ST80 Mods for Imaging

[Gina]

Another thing to consider is that (assuming I have my head around this properly) if you have three narrowband ST80s, one Ha, one OIII and one SII for example, the lack of colour correction effectively means you have three different focal length scopes and you'll therefore have three slightly different image scales.  That will need to be sorted in processing if it is sufficiently different to be visible.

James

I use RegiStar to sort out different sized images even if one set is binned.  It will also handle images at different angles and/or not properly lined up etc.  Best to be nearly right though or you lose some image area.

[Gina]

Hi Gina,

Just wondering if it's worth testing the basic scope out first before getting all the mods done? Would be a shame to go to all that effort and then find out the results just aren't worth it.

Could you rig it up to give it a go first?

Cheers

Stu

Sent from my iPhone using Tapatalk

Yes, I shall be doing that.  All I've done so far is to drill some holes in the ali plate and print gears and spacer.  I have only arranged for one ST80 too.  I only have the one ATM anyway.  I'll only buy another if this one proves useful   All the rest of the kit such as stepper motor and focussing remote control can be used for any scope.  I had to arrange stepper motor focussing as it would be impossible manually as it is.

[RobH]

I use RegiStar to sort out different sized images even if one set is binned.  It will also handle images at different angles and/or not properly lined up etc.  Best to be nearly right though or you lose some image area.

Registax will do a fine job matching it all up, but if you mix binned and unbinned images, the star sizes won't match. You need to make sure that you match the binning levels in your subs for each type of sub, in other words, match binning on all of your NB subs, and match it on your RGB too, although the RGB doesn't need to be the same as the NB, as if you are mixing the two, you'll most likely be replacing your NB stars with RGB ones. Just make sure that the stars in your RGB subs are bigger than your NB ones, whatever the binning level you choose.

[Gina]

I've been looking at the focuser drive.  With the stiffness of the focuser I decided things needed some improvement - I think the stepper motor might struggle.  So I've looked at three different approaches. 

Firstly, I have slackened off the spring on the focuser shaft reducing the pressure on the pinion and hence the tube on the PTFE pads, reducing the friction. 

Secondly, I've designed and printed a smaller pinion, offset from the motor shaft so that I wasn't limited by the diameter of the shaft.  This has meant I now have a 6t pinion rather than 9t and the pitch circle has gone down from 12mm to 8mm with the bottom of the teeth being smaller than the motor shaft.  The gear ratio is now 15:1.  I think this stands a very good chance of working.

Thirdly, I've been looking at the possibility of the planetary drive with extra output ring giving a gear ratio of 246:1.  I'm doing a test print to see how well it works.  I might scale up this Thingiverse model to handle the extra stresses.   I'll be interested to see how much backlash this gear system produces.  It's certainly a very interesting idea for large ratio gearboxes.  It may well have uses in other projects too.

[Gina]

Here's a photo of the focuser drive gears and stepper motor mounted on the ali plate.

[jarl72]

Thinking about the no-backlash gears, you could print a taller sun and annular gear and add two sets of cages+planet gears, preloading them with a (printed) spring. I haven't put on my engineer hat yet but I think it would reduce backlash.

[Gina]

Yes, indeed

[Gina]

I've designed and printed a carrier for a 1.25" filter to fit inside an extension tube next to the camera.  It consistes of a ring to take the filter and a tube fitting closely in the extension tube to hold filter in up against a ledge ing the extension tube.

Firstly, the two printed parts and a Baader OIII 8.5nm 1.25" filter.  The filter screws into the ring (middle) and the tube (right) is inverted and placed on top of the filter to hold it into an extension tube.

Assembled.

This shows the filter and mountings inserted into an extansion tube.  Firstly from the scope side and secondly from the camera side.

[Gina]

I now have the new power and focus control box pretty much finished and ready for testing.  The ST80 is mounted on the ali plate which in turn is now on the EQ8 together with the MN190 which I have set up for guiding.  The motorised focuser is all done at the scope end and all connected up to the Arduino.  I have the focus system remote control box to finish and then it's ready for testing.  So I'm hoping to do some testing tonight. 

I didn't have things ready before the light went so the hoped for setting up in daylight on the far hill and trees didn't happen.  OTOH there should be a decent moon to line up and focus on if it stays clear.  Then if there are no hitches (fat chance ) I may be able to do some imaging on the Heart Nebula if the moon isn't in the way - otherwise there are other DSOs I could try.

[Chris]

Lots of luck, hope it goes well

[Gina]

Thanks Chris

I've put an electric convector heater out in the warm room to make it more like its name   I used to have a fan heater out there but must have needed it indoors - now I can't find it

[Gina]

An unsuccessful test of the new power setup   There's a fault in the power and focus control box that only showed up when connected to the observatory power supply (13.8v Tantronics PSU).  I had tested it indoors on my bench PSU and all was fine.  The over-voltage crowbar triggered at 14.3v and all worked fine at up to 14v so there shouldn't be a problem with the fixed 13.8 supply. 

