to sell it on without too much of a discount would be advantageous.

A softer constraint is that it be as light as possible. A Newtonian telescope has the image plane which is far from the centre of gravity and well away from the symmetry axis of the telescope tube; this can lead to mechanical balancing problems.

Cursory rummaging around on eBay suggests that a 10-15 year-old DSLR camera body with a 3 - 10M pixel CMOS or CCD sensor is the most likely candidate but that leaves a rather wide choice and further advice would be most welcome.

[fivemack]

I think you are looking in exactly the right direction; I think the Nikon D100 is about the right point to look at, and that's a fifty-quid purchase from eBay for what was a £2000 professional camera when it came out. I used one happily (I was second owner) around 2006-2009.

It's 700 grams, but you're not going to get much lighter for DSLRs of that age - the Canon 300D is 50 grams less and maybe even cheaper on eBay, you're at the point where the T-ring adaptor is a perceptible fraction of the camera cost.

[xilman]

Originally Posted by fivemack

I think you are looking in exactly the right direction; I think the Nikon D100 is about the right point to look at, and that's a fifty-quid purchase from eBay for what was a £2000 professional camera when it came out. I used one happily (I was second owner) around 2006-2009.

It's 700 grams, but you're not going to get much lighter for DSLRs of that age - the Canon 300D is 50 grams less and maybe even cheaper on eBay, you're at the point where the T-ring adaptor is a perceptible fraction of the camera cost.

Thanks.

I just got a Canon EOS 10D with manual, charger and two batteries for £53 + £8 delivery on eBay. Also picked up a "1.25" Telescope SLR Camera T Ring and T Camera Adapter / 2x Fully Coated Barlow Lens Kit for Telescope Photography (Canon T ring+ ..." for another £23 new from Amazon.

If it all goes horribly wrong, the Barlow will still be useful for visual work (I don't have one already to go with my eyepieces) and I can probably sell the rest of the kit for most of what I've paid for it.

The Nikon would have been lighter and has a slightly larger sensor but slightly smaller number of pixels, so slightly poorer angular resolution. There weren't any for sale when I dispelled enough apathy so I went for the Canon equivalent instead.

[Dubslow]

Slightly hijacking this as a PSA to those certain few here who will be in the right place at the right time with suitable equipment: a UK redditor has calculated that the next SpaceX launch of its Dragon capsule to the ISS, occurring in ~one week, should have good relative visibility from the UK. He went to some lengths to acquire suitable orbital elements data and to extrapolate it for viewing information. Alas, he laments that he has not the proper camera and telescope equipment to get a good view of the event.

Which of course those here do have, in addition to plenty of experience locating things in the sky. Good luck.

[xilman]

Originally Posted by xilman

I just got a Canon EOS 10D with manual, charger and two batteries for £53 + £8 delivery on eBay.

It arrived this afternoon. The postage "stamp" (actually a printed label) was for £15.75 so the vendor seriously underestimated.

All looks well, though until the batteries have been charged (one completed recently) and the other stuff has arrived (expected by the weekend) it's hard to be sure.

[xilman]

A 2x Barlow lens, eyepiece adapter and T-ring ditto arrived yesterday so it was possible to take rather out-of-focus and over-exposed images of a nearby tree through (effectively) a 2032mm F/8 mirror lens.

Going to take quite a bit of practice before real astronomy can be done though.

[diep]

Originally Posted by xilman

A 2x Barlow lens, eyepiece adapter and T-ring ditto arrived yesterday so it was possible to take rather out-of-focus and over-exposed images of a nearby tree through (effectively) a 2032mm F/8 mirror lens.

Going to take quite a bit of practice before real astronomy can be done though.

i got all that as well (barlow and so on) yet regrettably i live in The Netherlands which is about the worst kind of nation for astronomy with my 250 mm telescope into anything.

Instead of barlow i had also bought some pretty expensive better optics to enlarge up to 480x or so effectively at a good night if i move the telescope to the military training field where only now and then some car lights cause a problem from a nearby road, it doesn't make sense to enlarge to more than 110x or so. anything above that won't show better.

Where my current telescope is a simple manual thing,
Idea here now is to modify one day as a hobbyproject a cheap trailer which has an automatic telescope on it. then drive a couple of hours direction France sometimes, park it somewhere at some old battlefield from world war 1, which seems to have some spots with very good visibility according to a map i saw of Europe there, then push a button and the t elescope automatically unfolds (stiff legs that get automatically out of the trailer that take care the entire construction is stiff on the ground and then enjoy the view...

p.s. note that for my viewing purposes a wide field is not so interesting to have. Even manual tracking what is many lightyears away is not so tough after a while yet with my workshop nowadays it shouldn't be so hard to use a few small motors to do that tracking automatically as well.

As for the telescope to build on that trailer - it's easy to scale it up a tad. A first and secondary mirror (ritchey chretien) are not overly expensive to buy. that won't be the utmost highest quality mirror then that you'd like observatoria to have - yet half a meter is just a couple of thousands of dollar.

