I am hearing about four-hour exposures. That is, frankly, nuts. There is NO point in doing more than five minutes with a DSLR. You do lots of <=5 min. exposures and stack. There are several excellent reasons to do so and no real downside apart from storage space (which is cheap).

I built a barn-door mount (this one) a number of years ago back in the days of film. It used a curved bolt. I was never able to do more than a couple of minutes with it with a 200mm lens. 300mm was out of the question. With film I needed to do 10-minute exposures minimum. For 300mm I ended up using an eq mount (this setup). I did up to one hour manually-guided exposures. Here is a 45-minute one.

Today, I get images with the same field of view as 300mm on 35mm film using Astrotracer on my K-3II and a 200mm lens. Astrotracer can handle a 300mm lens and I will try that as soon as I get a 300mm lens. Short exposures and stacking is the key. My barn-door mount is in storage.

Eeek! Not gonna lie - I just imagined trying to do Astro without the lcd image preview/review - and it gave me the fear! I would hate to do a bunch of shots only to get them developed and discover the exposure was off the whole time!

Very good point with the multiple shorter exposures - I think I'll aim for a fifteen degree transit range on my tracker - that way I can
Keep it tracking while taking a series of shots - and I can always shorten the boom if I want to track for longer.

Relating to that, would it be worth considering putting the camera in recording mode during a long track and then frame stacking? That said, I'm not sure how dim the image would be if I was doing that! Pretty dim I'd imagine (plus I'm losing resolution). I think I might have answered my own question there!

An incorrect exposure wasn't what I worried about back then. Half an hour at f/5 resulted in an underexposed slide or negative for most objects. So I almost always had to do more than that. I did manual guiding so I never went more than one hour, which was my "pain" threshold for that. Some people - true masochists - could do two and even three hours of manual guiding. Of course, when auto-guiders came out lots of people used them. But not I.

What we worried about back then was guiding errors, focus, airplanes and satellites flying through the field, optics fogging up, sky conditions changing (a little haze would wreck your image), etc. None of which we would know for sure until we got the film developed. DSLR users today have it easy.

Manual guiding sounds inconceivably fun!

Did you have any sort of focusing masks back then? I know the bahtinov mask is fairly recent (2005, though it doesn't seem to have peaked in google searches until 2008), but I can't find any dates for the Carey mask (though it was named after George Carey - I'm unsure if he was the inventor of the mask - which would mean it'd have to have been invented before 1906... but again, I can't find information on that (yet) - which would leave the Hartmaan mask/Scheiner disk (~1900)?

I did have a Hartmann mask but I rarely used it. Instead, I made a knife-edge focusing aid. When used correctly, it gives a perfect result. Unfortunately, from time to time something else went wrong and I would not get good focus (which I did not realize until I got the developed slides back weeks later). Most exposures, however, were good.

Woo! Just won an auction for an EQ-1 mount for £25! Sure, I'd have preferred an EQ-3 or EQ-5, but for £25 (including a reflector telescope with a number of different eyepeices) I can't really complain - and I can always upgrade it at a later date should I choose to! It should certainly support my camera and a few lenses (I think I read it can support up to a 3kg telescope, in which case I should be fine with my Camera)

I've found models on thingiverse for motorising the EQ1 mount - though, I want to give manual tracking a shot too - just to appreciate what came before me! I'm going to try both directly mounting the motor on the mount, and mounting it elsewhere and driving it via a flexible shaft to try and reduce vibration. Will try both, since I can and let you know how I get on

Time will tell if my £25 was worth it!

Manual tracking? Hey, even I didn't go THAT far!

Hahah - my doing this is your fault! You mentioned it and I was like "Woah, that sounds terrible... I just have to try it!"

---

Now, I was just thinking - this EQ-1 mount isn't the sturdiest - as I put weight on it (say, my DSLR + a 3x teleconverter + a 2x teleconverter + my big sigma lens + the wooden mount) - the whole system becomes more susceptible to movement because we lower the moment of inertia.

Given the long focal length (I mean, what is that... 300x3=900,x2=1800, x1.5=2700mm... that's crazy!) any error is going to be multiplied drastically. As such, I don't think I'm going to get away with this setup - BUT I'm going to try anyway!

But, this does raise an interesting question... you see, I used to be into archery - I had a 30lbs recurve bow with a stabiliser on it.

Now, the stabiliser worked by increasing the moment of inertia (by adding mass far in front of the bow) and by damping vibration (by having that mass on a spring so that it'd move in the opposite direction) - the end result was that you would require more torque to move the bow by a given angle - thus increasing the stability of the system (at least in terms of vibration)

Do you reckon something like that would work on a telescope? For instance, I know short and fat telescopes are less stable on a given mount than a long and thing telescope of the same weight (and focal length) in that the longer telescope has a higher moment of inertia.

Of course, this would have to be offset by using higher torque motors to drive the telescope... which may introduce more vibration - swings and roundabouts!

I may need to test this hypothesis once I get my EQ-1... I also need to figure out a way of sighting along the polar axis... which I should really call by it's proper name, which is... right ascension?

