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Alt/Az imaging with guiding (images included)

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#1 jesco_t

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Posted 07 March 2022 - 04:31 AM

I wish I had an EQ mount, but my balcony is small and Polaris nowhere to be seen from there.

 

So I decided on getting an alt/az mount. It's a SkyTee2, motorized with OnStep. For visual it's a joy with my C8. Planetary also works nicely. For DSO tracking is good enough for 5-10s subs at 1280mm. Quite nice already, but I realized that OnStep allows pulse guiding also in alt/az configuration. I tried it out and get around 1.5" RMS error. Coupled with my 3" sky FWHM this allows for round stars. PHD2 is amazing and can deal with the field rotation of the guide FOV from south-east to west without deterioting the RMS.

 

Of course field rotation is still annoying. In the south I am limited to 30s, south-west 60s and west 90s subs. But that's already a lot to work with. More tricky is the long-term rotation of the FOV around the guide star which makes getting the FOV setup properly tricky.

 

Overall, it's of course nothing for pixel-peeping. But for my occasional imaging for the fun of it, I am pretty happy.

 

Some examples...

 

NGC 7331 (9 hours of 30s subs)

 

NGC 7331 Galaxy

Edited by jesco_t, 07 March 2022 - 09:18 AM.

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#2 jesco_t

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Posted 07 March 2022 - 04:33 AM

M 33 Triangulum Galaxy (11.5 hours of 30s subs)

 

M33 - alt-az imaged with Celestron C8 and Nikon D3400

 

 


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#3 jesco_t

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Posted 07 March 2022 - 04:36 AM

Leo Triplett (4.5 hours of 30s and 60s subs)

 

Leo Triplett (4.5h alt/az imaging with C8)

 

Here, getting the FOV right of tricky. In principle it should fit, but field rotation around the guide star (the one on the right side, half-way between top and bottom, made it necessary to do a mosaic here.

 

I hope nobody is offended by the number of images. I just want to show people that nice results can be had, even when doing things out of the box.

 

Jesco


Edited by jesco_t, 07 March 2022 - 04:49 AM.

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#4 coinboy1

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Posted 07 March 2022 - 07:54 AM

Very nice! Especially in Alt-Az mode. Great experimentation for sure. I think a field rotator would allow much longer exposure times but with these new low read noise cameras, lots of short exposures definitely work! 



#5 vidrazor

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Posted 07 March 2022 - 08:31 AM

I wish I had an EQ mount, but my balcony is small and Polaris nowhere to be seen from there.

So I decided on getting an alt/az mount. It's a SkyTee2, motorized with OnStep. For visual it's a joy with my C8. Planetary also works nicely. For DSO tracking is good enough for 5-10s subs at 1280mm. Quite nice already, but I realized that OnStep allows pulse guiding also in alt/az configuration. I tried it out and get around 1.5" RMS error. Coupled with my 3" sky FWHM this allows for round stars. PHD2 is amazing and can deal with the field rotation of the guide FOV from south-east to west without deterioting the RMS.

Of course field rotation is still annoying. In the south I am limited to 30s, south-west 60s and west 90s subs. That's a lot to work with. More tricky is the long-term rotation of the FOV around the guide star which makes getting the FOV setup properly tricky.

Overall, it's of course nothing for pixel-peeping. But for my occasional imaging for the fun of it, I am pretty happy.

Some examples...

That's quite amazing what you've managed with that mount. Great post work too.

 

Because you're in heavily light polluted skies, those short exposures actually work in your favor, but your camera shutter is going to pay the price. :)

 

Can that mount be converted to an EQ mount if you put it on a wedge or alt-az base?

 

BTW, I see you're running your sessions with Ekos. If you're not familiar with it, Ekos can polar align even if you don't have Polaris in sight. Just make sure to download all the astrometry files it needs to work with.
 



#6 deansjc

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Posted 07 March 2022 - 11:03 AM

Great job with a limited setup.  Even a simple fixed elevation wedge for your latitude would take you one extra step.  And I mean simple, as long as it is rigid to support the substantial weight of your rig.

 

Very nice shots!

 

John



#7 jesco_t

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Posted 07 March 2022 - 03:37 PM

Thank you guys for the nice comments.

 

Very nice! Especially in Alt-Az mode. Great experimentation for sure. I think a field rotator would allow much longer exposure times but with these new low read noise cameras, lots of short exposures definitely work! 

I have thought long about this. It's actually not that hard to build one. I have a 3D printer and OnStep support derotators. But I don't think I would gain anything. The problem is that the field rotates around the guide star which is not in the center of the FOV of my DSLR. So my derotator wouldn't actually counter the field rotation, but just add another albeit linked rotation. It might help a bit, but I am not sure how much. A simple derotator would need a mount with encoder for precision unguided tracking, which kind of defeats the initial premise, I am afraid.

 

 

Can that mount be converted to an EQ mount if you put it on a wedge or alt-az base?

 

BTW, I see you're running your sessions with Ekos. If you're not familiar with it, Ekos can polar align even if you don't have Polaris in sight. Just make sure to download all the astrometry files it needs to work with.
 

There's no wedge that fits the GP/EQ5 tripod connector - I couldn't find any so far. But I am also limited in balcony space - a full EQ mount slewing a C8 has much wider and stranger range of motions than an alt/az mount. I am sure I would hit stuff frown.gif

 

Great job with a limited setup.  Even a simple fixed elevation wedge for your latitude would take you one extra step.  And I mean simple, as long as it is rigid to support the substantial weight of your rig.

 

Very nice shots!

 

John

My balcony prohibits this. But... there's hope. I am moving to a house with a garden (yay!). So I am thinking about building myself a plywood wedge - I think it should be strong enough. I can just re-flash my controller to EQ mode. I already found some links depicting simple but apparently rigid wedges made from plywood. My summer project smile.gif

 

Jesco
 




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