A good question is how to build a construction on the trailer such that during transport the telescope doesn't get damaged :)

Another question is how to build other stuff on the trailer as well (small bathroom especially). that small trailer in CAD here probably soon will grow to some sort of huge heavy dreadnought :)

[xilman]

Originally Posted by diep

As for the telescope to build on that trailer - it's easy to scale it up a tad. A first and secondary mirror (ritchey chretien) are not overly expensive to buy. that won't be the utmost highest quality mirror then that you'd like observatoria to have - yet half a meter is just a couple of thousands of dollar.

A good question is how to build a construction on the trailer such that during transport the telescope doesn't get damaged :)

Another question is how to build other stuff on the trailer as well (small bathroom especially). that small trailer in CAD here probably soon will grow to some sort of huge heavy dreadnought :)

Only a very quick response. More to come later I hope.

The R-C design is compact and gives excellent images over a wide focal plane. It does so at the cost of requiring extremely precise collimation. The reason being is that the secondary is hyperbolic and so has a centre of symmetry which must lie precisely on the optical axis and the rotational symmetry axis of the hyperboloid must coincide which the optical axis of the primary.

In a traditional observatory collimation, once achieved, tends to persist. I have real doubts whether the same could be said for a system bouncing around in a trailer.

You might wish to consider a Dall-Kirkham which has a spherical secondary, therefore no axis of symmetry, and much easier to get and maintain collimation. The modified Dall-Kirkham uses a lens system to achieve a wider field of view.

In a bright sky, where low focal ratios do not perform as well, you may also wish to consider a classical Cassegrain or transfer Cassegrain for a large image scale in a short tube.

[diep]

Originally Posted by xilman

Only a very quick response. More to come later I hope.

The R-C design is compact and gives excellent images over a wide focal plane.

the wide focal plane is not the main reason to think of Ritchey-Chretien.

Of course astrography is one of the reasons yet the real main reason is that the image gets out of the telescope at the back of the telescope which for logistical and design reasons (in my limited design world) is simple. If you look through it without camera attached it's much easier. I want to sit somewhere :)

At the newton i got now, i need to adjust position too much.

> It does so at the cost of requiring extremely precise collimation.
> The reason being is that the secondary is hyperbolic and so has
> a centre of symmetry which must lie precisely on the optical axis and the
> rotational symmetry axis of the hyperboloid must coincide which the
> optical axis of the primary.

It's very good you mention that the secondary mirror needs precise collimation.
I didn't realize it needed it that much better than a newton.

As for the hyperbole versus parabole mirrors, that has to do with size of the mirror. For example we can calculate that for the hubble telescope, the "public" hubble, if we calculate the difference between a hyperbole and parabole then the largest difference is far under 0.1 nanometer between the 2. Obviously they couldn't produce it that accurate so obviously it would've been very simple to test the primary mirror of the hubble telescope by just checking whether it was a parabole.

For the size of mirrors i would be able to use there is however a clear distinction which is why i wouldn't want to produce those mirrors myself.

>In a traditional observatory collimation, once achieved, tends to persist. I have >real doubts whether the same could be said for a system bouncing around in a >trailer.

as for the collimation i intended to have that automatically adjusted. it is obvious to me now some big testing is needed there whether this is simple to do for either sort of design that gets chosen.

should be no problem i suspect if you take the same stars and do some camera vision techniques. Just some work to get it all automated is my guess. Historic words of course! :)

> You might wish to consider a Dall-Kirkham which has a spherical secondary, > therefore no axis of symmetry, and much easier to get and maintain collimation.

Very interesting. I will take a look at it.

The modified Dall-Kirkham uses a lens system to achieve a wider field of view.

A wide field doesn't have my full attention as of yet (might change one day). That's more interesting if you want to detect UFO's i suspect or stuff that's going around the planet Earth or for some E-Elt type observatory where you want to do new discoveries. Obviously as a layman, not even amateur, it's wishful thinking to discover something new.

>In a bright sky, where low focal ratios do not perform as well, you may also wish >to consider a classical Cassegrain or transfer Cassegrain for a large image scale >in a short tube.

[/quote]
Yes i did consider already a classical Cassegrain.

The sort of mirrors i didn't decide upon yet.

Obviously i first need to make some money selling 3d printers to start this hobbyproject with good odds i open source the design of this telescope project and openly spread the CAD files. With some more budget might even be possible to experiment with different sorts of mirrors then (the experiments can be done with smaller mirrors of course).

Yet you can be sure there is going to be auto collimation. Simply protecting the telescope mirrors during transport i might already need to modify the secondary mirrors position anyway.

Adjusting that secondary mirror probably will happen with some accurately produced bolts and you do not want any sort of damage to those bolts when transporting that telescope as then the autocollimation proces is going to be more complex, in short eat too much time. it probably means decoupling the entire secondary mirror. It would be possible to calculate all this though. A possible solution is using very thick bolts (that do not deform during transport). All this can be easily calculated how many G's it's supposed to take during transport.

A fully automated system would then need to put it on a spot where some motors then do the auto collimation. With some fist rules as what temperature each mirror is and the temperature of the tube and having a table ready at which distances to position it based upon the table after which auto collimation proces takes over, it might be possible to do such collimation pretty fast, even when the mirrors , especially primary mirror, still has to cool down.

