Stub Mandrel

Here we go:

I don't want to put it on wet grass... to photograph it. Perhaps tomorrow evening?

I'm cautious of making a definitive statement but everyone who has seen the stacked images seems to agree that the images in different colours are coming out at slightly different scales giving radial colour fringes, which suggests chromatic aberration. Olly (I think) pointed astigmatism making stars in the corners seem squished and Vlaiv noticed the star shapes varied between R G and B. It's hard to believe with so many independent voices saying that the fault is with the optics, not the stacking. Also, I think I've never had misaligned stars more than once or twice and have always been able to improve them by getting rid of bad subs or changing stacking settings. I know you can get rid of artefacts like this through processing, but they will rob the image of small high-contrast details. More importantly, when you pay the premium on a multi-element apochromatic scope isn't getting rid of these effects exactly what you are paying for?

It's finished! All I need now is a clear evening to test it. Photos to follow.

D'oh! Tilting the platform from side to side is pretty pointless... it needs to be slewed not tilted for azimuth adustment. Never mind, a single adjustable foot at the pivot end would be tricky to implement anyway as the pivot is in the way. Slewing will have to be physically moving the platform, which should be fine in practice.

And now done a test with the scope (passed) and made two adjustable two feet for the north end and paddle for the limit switches, now printing a first try at a switch housing. I'm going to have a fixed foot at the south end (with a curved foot) so (to a first approximation) if the two feet are turned by the same amount it will adjust the polar alignment in AZ only, turning them the same amount in opposite directions will adjust it in azimuth only. I feel child-like anticipation!

I've just done a test, using a leisure battery instead of the telescope. Not as heavy, but placed right at the front to put all the weight on the rollers and way below the CofG so much less balanced. All went well, except near one extreme (probably beyond the range I actually need) it stalled but the motor current was limited at only about 200mA, I upped it to 400mA and it worked perfectly. The stepper is a 900mA one so I've got plenty of headroom if needed. Left to sort: Adjustable feet (printing one now) limit switch arrangement. Properly wiring up

Well... the last few weeks have been a crash course in C++ which I always said I wouldn't bother with. So where am I? I have an all-new set of sectors and rollers but need to remake the upper platform. I will need feet, two of which at least are adjustable for polar alignment. I need to refine the control box and also print some bracket/boxes for the limit switches. I've actually used small (good quality) push buttons rather than microswitches as exact repeatability isn't needed. The main change to the control box will be adding a housing for a little piezo sounder. Quite pleased with the control system so I'll summarise it: Arduino Uno with a16x2/buttons shield I've had in a draw for a few years. Stepper driven via a DRV8825 in 32 microstep mode. The ~12mm diameter roller rotates in about 37 minutes, but this will no doubt have to be tuned, software allows for this. Then limit switches and sounder. Starts up and makes a pretty close approximation of a BBC Micro beep 😉 If eeprom is untainted it stores the default microsecond and millisecond delays for three rates (sidereal, solar, lunar). Then loads these values into RAM. Then loads up sidereal rate and starts tracking. When in track mode it displays the rate and delay length to 0.1ms (default is about 68ms). Also up down arrows to show what to press to change the delay in 0.1ms steps. A mode select button cycles through track and three other modes. Tracking continues unless overidden by another movement. Next mode is to choose the rate, just cycles through the three and pause. Move mode has up and down buttons to make small movements (will need fine tuning) to help with centring an object in RA or polar aligning. Left and right buttons get the platform to move to the end of travel as determined by the limit switches. When 'homed' it starts tracking again, but f it reaches the other end it just stops unless manually moved off the limit switch. Final mode is the clever bit, you can save the current rate into eeprom as a new default, or you can restore the default for the current rate from eeprom. A third button allows you to restore the 'factory' values if you have totally muddled them up. Unlike the other modes which are 'sticky' this one shows a brief confirmation message then drops back into tracking. Other refinements are a 'beep' on saving data or hitting a limit switch. So quite excited really and looking forward to building it all up in time to catch Saturn and Jupiter as they approach opposition, and, of course, Mars later in the year.

Well I'm happy to have worked out a way of spotting the fault, rather than just swapping the board and never figuring it out. Now the hard part - the program!

Standard NEMA 17, 200 steps. Current limit set to about 280mA, everything works as expected. All I can think of is that the DIR input of the stepper board is fried. That could explain why it isn't reading the full 5.0V I get with teh board disconnected. Hmm... the DIR pin reads 300R to GND, while the STP pin is MUCH higher, with the UNO disconnected. I'll try another board./ .... Yes, just swapped the stepper board for another one and it works. Sorry if I wasted your time, but sometimes explaining a problem to someone else is the best way to solve it!

I'm trying to program an Arduino Uno to operate an euqatorial platform. I'm using a DRV8825 for the first time as the 32-microstep resolution will be helpful. Try what I can, the stepper will ONLY rotate in one direction. I have got it stepping once per millisecond on the STP pin, but whether I have the DIR pin high or low (I've checked with a voltmeter, it's either 0.00V or 4.76V) it resolutely steps in the same direction. I can't figure this out, even a mis-wired stepper shouldn't do this - you can reverse a stepper by wiring a coil backwards but not make it unidirectional, as far as I know?

Looks good, I made a cabinet base out of 1/2" ply and ended up adding a pieces on 40x20mm pine up one side of the front as a plain box wasn't rigid enough, that looks nice and chunky but if it still moves you could use gorilla glue to put wooden panels inside the legs.

I use a 3D printed one.

Managed 34 5-minute Ha subs last night, 150PL, ASI 1600MM, and got more than I expected, considering the combination of murk and light pollution. I think the corners are vignetted, despite using a flat. Now I need OIII and SII. <edit> revised with a version to address vignetting.

A few nights ago Antares was such a brilliant red I made the ages-old mistake of thinking it was Mars...

Makes a big difference, I would have put planetary imaging aside if I hadn't got it. I was convinced my setup was broken in some way, but it was just atmospheric dispersion.

I'm afraid I had to use Google Translate as my grasp of Flemish is tenuous. You may be horrified to discover that Google treats it as Dutch... Actually, it does a very good job (which isn't surprising given the shared roots). I am impressed by the website and will definitely head for Urania if I visit Belgium again.

Yes, but it's glued at the neutral points, its also 3/4" x 6" mirror so a lot more rigid than say a 3/4 x 10"

Had to go back and see what I could find. This is the RGB image I ended up with. Compare with RGB image a couple of weeks later: Perhaps a lot of the difference is the processing?

Just an observation, it took months of work to sort the secondhand 3m dome on our club observatory. So many things to put right...

Did a test last nigh. At 11:45 it read 19.22 and at 1:15 it read 19.40, aimed at the darkest patch of sky (CO suggests I am 19.66, but that would be during astro dark). Without access to the exact formula for calculating Lux, I can't be sure of the resolution but after changing my code to take four readings and average them the final SQM readings seem to have a resolution of about 0.02 right down to well past 22.

You haven't got a clue what you're doing and are so stressed that when it's all over you aren't sure if it was worth the effort ...

It doesn't make spikes, a star test suggests that it makes out of focus stars a bit triangular but I need to do a proper test on a bright star now I've flocked it as it may be caused by reflections off the white tube.

@Heskyyyyy the short answer (borne out by @The Admiral's graph above) is that most folks use ISO800 for the older Canons.

I understand, having seen a hobby become (in part) my job, I know what happens!