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SharpCap 2.9 - Polar Alignment


Andyb90
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Well I managed to get Sharpcap 2.9  to Polar Align my AstroTrac. SC didn't like the SSAG as it kept erroring out so I had to switch cameras to my Altair Astro GPCAM which worked sort of okay. Had the initial solved in the first image then took ages to solve the second and then didn't seem to register the changes. Did this a few times and eventually got better and better PA. Stopped when I got it down to 6" as I believe the AstroTrac is only accurate to 5" unguided.

Anyway hooked up the Nikon D7000 and the 300mm lens and although the images are poorly focussed they show no trailing at 300 second images. Will try again tomorrow night 

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Have managed a second night and got PA Error down to four arc minutes. Downloaded the latest build of SharCap 2.9 and it seemed to solve quicker. What I noticed was the boxes around the stars didn't have four sides to some of them. What I am finding is if you go through the PA routine a couple of times it gets more accurate. Below is a picture I took of M45 300s x 10 unguided with the AstroTrac

IMG_0803.jpg

Pleiedes.jpg

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Just to say to everyone that getting the polar alignment error down to a few seconds of arc is really unnecessary (although fun...)!

If you took a six hour exposure then the total amount of drift due to polar alignment issues (as opposed to other things like tracking rate inaccuracies) would be roughly the same as your polar alignment error. If you are taking 5 minute exposures then the drift due to polar misalignment will be 72 times smaller, so getting within 1-2 minutes of arc should be fine for 5 minute exposures.

cheers

Robin

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  • 2 weeks later...

Although I was pretty pleased with the Pleiades image. It was unguided with around five minutes polar alignment error it was unguided. This image was 

combined using five minute exposures which for 300mm is on the upper edge of the envelope for this mount I am seeing trailing in the 769 Tatjana blog

images. This you can see on my blog on my website

http://www.digitalastroimager.com/my-blogs/

I eliminated flex and bounce  as much as possible and will see if I can improve PA and run with the autoguider next chance I get. Astrotrac claim five arc seconds peak to peak with this setup (RA onlly).

It is fun getting good PA though!!!:headbang:

Regards

 

Chris

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8 hours ago, Crose01 said:

Although I was pretty pleased with the Pleiades image. It was unguided with around five minutes polar alignment error it was unguided. This image was 

combined using five minute exposures which for 300mm is on the upper edge of the envelope for this mount I am seeing trailing in the 769 Tatjana blog

images. This you can see on my blog on my website

http://www.digitalastroimager.com/my-blogs/

I eliminated flex and bounce  as much as possible and will see if I can improve PA and run with the autoguider next chance I get. Astrotrac claim five arc seconds peak to peak with this setup (RA onlly).

It is fun getting good PA though!!!:headbang:

Regards

 

Chris

Hi Chris;

I am using the Astrotrac with a Nikon D7200 and the cheaper Nikkor 70-300mm F3.5-5.6 telephoto lens. Your experience and comments piqued my interest. After dropping the Astrotrac polar scope, I had no luck collimating the scope and replaced this with the Polemaster, and have had good success so far with this setup. I'm running without the declination arm, and I am getting luck with 240mm FL, and two minute exposures. I'm looking forward to pushing to 3-4 minutes next time out. Love your pic.

 

Pleadies stacked.jpeg

IMG_20161123_014035.jpg

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Regarding those with AstroTracs...

I have yet to try the PoleMaster/SharpCap on my AstroTrac...it is on my TODO list.

I peaked at 180 seconds at 500mm focal length using the supplied, but well collimated, polar scope. I was unsure if it was tracking error or polar alignment error that was the limiting factor, I suspect PA error as well as flexure. Flexure from the tripod foot as it is only one point of contact and with my 500mm f/4 lens it definitely flexes ever so slightly and is the reason I now fix it at the front of the lens as well. The speed of tracking of the AstroTrac changes over time (tangent function) so if it isn't granular enough you could end up with oblong stars. I did once measure the stepper motor frequency of the AstroTrac and it did change every so often in discrete "steps". These comments are from my notes...

228.581Hz initial frequency
228.414Hz after about 3 minutes
228.08Hz after about 10 minutes
227.914 after about 12 minutes

So it would be good to put this to rest and figure out what was limiting me to 180 second (reliable) subs at 500mm focal length.

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Hi. First post!

I've been using Sharpcap 2.9 and a Touptek camera  and a 50mm F4 finderscope (~200mmEFL, camera same as Altairastro GPcam AR130M) to polaralign my CI-700. I have an accurately aligned 7x50 PA scope and Sharpcap confirms that it aligns to about 3min from the NCP (which I can confirm visually from the starfield in the PA scope.

