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Startup idea - cheap camera rotator


spaceman_spiff

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Hi all!

I was chatting with my brother earlier today about a possible astronomy kickstarted project. He asked if there are any gaps in the astronomy/astrophotography market. After some thought, I mentioned about making a relatively cheap (<£100) camera rotator that would correct the field rotation in alt-az mounts. This could open up long-exposure AP to people who otherwise would have to invest in an EQ mount. I noticed that even second hand field rotators sell for upwards of £400!

The idea would be to make a solid (plastic) body with a fixed connection to the telescope but a rotating connection to the camera. You would input the declination of your target (or select a target from a list of pre-written ones), time and coordinates and it would begin rotating the camera at the correct speed. Afterwards you could reset the rotator to the starting position.

Things to mention:

- Cable wrap - I could program a limit of rotation.

- Problem with flats - since the camera is rotating relative to the optics of the scope, flats will be useless. 

- Issues near the zenith, the rotator may not be able to track targets that go near the zenith.

So what do you think, is this a viable idea. Between my brother and me, we have enough experience to program a stepper motor and produce a prototype. But will anyone want it???

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To add to that there's the other Dragons Den critical idea: Has anyone else done it before?    

In this case I'm afraid yes, people have beaten you (and me, I had the same idea!) to it in the form of the commercially available product known as a Field Derotator.

They aren't cheap mind, so if you are DIY minded it could be worth making your own. Here's a guy who's open-sourced his design as a starting point: https://github.com/cytan299/field_derotator

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First thought for me - how would you control rotation speed? Surely it won't be fixed, but dependant on pointing coordinates?

Maybe some research into the professional observatories, like Paranal, or Hawaii, to see how they do it? These are mostly alt-az with rotation devices.

Even the new E-ELT looks like it'll be alt-az.

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Thanks for the replies guys!

I was thinking of aiming it at the beginner/intermediate level - people who have a goto az mount but want to do a bit more than visual astronomy. The problem is that most people at that level will not have much idea about what add ons do what (at least I didn't at that point in time). I know derotators are on the market but they are priced towards the advanced amateur astronomer.

Simple back of the envelope calculation put materials and packaging at about £80 so I could make a small profit selling at £100.

The rotation speed can be controlled - the motor will be a (possibly Arduino controlled) stepper motor. The rotation will need to be calculated from two sources, a stars (apparent) rotation around the polar axis (increases with declination) and the flipping action of the az mount around the zenith. Both will need to be factored in. For me this is part of the fun n programming the motor speed!

My point is, if I could make a working product that sells for around £100 would that interest people?

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14 hours ago, Ajhayter said:

To add to that there's the other Dragons Den critical idea: Has anyone else done it before?    

In this case I'm afraid yes, people have beaten you (and me, I had the same idea!) to it in the form of the commercially available product known as a Field Derotator.

They aren't cheap mind, so if you are DIY minded it could be worth making your own. Here's a guy who's open-sourced his design as a starting point: https://github.com/cytan299/field_derotator

Brilliant link, thanks! :happy7:

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2 hours ago, SteveNickolls said:

Count me in and very good luck in your endevours.

Cheers,
Steve

Great! there's some interest already! Once the maths and programming is sorted out, actually making a body should be relatively simple (we have a 3D printer). I was thinking of drawing up some plans in the next few days.

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I'd have thought that developing a well engineered wedge might be a better bet , I know the Meade wedges were/are notoriously bad to accurately adjust .

That said I can't see either being brought in within the <£100.00 mark but good luck with the project.

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A very interesting topic, thought provoking even !  A few random comments :-

I had not thought about the Flats Problem before :) how do the professional alt-az-rot scopes manage,, good optics that dont need flats I suppose ?

If @Gina's experiments with the new breed of cmos continue to show the promise of 30sec exp,  and with our "No-Eq" chums showing 30upwards seconds being good, then the need of a de-rotator diminishes, especially for the beginner who may be more happy than the more advanced to stack loadsa frames :) ?

There may be undesirable mechanical constraints on tilting an Alt-Az on a wedge, especially those built down to a budget. I am thinking about the overhang of the arm and the torque on the central pivot/ gears ? Not sure if repositioning of the ota to get balance back over the pivot would be possible/adequate ?

Not meaning to be negative or controversial, just thinking aloud :) whilst it is raining again outside :( Carry on up the good works.

 

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Don't worry about being negative! I haven't even started drawing plans up yet! I would say the biggest problem with the idea is (as you said) the quality of non-eq photography these days, which reduces the need to add complexity and cost to the scope in order to achieve longer exposures.

Regarding the flats problem, I have been thinking of using a reference mark on the scope (like a small dot) somewhere on the image. As the camera rotates, this dot will draw a curve and I could use this curve to transpose a flat frame to the required angle. Sounds complicated but using matrix algebra it really should't be that tricky.

The wedge idea is interesting but I would worry about the stability of a cheap wedge (made of 3D printer plastic) supporting lots of equipment!

Dan :happy7: 

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An interesting thread - thank you :)  Since I have EQ mounts, image rotation doesn't apply like it does for Alt-Az.  However, I would like camera rotation to allow me to orientate the camera to make best use of the frame shape with respect to the object.  With my current imaging rig and the DSOs I'm thinking of imaging, the FOV is a fair bit larger than the DSO but I quite expect to be working with closer fits in the future and I have been thinking of a remote controlled camera rotator.

