ST80 Mods for Imaging

[Gina]

I have decided to try the ubiquitous ST80 guide scope for imaging.  Mainly in a dual or triple imaging rig with the Esprit 80ED Pro for Ha and the ST80 for OIII.

The main difficulty for imaging (and even for guiding) is the focuser which is only single speed with quite a coarse rack and pinion drive system.  The manual control is by serrated plastic knobs of about 40mm diameter which are totally impractical.  I shall be overcome this with a large, 130mm diameter focus gear driven by a 10mm pinion on a 28BYJ-48 stepper motor.  This itself has a 64:1 reduction gearing built in so the 5.6 degree steps will be reduced by 64x13=832 to 1.  ie. the steps will be 5.6/832 = 0.0067 degrees.  And in fact I use half-stepping in my Arduino sketch so the resolution will be 0.003 degrees.

I decided on using spur gears rather than timing belt as these are easier to 3D print in plastic.  There is already some backlash in the stepper gearbox which exceeds that of the printed gears so nothing is lost on this score.  I approach focus from one direction only to avoid backlash issues.

I also plan to carry out the improvements given in this video.

[andyboy1970]

I have imaged with the ST80, great little scope.

Have you got a Baader fringe killer or similar as CA is quite bad on brighter stars.

[Davey-T]

I use the ST80 for guiding as is with rubbish focuser, don't think it matters as PHD doesn't seem that fussed about the guide stars being perfectly focused, not sure if it's worth the effort of improving it although I am going to try the Quark in it if it stops raining so depending on the result of that I may have a go at modding  it.

I find it's not so much the backlash more the fact that the focuser flops up and down when reversing direction

Dave

[Chris]

I hope it goes well Gina, I wonder if there's any filter which will help with the CA without killing the blue channel too much? I doubt it I guess or more folks would be imaging with them., anyway probably not as important for narrow band so you might be fine

I've been on the look out for a cheap small and light ED frac to stick ontop of the C8 but the way things are going I'll be settling for an ST80

Its a shame you dislike diff spikes else you could run 3x 130pds's at f/4.5 with .9 CC's for much less than the price off one Esprit !  

[Gina]

I have imaged with the ST80, great little scope.

Have you got a Baader fringe killer or similar as CA is quite bad on brighter stars.

I'm hoping that using NB filters will take care of this.  Time will tell

[Gina]

I use the ST80 for guiding as is with rubbish focuser, don't think it matters as PHD doesn't seem that fussed about the guide stars being perfectly focused, not sure if it's worth the effort of improving it although I am going to try the Quark in it if it stops raining so depending on the result of that I may have a go at modding  it.

I find it's not so much the backlash more the fact that the focuser flops up and down when reversing direction

Dave

Yes, I've used it for guiding in the past - found you needed the focus reasonably near to see any stars at all    The focuser on mine is quite tight and I have had no problem with flop.  The video in the link in my first post shows how to clean, lubricate and adjust the focuser amongst several other improvements.

[Gina]

I hope it goes well Gina, I wonder if there's any filter which will help with the CA without killing the blue channel too much? I doubt it I guess or more folks would be imaging with them., anyway probably not as important for narrow band so you might be fine

I've been on the look out for a cheap small and light ED frac to stick ontop of the C8 but the way things are going I'll be settling for an ST80

Its a shame you dislike diff spikes else you could run 3x 130pds's at f/4.5 with .9 CC's for much less than the price off one Esprit !  

Thank you Chris

Yes, I'm hoping NB filters will overcome the CA

The ST80 is so small and light that it's quite a delight   And at 400mm FL it matches the Esprit 80ED.  I like using shorter FL with the increased FOV

The 130PDS is great value I agree.  The Evostar 80ED Pro is also good value and I already have two of them.  Pity the FR/FF costs so much   510mm FL at f6.3 isn't bad.

[Gina]

Here are some pics of the focus gearing.  The first big gear design proved to have too little material for the gear teeth so I changed it to widen that area.  The cross-section shows the hub design to fit the ST80's focuser shaft.  The fixing is with an M4 screw into the end.

Big spur gear with 130 teeth and 130mm pitch circle diameter - about 134mm OD.