The problem arises when I try and adjust in the alt  or azimuth direction; Sharpcap always reports that the error in the opposite axis increases even as the error in the axis being moved decreases. I have thought that maybe this has to do with Arizona not using DST or maybe the touptek sensor is being read out in reverse - (another USA user with a Touptek is reporting the same issue) . Anyways,  Sharpcap PA seems like a great idea and does accurately report the PA error even if I can't use it to directly refine my PA.  I have tried this many times and carefully followed the steps with the autoadvance to next step turned on and turned off but always with the same result.

 

thanks.

Edited by DuncanM
sp
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@DuncanM

Actually the numbers are just 'nice to have' you can do the polar alignment without using them at all as long as you put the indicated star into the target crosshairs by adjusting the alt/az bolts. The distances in Alt and Az are based on an estimation of your longitude from your time zone (15 degrees west for every hour behind UTC). If your true longitude is more than about 15 degrees out from the estimated longitude then the alt/az figures aren't going to be as accurate.

cheers,

Robin

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Well, I've finally settles on a PA hardware setup fro my AZ EQ 6. It's a ZWO ASI 120MC, with a Nikon E series 50mm f1.8:

polar1.jpg

I've made an adaptor out of a waste pipe stop end that fits in the polar finder hole of the mount, and the camera screws onto it nice and firm.

With one second exposures I can get a solve in no time at all, and regularly get well under an arc minute, with the resulting PHD trace speaking for itself:

phd.JPG

Hats off to Robin

 

Huw

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Great to hear that people are getting good polar alignment results :)

I think I have an idea for another polar alignment technique that would work when you can't see the pole - basically accelerated drift alignment with the same sort of guidance for the adjustment stage that you get with the current polar align in SharpCap. This would require a view of about 120 degrees of RA at some particular declination and plate solving software (like Astrotortilla). Just an idea at present, so it won't make it to the next version of SharpCap, but maybe later in 2017.

cheers,

Robin

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  • 4 weeks later...
On 12/13/2016 at 15:21, Crose01 said:

Nice setup how did you attach your red dot finder to your camera?

 

Regards
 

Chris

Chris, sorry for the late reply - The red dot finder hot shoe adapter is 3D printed, and I purchased it online at an astronomy dealer. They no longer carry it, but is similar to this link

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p5641_Lacerta-adapter-for-mounting-red-dot-finders-to-camera-flash-shoes.html

Huge timesaver!

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I'm finally ready for first light with my now completed set-up. Completed for the moment anyway.  :wink:

I'm intending to start by imaging with a Nikon D7200. I'm looking for a S/H D600 which I'll get modded. My current lens are Samyang 14mm f2.8 (for the Milky Way etc), Tamron 24-70mm f2.8, Tamron 70-200mm f2.8, Sigma 150mm f2.8 Macro and a Nikon 300mm f4 + 1.4 T/C.

I have a HEQ5 Pro with a side-by-side plate to mount the camera, with flash mounted green laser pointer, and my WO 50mm guide scope with ZWO ASI120MM-S to use for drift aligning (?) and when I get into guiding. Initially I'll be doing tracking only till I get the hang of what this astro stuff is about. I have a lap-top with a USB3 port available.

I should say at this stage that I'm reasonably competent with terrestrial photography but have only dipped my toe in the water with the astro stuff.

I'm pretty enclosed at home but I think I have enough clear view to the South and East to get a reasonable alignment with SharpCap and M42 and M45 are very visible at present.

OK, newbie questions: 

- Do I need to have any other drivers other than what came on the ZWO disc? It has SharpCap V 2.9 and Firecapture V 2.5.10 on it.

- The WO 50mm 'scope has a FOV of 5.7°. Is that OK for drift aligning?

- So I'll use the guide scope for my drift aligning and then plug in the camera for imaging?

- What is a good, easy to follow program for my imaging capture with a DSLR?

This has been a twelve month process getting to this point with a dud HEQ5 (returned) an NEQ6, too heavy, and the frustration of finding the bits to mount a DSLR camera.

I'm sure I'm over simplifying the whole process so any tips would be most appreciated.

 

Edited by DarkKnight
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Kev,

just install the ZWO drivers from their website or the CD and also SharpCap and you should be fine. Your guide scope has a 5.7 degree FOV with the eyepiece it comes with, but it will be nearer 1 degree with the camera, which is spot on for SharpCap polar alignment (which isn't really drift alignment - not sure if there is a name for it - perhaps photo polar alignment?).

If you are going to be guiding then you will need PHD2 as well for the actual guiding.

There are a number of programs for DSLR control - APT and BackyardEOS for Canon, both now have a Nikon version or some Nikon support too I think (APT Nikon might still be in beta).

Robin

 

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Many thanks for taking the time to assist Robin, much appreciated. :thumbsup:

And of course SC is different to drift alignment.  :o  Perhaps we could call it PSPA, an initialism for 'Plate Solving Polar Alignment'.

Now all I want is a clear sky. The only clear night we've had recently it was 30°C (86°F) at 2200 and the sky was shimmering like a desert mirage. Today it is grey and drizzling. One thing I have learnt is not to miss an opportunity to shoot when you have good seeing, as those times are rare.