The real big question i have no answer to as of yet is whether transport might move dust that got into the tube of the telescope somehow and then somehow lands onto one of the 2 mirrors at a spot i do not want it.

That consideration might constrain me to a very specific choice what sort of mirror setup to use.

As a telescope with a relative easy to clean front that closes the tube and takes care no dust can get into the tube, might be a superior idea for a telescope you transport a lot over the road.

[xilman]

Originally Posted by diep

Obviously i first need to make some money selling 3d printers to start this hobbyproject with good odds i open source the design of this telescope project and openly spread the CAD files. With some more budget might even be possible to experiment with different sorts of mirrors then (the experiments can be done with smaller mirrors of course).
...

As a telescope with a relative easy to clean front that closes the tube and takes care no dust can get into the tube, might be a superior idea for a telescope you transport a lot over the road.

The telescope I am in the process of buying was designed by an engineer who is also an amateur astronomer. Some of the design documents are freely available on the web. I do not know whether they are open source though, nor whether everything needed is available. I can ask him if you wish.

His is a transfer Cassegrain with 0.4m aperture and a fully enclosed tube which, as you say, keeps the dust out much better than classical pure reflecting telescopes.However, the cleanliness comes at the cost of a full aperture optically flat plane window and its small but noticeable light loss.

http://www.tacandeobservatory.com/ is the home page of Joan's technical pages. The telescope design can be found on page 32 of http://www.astropalma.com/observatory_handbook_I.pdf

[diep]

Originally Posted by xilman

The telescope I am in the process of buying was designed by an engineer who is also an amateur astronomer. Some of the design documents are freely available on the web. I do not know whether they are open source though, nor whether everything needed is available. I can ask him if you wish.

His is a transfer Cassegrain with 0.4m aperture and a fully enclosed tube which, as you say, keeps the dust out much better than classical pure reflecting telescopes.However, the cleanliness comes at the cost of a full aperture optically flat plane window and its small but noticeable light loss.

http://www.tacandeobservatory.com/ is the home page of Joan's technical pages. The telescope design can be found on page 32 of http://www.astropalma.com/observatory_handbook_I.pdf

Very interesting to take a look at. Especially i like to see also the calculations regarding the mirrors.
That would be any calculations. Stiffness of the mirrors under gravity and the distance calculations and whatever there is to calculate with those mirrors :)

Note that for my Diepsize (swallowing another word) that aperture ain't large enough!

I'll go for the maximum that fits on a small doubl axis trailer and is somehow economically justifying the effort to design something for in CAD.

Carrying out a project that costs 20k euro, in labour and material, in order to mount a 2000 dollar cassegrain primary mirror and secondary mirror set on it of 40 CM, is basically wasting 20k euro.

Even then it's nice to see such designs. Whatever sort of telescope it'll turn out to be, it'll be a cassegrain sort of design in the end of course.

I do not pretend to be an expert in any sort of CAD software - yet that's my daily job - designing new objects.

40 CM is getting close to what is interesting though.

So building a design that is shockproof on a trailer is some interesting challenge in CAD that'll keep me busy one day for a few nights. I do like closed tube designs though :)

Yet all that will depend also upon what sort of design gets out of the design pipe. In case it gets very long telescope it won't fit with sureness on the trailer in 1 piece so it has to be folded then. realistically the sort of trailer i look at can carry at most 3 meters length and reduce some of that for the construction itself. legs that automatically get out and so on.

we'll have to see what is left in the end for the telescope tube itself and then see what sort of mirror fits in and how stiff the trailer construction can get made.

Possibly the entire trailer construction is easier to have it get welded and then after welding put the entire trailer (all rubber and wood stripped away of course) in an oven to stressrelease all the steel.

I have on paper (just mathematical paper) also a R-C design here where i chose the parameters such of the primary mirror that it can be grind as a parabole, as the difference between a hyperbole and parabole is far under 1 nm which is impossible to grind to that accuracy (the accuracy of the grinding is going to be factors factors worse than that of course).

Then it's matter of designing a grinding apparatus (i won't be grinding it by hand - i'm bad doing things by hand) and a simple test for the primary mirror.

Yet total cost of something like that won't be cheap (would need to grind more mirrors to distribute the cost - especially one of the major costs is the sort of material you choose to build the thing from and very expensive is an accurate oven to melt the material and cool the mirror down slowly). Probably that eats too much time.

As i'm in the business of designing CNC machines using linear motion, designing an accurate grinding apparatus shouldn't be a problem - one that is accurately emulating the human manner of randomly grinding away material. .

Yet the labour to operate the apparatus and carry out the grinding i would need to hire someone for.

Yet can do larger mirrors then up to 700-800 MM then. Above that is really complicated and the mirrors get really heavy. The cost of setting all this up might not be interesting to grind just 1 mirror - but all scenario's will be on the table.

Maybe have him grind after the first test mirror, which i use myself, some very accurate ones and donate those to some universities or something.

It's easy to speculate right now. I need to work further in CAD here for a 3d printer :)