I connected the power up in the observatory and switched on.  Checked the volts at the power box at the scopes and that was fine.  Supply current was 2.4A with the dew heaters connected which did seem a bit high but not OTT.  I spent some time looking out the various USB cables and connected the USB hub power.  No blue light came on on the USB hub - so a problem   Then I noticed smoke coming from the power box and immediately switched off.

So yet again, tonight's clear sky must be discarded - have to sort the problem out before I can connect any cameras.  I think setting up and testing really needs daylight - there is just not sufficient lighting in the scope room for checking electrics at night.  Think I'm trying to run before I can walk

[Gina]

Actually the clear skies didn't last and it has now clouded over.

[kev]

Just a quick comment. Ive got a st80 that i use as a travelling scope, so this thread is most useful. Mine regularly goes around the uk with me on work journeys and has also done menorca, Tenerife, and gran canaria.

Sent from my iPhone so excuse the typos!

[andyboy1970]

If CA causes the R,G&B to focus at different points wouldn't this be overcome by using RGB filters to image refocusing for each?

Sounds far to simple to be true!

[JamesF]

If CA causes the R,G&B to focus at different points wouldn't this be overcome by using RGB filters to image refocusing for each?

Sounds far to simple to be true!

Well, the actual physical effect is that all frequencies come to focus at slightly different positions.  There's no specific division between them.  So, light at one end of the passband of the red (say) filter will not focus at quite the same position as light at the other end.  The question is how far you would be out.  It might be possible to measure that experimentally.  I'd guess though that with an ST80 you'd probably see CA effects across the passband of a R, G or B filter.

Of course if this approach does work for NB imaging with the ST80 then it may well also work with the ST120

James

[Gina]

I now have my main imaging rig back on the EQ8 mount   That's MN190 with Lodestar X2 for guiding, Esprit 80ED no.1 imaging scope with EX460 imaging camera and ST80 with second EX460.  I have been aligning the two images by adjusting the mountings of the ST80.  Not quite lined up but probably adequate for a test run which I'm hoping to do a bit later this evening.

If the ST80 results look encouraging I shall probably devise a better way of aligning than loosening the bolts holding the rings onto the ali plate for one way and shims for the other.  A rather crude system.  It needs screw adjusters really.  I guess I could provide remote control with stepper motors   I'm already using an Arduino Mega 2560 to provide remote focussing for four focusers and there are plenty of spare I/O lines for extensions   When I eventually get three refractors and cameras for imaging I could use four additional stepper motors to adjust the alignment.  Now that will really be making good use of the Mega - using 32 of the 54 digital I/O ports

[Gina]

Unfortunately my new remote focussing system fails to work in the observatory though it works perfectly indoors so until this is sorted out the ST80 imaging run is on hold

[jarl72]

Bummer. Which are the symptoms?

[Gina]

Wrong operation of the stepper motors.  A couple of them were running continuously when they weren't supposed to be going at all

I have installed the Arduino IDE and will try to fault find in situ tomorrow.  Too cold to work in the scope room tonight   Not enough light either.

[Merlin66]

I use a achromatic doublet in the spectroscope.

The focus can vary dramatically from wavelength to wavelength.....

The attached Edmund Optics chart gives fair idea of what to expect.

[Gina]

Thanks for the definitive information Merlin  

My idea is to use an achromat for narrow band only using 3nm and 5nm filters so IMO that problem should effectively not apply.  A worse problem might be field curvature and that's what I want to check.  I tried manual focussing last night while looking at the image of a star field but it was too cold next to the scope to get good enough focus and I had to give up.  On such a cold night it needs remote focussing both to be in the warm and get close to the laptop screen.  And of course, more precision than I can get manually even with the huge gear wheel I have on the focuser shaft.

[JamesF]

Thanks for the definitive information Merlin

Indeed.  That's an interesting graph.  It suggests to me that using the same arrangement for imaging with a green filter might be possible (assuming the ST80 performs similarly), but the red end looks a bit iffy and blue/violet would probably be a complete disaster.

James

[ollypenrice]

Registax will do a fine job matching it all up, but if you mix binned and unbinned images, the star sizes won't match. You need to make sure that you match the binning levels in your subs for each type of sub, in other words, match binning on all of your NB subs, and match it on your RGB too, although the RGB doesn't need to be the same as the NB, as if you are mixing the two, you'll most likely be replacing your NB stars with RGB ones. Just make sure that the stars in your RGB subs are bigger than your NB ones, whatever the binning level you choose.

Rob, not Registax but Registar. (I consistently open the wrong one from the Desktop even after eight years!!)

I think this was a typo?

Personally I think the fly in Gina's ointment is likely to be field flatness in the ST80. It has a mightily curved field and may struggle to give decent star shapes across the chip. But it takes more than a tough problem to defeat Gina, as we know!!!   

Olly

[Gina]

That's a lovely comment Olly, thank you