I'm thinking an adjustable bracket that just gets glued or even screwed onto the mount (because drilling and tapping holes in a telescope mount could never go wrong! I'd hate to screw it up!... Ha!...) The bracket will hold a little sighting scope that can be adjusted with set-screws for windage and elevation (to make sure it is properly aligned with the axis) which can be glued down once it's set. Not sure what I'll use for a sighting scope yet (finderscope is probably too low in terms of magnification, maybe a monocular? Might see if I can get a small rifle scope or something if all else fails)

I doubt it will be perfect (I'd much rather have a proper coaxial polarscope, but, that requires a more expensive mount! and I don't have the budget for that!) - but hopefully it will be enough to allow me to use that monster teleconverted lens for maybe a 10-30 second exposure. Anything above that is a bonus! I'd be happy with a 5-second exposure at that focal length if I'm being perfectly honest!

Although I'm buying it for the mount, it comes with a refractive TASCO Galaxsee telescope (model 46-060675, 60mm objective, 900mm focal length with a number of eyepieces), so that will be fun to play with too - will certainly be a good learning aid!

My EQ-1 and telescope arrived last night - can't wait to give it a proper go!

The telescope isn't great - TASCO-Galaxsee-46-060675 - which is a 60mm x 900mm refractive scope.

The aluminium tube is dented - possibly during transport - I would knock it out but I'd be concerned about deforming the tube further - I'll just leave it since it appears to be asymptomatic. I may replace it with a fibreglass tube some day, but in the meantime I'll live with it. I took everything apart and gave it a good cleaning out and recollimated it with

It came with 4mm, 12.5mm and 24mm SR eyepieces, a 2x and 3x barlow, and a 1.5x erecting eyepiece.

What I'm really interested in, though, is the EQ-1 mount that it comes with: I'm hoping to convert this to electronic control and maybe move the mount from the lightweight aluminium tripod to my heavy-duty wooden one. Sure, it'll ruin it's portability, but I have the O-GPS1 for astro-on-the-go, and I plan on making the conversion reversable anyway.

I also need to come up with a way to make more polar alignment more accurate - I'm thinking that a small scope mounted on the side will be the way to go: just a matter of getting it properly lined up and firmly fixed to it!

Electronics-wise, I plan on running a stepper motor from a microcontroller - I want to try the gearing directly first, then I'll move to a 4:1 or 5:1 gearing, and then a worm gear connection if all else fails. It's going to be a bummer to calibrate, but once I get it running it should be able to track for as long as I want (polar alignment notwithstanding).

So, all in all, £25+P&P well spent, I think!

Bring on the clear nights!

The problem is, it isn’t as easy as one might think. Years ago I built a micro controlled motor drive that according to my calculations should have been very accurate, but it wasn’t. Turned out that the final worm gear had a periodic error so no matter how accurate the motor rpm was the final movement oscillated slowly. 10 minutes slightly to fast, then 10 minutes to slow.

Maybe I should’ve bought a better worm gear.

Not as easy as I think? Aye, that is not good, because I didn't think it's going to be easy to begin with!

So - let's look at this logically! My goal is a 10-minute exposure: but I have heard the I've heard the precision of the gearing on these EQ-1 mounts isn't too great. As you said, there will likely be periodic error in every possible gear interface in that drive chain.

That error is unknown at the moment - I was hoping to cross that bridge when I came to it by wishful thinking keeping my fingers crossed that the error in my mount was small enough to get away with.

Let's face it, Murphy will get you every time: and things won't pan out that simply!

As you said, it isn't as easy as one might think (or hope!) - especially if we want to track for any real length of time - so if we can't replace the gearing with precision-machined parts, we are going to need to perform error correction.

So, with that in mind, I reckon we can either use relative positioning with error correction based off a lookup table... or use absolute positioning and compensate for the error as it occurs.

Although we could use relative positioning against a calibrated lookup table of actual position vs time or number of steps... (and I can assure you I am not generating that table by hand!) - it isn't a good system: errors will still be accumulative, and it requires a "home" position to be installed on the Right Ascension.

Not only that, but in order to generate the error-lookup-table, I would need to find a way of accurately recording the position of the RA against the number of steps the motor has gone through, and if I'm doing that, then I might as well develop a positioning mechanism that can be used to provide live feedback of where the RA actually is and compensate for error as it occurs via a PID control loop.

All I need to do is find a way of precisely measuring the position of the RA... I'm not confident that this is going to work... but there must be some way of doing it! The question then becomes - "is it worthwhile to spend the time figuring out some way of doing it?". It may very well transpire that when it comes down to the cost in man-hours of work I would need to do, it may have been "cheaper" to just buy a more expensive motorised equatorial mount in the first place! Even worse, even if I do manage to come up with something, the system can only be as accurate as that positional feedback!

Time will tell whether this is just a big (fun?) boondoggle :P

I have a bad feeling about all this... for now, let's just hope that any periodic error is slight enough that I can get away with it... I have a sneaking suspicion that Murphy is going to get me though!

--EDIT--
I wonder if I could bore a hole in the bottom of the latitude-RA assembly and extend a shaft out the bottom of the RA axis... that way I can attach an optical encoder to the shaft with (relative) ease.