In my case, the exact gear ratio between stepper motor and camera is not important since it'll be me in the feedback loop rather than an OAG guiding system.  This means I can use one of those ubiquitous cheap little 28BYJ-48 stepper motors with their strange ratio gearbox.  Yes, yet another Gina Project for the future :D  But the promise I see in the ASI1600MM-Cool camera may mean I won't be working on my triple imaging rigs.

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16 minutes ago, spaceman_spiff said:

Don't worry about being negative! I haven't even started drawing plans up yet!

tsk tsk, what's keeping you, it's been a whole 19hrs since :laughing4:

The guiding problem : there are two (at least?!) needs, one is the conventional one of assisting the drive/tracking accuracy to keep on target, and second is to assist the de-rotator accuracy. Do we need both ? Assuming that the tracking of the al-az is good enough ( and our No-Eq chums suggest it is so ) then an oag to correct the rotation only needs 1degree freedom - 90deg (tangential) to the radius, taking into account the  umm yes well err the reference axis ?? my head hurts ;) , pass .

Oh ah maybeeee - the guide star at the periphery would be detected to cross the radius and just one of the conventional correction/drives/motions in RA or in Dec.  would be applied to the rot motor to bring the guide star back. ( the software guru would have to think in polar, not cartesian, coords :) )

On the other hand : if we calculate, for the sensor being used, what the permitted angle is - tan[(pixel-to-pixel)/max.sensor axis] we would know what rotation to give it once every 15?sec and we could do that like the barn-door peeps do, maybe.

Wonder if it is still raining  , , ,

 

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2 hours ago, Steve Ward said:

I'd have thought that developing a well engineered wedge might be a better bet

Only if you want the hassle of of an EQ mount, which some of us don't! One might as well buy an EQ mount in the first place and I expect it would perform better from day 1.

Ian

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Good luck with your idea but somehow I don't think it would live up to expectations. With Alt-Az imaging the principal limitation to long exposure duration is not field rotation, because even where field rotation allows, it is the mount's general tracking ability that lets it down. Those who've tried to extend the exposure much beyond a minute with Alt-Az mounts priced at the bottom end find that there is star streaking. Unfortunately, what is not known is whether this would be a problem in higher end Alt-Az mounts such as the Az-EQ5/6, iEQ45 PRO GOTO, or the iOPTRON AZ PRO GOTO, because folk who are prepared to spend that much on a mount just don't do Alt-Az imaging! I would only consider such a device if I was certain that the tracking issue would be solved by spending more on the mount.

As for flats, I could see that might be a problem unless any vignetting was absolutely axially symmetric. I'm not sure how it would affect the elimination of dust-bunnies. I reckon I'd need to think a bit more on that one!

Ian

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One idea I would want to see in this project is ability to use not only in dobsonians but for example, fully automated setups that would use rotator for composing (adjustable rotation speed).

Also many things have specific connection like T2 or M48 permanently in them. Instead of having multiple versions could you perhaps make one that has changeable plate on both sides with male / female thread options available? If price isnt astronomical and build quality is good I would be very intrested to buy one. One thing you can also make yourself to stand out is to have equipment that works in freezing temperatures.

-V

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15 minutes ago, The Admiral said:

what is not known is whether this would be a problem in higher end Alt-Az / was certain that the tracking issue would be solved by spending more on the mount.

Was just about to post similar about our discussions on streaking elsewhere ! Had to chuck it all away :icon_salut: That gets to the heart of my dilemma - do I go for an EVO or a Discovery, still not decided and winter is nearly upon me !! :(

But as Gina said and V just now, there are other reasons for a gizmo like this but perhaps not supersophisticated, even just the ability to conveniently adjust the rotation (indexed?) between runs/stacks/groups during Nige-type long sessions, as we also discussed elsewhere.

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5 minutes ago, Gina said:

What's the matter with an EQ mount?  Is the Alt-Az easier in some way?  I'm not being awkward - I've never used an Alt-Az mount.

Well, if you've got an observatory where you can set it up properly and leave it, nothing, and clearly many advantages. But for myself, I have to drag everything down from upstairs and take it into the garden each time I want to use it. And I can't see the Pole Star so alignment would be that bit more complicated. And I want to spend what little time I have using the gear rather than setting up. Also, I'm not after salon-class images, but take great satisfaction in being able to reveal an object which to to the eye is but a grey smudge. It is I suppose what could be regarded as entry-level astroimaging which doesn't cost an arm and a leg. Perhaps I should turn the question around, "What's the matter with an Alt-Az mount?"  I'd like to squash what appears to be a prevalence of view that one must have an EQ mount if one wants to embark on astroimaging. That is simply not the case. I'm not suggesting, Gina, that you are questioning that in your post, but it does give me a chance to get on my hobby-horse :icon_biggrin:

Ian

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31 minutes ago, VilleM said:

One idea I would want to see in this project is ability to use not only in dobsonians but for example, fully automated setups that would use rotator for composing (adjustable rotation speed).

Also many things have specific connection like T2 or M48 permanently in them. Instead of having multiple versions could you perhaps make one that has changeable plate on both sides with male / female thread options available? If price isnt astronomical and build quality is good I would be very intrested to buy one. One thing you can also make yourself to stand out is to have equipment that works in freezing temperatures.

-V

This is an interesting idea...I could easily write a program with two main modes - de-rotation and framing. In the second mode, the rotor would move at faster speeds to attain the correct orientation and would then stop for imaging. If I use a stepper motor then I can track how far it rotated moved the camera...possibly even giving the user an angle reading (useful for taking flats later).

As for the interchangeable plates, it's a good idea...but more parts = more cost. I think the prototype will just be the barebones mechanism. I was thinking of using my freezer for testing it at sub zero temperatures!

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