This shows the cross-section and the shape of the inside of the boss to clamp onto the focuser shaft.

Design modified for extra strength.

Motor Pinion.  10t and 10mm pitch circle diameter.

[Gina]

Now for some photos.

Focuser Shaft.  8.4mm OD with conical section and M4 tapped hole in end

Large Focuser Gear

Large Gear on ST80 with the latter fitted onto the ali plate with tube rings

Motor Pinion

Pinion on Stepper Motor

Spacer for fitting stepper motor to ali plate

Stepper Motor with Spacer and Pinion

Stepper Motor placed on spacer showing how it will be fitted to ali plate

[Dave In Vermont]

WOW!! Incredible posting/reporting Gina! That belongs in a hard-bound book.

I'm thinking about employing planetary-gears for my ST80 - like what are on the Sturmey-Archer 3-speed hub for bicycles. Toying with the idea for now.

Happy Improving!

Dave

[jarl72]

Nice
Have you checked cycloidal reducers (http://www.thingiverse.com/thing:8348) or a planetary one (http://www.thingiverse.com/thing:7390) ?

Also: are you familiar with the backlash-reducing gears (essentially two regular gears with a torsion spring between them)? If you think of it it looks like the perfect match for a 3D printer (the two gears and the spring can be printed as a single part).

(Now that a I think of it, a planetary reducer on a wide FOV telescope is a nice mind-bender )

PD: Some things you find on internet are really interesting:

[SkySurfer]

Hi,

I use an ST80 for guiding only at the moment, but wondered about some imaging through it, I upgraded the focuser to a new single speed 2" crayford version, which is ultra smooth and can hold up to 3kg on the back without any flop, bought it from Teleskop service for £80

Seems a lot considering I only paid £99 for the full ST80 and accessories, but that's me, bit of an impulse buyer I guess.

Regards

SS

[SkySurfer]

Hi,

I use an ST80 for guiding only at the moment, but wondered about some imaging through it, I upgraded the focuser to a new single speed 2" crayford version, which is ultra smooth and can hold up to 3kg on the back without any flop, bought it from Teleskop service for £80

Seems a lot considering I only paid £99 for the full ST80 and accessories, but that's me, bit of an impulse buyer I guess.

Regards

SS

This is the one I bought

SS

[Gina]

WOW!! Incredible posting/reporting Gina! That belongs in a hard-bound book.

I'm thinking about employing planetary-gears for my ST80 - like what are on the Sturmey-Archer 3-speed hub for bicycles. Toying with the idea for now.

Happy Improving!

Dave

Thank you Dave

I've printed planetary gears as an excercise   And I used to have a bicycle with the 3-speed S-A hub

[Gina]

Nice

Have you checked cycloidal reducers (http://www.thingiverse.com/thing:8348) or a planetary one (http://www.thingiverse.com/thing:7390) ?

Also: are you familiar with the backlash-reducing gears (essentially two regular gears with a torsion spring between them)? If you think of it it looks like the perfect match for a 3D printer (the two gears and the spring can be printed as a single part).

(Now that a I think of it, a planetary reducer on a wide FOV telescope is a nice mind-bender )

PD: Some things you find on internet are really interesting:

Thank you

That extra outer gear giving a fantastic gear ratio is a great idea - I shall bear that in mind   However, I think my current design will be suitable but if the ratio is too small I'll look at that planetary with extra outer gear.

I have used spur gears split in two with a spring between to prevent backlash in the past.

[Gina]

If I get enough reduction ratio as I think I will with the spur gear and pinion this will give less backlash than those high ratio planetary gearboxes.

On the Esprit I have a 4:1 belt drive ratio plus the 11:1 built-in dual drive ratio giving 44:1 between the stepper motor and R&P pinion.  Now I don't know how the two R&P pinions compare for size but a quick test turning the coarse knobs would seem to show that they are comparable.   I'm using 5 half-steps per focus step for the Esprit and this gives perfectly adequate resolution.  I could reduce to just one half-step if needed for the ST80.  That gives me an additional 5:1.  So with the ST80 the current 13:1 and 5:1 gives 65:1

Unfurtunately, my UP printer has developed a fault resulting in no print bed heating so I'm getting curl and a reduction in printing accuracy.  Whilst the pinion printed fine, the large gear has curl and malformed teeth on very close inspection.  So I've changed the design to a slightly smaller OD large gear and 50% larger teeth to see if I can get an adequate print.  The result is that the large gear has 90 teeth and the pinion 9 teeth giving a 10:1 ratio rather than 13:1.  However, by reducing the stepper motor steps for each step of the focus control from five to one I can get a ratio of 50:1 - as compared with the Esprit's 44:1 which should be fine

I'm hoping the new design will reduce the curl by using less ABS in the large gear and the larger teeth will be less affected by the poorer print definition.