--Update--
No, you cannot extend the RA shaft, it is held in with a bolt on the bottom where I wanted to put the encoder rather than a bolt from the top.

---

So, my other problem is polar alignment. The EQ-1 doesn't have a polarscope - and I'm not convinced I'll get good enough results for photography if I just side-mount one, even if I do get it precisely aligned to the RA axis.

So - that leaves me a couple of interrelated questions:

1. Is there software that can help me manually polar-align my mount?
2. Will that software be able to work with the Pentax K-50 (my camera)?

Of course, I can also use drift-alignment! I'm just curious if there is anything maybe a little easier out there!

I heard that the PE on that mount could be as high as 130 arc-seconds. Ouch. A home-brew PEC solution would certainly help. That's what you are re-inventing, and aside from the problem of making it permanent, which is highly desirable, it should not be so hard for someone who can program a microcontroller. You could also add your own motor or you could start with something like this. Open it up and add your own extra electronics or, if no room inside, add an interface to an external box or a hand control. Lots of fun for do-it-yourselfers.

Re polar alignment software there is Polemaster and SharpCap Pro. Or you could drift align for free. With enough practice you can get it down to about 15 or 20 minutes if you want really high accuracy or ten minutes for something good enough for a 200mm or 300mm lens. I did it all the time when I used a fork-mounted telescope (this one)

There is also the KStars/Ekos polar-alignment routine, which uses plate solving via your camera to accomplish the same thing as PM and SCP. First you point the scope at the Pole. Then take an image and rotate the mount 30º or so in RA. Repeat twice. It then plate-solves the images and does the math to ensure that the axis of rotation is aligned with the celestial pole. Open source. Runs on a $35 Raspberry Pi, for that matter.

I went through the exact same thoughts and landed in that I need to guide the mount with a camera looking at a star. So I bought a raspberry pi and a raspberry cam, removed the tiny lens and 3D-printed a new mount (Pentax K of course) attached a 50mm M-lens. Then found some software meant for tracking, 3D printed a house for the camera + raspberry + battery, modified my home built drive to take correction pulses and got everything to work in a lab-enviroment. Then I lost interest.

Reinventing the wheel, and developing a system to reliably turn slowly - I like it! Just my style!

My plan is to go down the route of using a toothed pulley and a mount similar to this (in fact, that particular model is my first port of call - only I'm going to use aluminium sprockets) - I want to modify it so that it's weather resistant (electronics in a proper enclosure) - and add some feedback.

My current problem is figuring out how to get the positional feedback. From disassembling (and cleaning and lubricating) the mount last night, I can see we can't extend a shaft out the bottom to an encoder - so as I was laying in bed thinking about it, I wondered about putting three IR interrupters lined up through the teeth at uneven intervals, effectively using the RA drive sprocket as the encoder wheel. However, I'm having doubts as I suspect that simply ensuring the right number of teeth pass a certain point will not actually correct for periodic error - since there may also be periodic error in the sprocket itself (for instance the tooth pitch may change slightly, or the sprocket may be off-axis)

---

That swings us round to the solution that @Gimbal came up with: a guiding camera. Now, a guiding camera attached to the mount would also tie in the with suggestion of @fewayne for using the KStars/Ekos and a raspberry pi - as I would be able to kill two birds with one stone and tackle polar alignment at the same time as error correciton/guiding.

Quite a lot of pros for that one, but the cons are in the complexity of the system, and weight gains. I'll need to look into it more to see if I can reduce the size of the entire camera system - I'm thinking that instead of using a Raspberry Pi camera module (though I could certainly go down that path as well), I could make use of the USB image sensor I bought for my light microscopy - it's a 22mm fit but has a bunch of adapters. Maybe I could focus it through a little monocular? I'll need to figure what sort of focal length and FoV I need for polar alignment and guidance - monocular would be light... but the FoV and objective lens size are probably too small - I think it's a 25mm objective - it isn't exactly a light bucket! I'd probably be better mounting a lens on it, actually... Hrm... Will see how it turns out with the monocular first. I suspect poorly!

---

If I want to stay on the optical encoder path, I would probably need a very fine encoder ring that would be glued to the bottom of the RA sprocket, and a reflective IR path for counting the pulses.

That should be fairly simple in the scheme of things (bespoke encoder ring notwithstanding) - just a matter of making sure it's turning at the right rate. The cons for this one is that the encoder ring will have to be kept clean (preferably with an enclosure) and the system will only be as accurate as the finest markings on that ring - and I am not sure I can get an encoder ring fine enough for the purpose - which obviously puts a damper on that idea.

---

I think I might end up going for the camera option - I think the benefits in terms of polar alignment and autoguiding may possibly outweigh the additional complexity.

BUT - in the meantime, I need to start with getting the drive assembly running - let's see what size of error we're dealing with. Who knows, it might be tiny and I won't need any error correction! .........The error is going to turn out to be freaking ginormous, isn't it!?


Next step will be trying something like PECPrep on the laptop and the USB camera on the next clear night to figure out the error. Probably easier than trying to measure by hand or by taking long/multiple exposures and working it out from there.

Thanks for the suggestions folks!