[jgs001]

If you find you do have focuser slop issues, then apply some teflon tape (~£10 for a 30m roll) along the drawtube in line with the felt pads in the barrel. Mine was really sloppy to the point with a 2x barlow and a webcam, I'd have to adjust the aim with the mount when I tried to focus. With the teflon tape fitted, I had to go to a 5x barlow to see the slop

[Gina]

The large gear has printed well   I used the transparent ABS which prints a bit better than coloured ABS, which helped.  Not as pretty but prints better

[JamesF]

I wonder if you might be able to estimate the amount of CA you're likely to experience?

Say you took an image of the Moon through the scope using a red filter and then a blue filter.  The offset/change in size when overlaying the two should give you an idea of the total CA across the entire visible spectrum.  Although it's painfully obvious when using the scope visually, I bet it isn't actually that much in terms of pixels.  And that would be across a range of, what, say, 400nm?  If the OIII filter has a bandpass only 3nm wide I'd have thought the misalignment across that sort of range is going to be minimal.  Perhaps if you were using a 35nm Ha filter it might show up, but with filters with much smaller pass bands I don't think I'd expect to find anything significant.

James

[Gina]

If you find you do have focuser slop issues, then apply some teflon tape (~£10 for a 30m roll) along the drawtube in line with the felt pads in the barrel. Mine was really sloppy to the point with a 2x barlow and a webcam, I'd have to adjust the aim with the mount when I tried to focus. With the teflon tape fitted, I had to go to a 5x barlow to see the slop

I have had the focuser apart and serviced it.  Not that it needed it really.  There was a very slight slop which could be easily taken out with the adjustment grub screws.  The pads are actually PTFE already and lovely and slipery.  It's possible that some polishing of the focuser tube might help make it move easier but I won't bother unless I find the stepper motor won't drive it.  In the worst case I might have to increase the gear ratio and use multiple steps of the stepper motor.  I could add an extra stage of gearing if necessary.  The focus mechanism was already lubricated with the ubuquitous white lithium grease.

[Gina]

I wonder if you might be able to estimate the amount of CA you're likely to experience?

Say you took an image of the Moon through the scope using a red filter and then a blue filter.  The offset/change in size when overlaying the two should give you an idea of the total CA across the entire visible spectrum.  Although it's painfully obvious when using the scope visually, I bet it isn't actually that much in terms of pixels.  And that would be across a range of, what, say, 400nm?  If the OIII filter has a bandpass only 3nm wide I'd have thought the misalignment across that sort of range is going to be minimal.  Perhaps if you were using a 35nm Ha filter it might show up, but with filters with much smaller pass bands I don't think I'd expect to find anything significant.

James

I am going to try using a Baader 8.5mn OIII filter initially (which I have spare) though I expect to use my Astrodon 3nm OIII filter for real imaging.  I could certainly take red and blue images of the moon - if we get a clear night while the moon is out and after I get the rig set up

I very much doubt that the ST80 would be suitable for RGB imaging but the only time I use LRGB filters is for galaxies which mainly need the MN190 to get a big enough image.  So "single shooter"   However, the Evostar 80ED might be suitable for galaxies and I might just make up a trio of those sometime in the future   Might just manage that before galaxy season comes round again.

[RobH]

You really are industrious Gina...hats off to you !

You're right that you will get around CA problems by using it as a narrowband scope. You'll need to use a field flattener too though, as you'll be getting field curvature on a chip the size of the 460 chip.

[Gina]

Thank you Rob   I'll have to see how bad field curvature is and maybe get a flattener.

[JamesF]

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

[Stu]

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

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