Thanks again.

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On ‎22‎/‎12‎/‎2016 at 19:33, DuncanM said:

The problem arises when I try and adjust in the alt  or azimuth direction; Sharpcap always reports that the error in the opposite axis increases even as the error in the axis being moved decreases

I think this could be due to your tripod not being level.  This could cause an adjustment in one axis to change the other.

 

Barry

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Hmmm - using the new Directshow pipeline in 2.10, I’ve lost 520fps from my camera... if I switch to original DirectShow pipeline and restart, I’m back up to 500-600fps. Switch to new SharpCap pipeline (no other settings changed) and I’m immediately dropped down to 30fps or less. (Opticstar PL131M)

Edited by Marci
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42 minutes ago, Marci said:

Hmmm - using the new Directshow pipeline in 2.10, I’ve lost 520fps from my camera... if I switch to original DirectShow pipeline and restart, I’m back up to 500-600fps. Switch to new SharpCap pipeline (no other settings changed) and I’m immediately dropped down to 30fps or less. (Opticstar PL131M)

I suspect that the frame rate you were seeing under the old directshow code wasn't real - if you look at the specs for that camera, it's rated for only 22fps @ full resolution and only hits 220fps @ 320x240 crop, so 500+fps isn't going to be a true rate!

cheers,

Robin

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On 1/26/2017 at 04:20, Barry said:

I think this could be due to your tripod not being level.  This could cause an adjustment in one axis to change the other.

 

Barry

The tripod is perfectly level. Many other users in the southern USA are reporting this issue as well.  However, once I decided to just concentrate on reducing the error it all went well.

 

Does anyone know if Sharpcap uses the true or refracted pole? For users in low latitudes it becomes more important.

 

thanks

 

 

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@DuncanM

There is currently no correction for atmospheric effects in the polar alignment routine - I don't think that would be too difficult to add as long as I knew the observer latitude and longitude.

The lat/long is most likely the cause of the adjustment directions being out too. I estimate your longitude based on PC time zone - so if your TZ is EST (UTC-5) then I estimate your longitude at 75 west (and your latitude as 45 degrees). This gives me the info I need to convert a difference in RA/Dec to a differences in Az/Alt. If your PC isn't set to your local timezone then my guess of longitude will be wrong and the offsets will be wrong, although I think you'd have to be out by 30 degress+ to see a big problem.

I will try to find some time to put a way to set Lat/Long in the 2.10 version (and show the estimated values)

cheers,

Robin

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4 hours ago, rwg said:

@DuncanM

There is currently no correction for atmospheric effects in the polar alignment routine - I don't think that would be too difficult to add as long as I knew the observer latitude and longitude.

The lat/long is most likely the cause of the adjustment directions being out too. I estimate your longitude based on PC time zone - so if your TZ is EST (UTC-5) then I estimate your longitude at 75 west (and your latitude as 45 degrees). This gives me the info I need to convert a difference in RA/Dec to a differences in Az/Alt. If your PC isn't set to your local timezone then my guess of longitude will be wrong and the offsets will be wrong, although I think you'd have to be out by 30 degress+ to see a big problem.

I will try to find some time to put a way to set Lat/Long in the 2.10 version (and show the estimated values)

cheers,

Robin

Can I ask if this PA alignment feature will work with a wedge mounted SCT... or does it have to be an actual equatorial moun..? :)

Edited by SkyBound
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On ‎30‎/‎12‎/‎2016 at 12:39, rwg said:

Great to hear that people are getting good polar alignment results :)

I think I have an idea for another polar alignment technique that would work when you can't see the pole - basically accelerated drift alignment with the same sort of guidance for the adjustment stage that you get with the current polar align in SharpCap. This would require a view of about 120 degrees of RA at some particular declination and plate solving software (like Astrotortilla). Just an idea at present, so it won't make it to the next version of SharpCap, but maybe later in 2017.

cheers,

Robin

Hi Robin,

just wanted to add my thanks to you for the effort you're putting into SharpCap.  I'm a first time user of any sort of guiding, so when I come across v2.9, I was really pleased and looking forward to giving it a go.  I've only had one clear night to use the software with an Altair 50mm guide scope and GPCAM mono.  But I got it working.... mostly.  I went through steps 1 and 2 where the Next button becomes available to click to move you onto the next stage once the plate has resolved.  However, I have a question on the 3rd step.  I got to the point where I was moving the mount and got the accuracy down to a few seconds on each axis, but I couldn't make the Next button become available to progress.  I was just wondering if I had reached the end of the process, or, if I needed to do something else?  Do you get a definitive Alignment Complete message or such like?

I'm very much still in the learning and discovery phase, but it did have me a bit stumped.

Thanks once again.  It's a fantastic piece of software,

Tony

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@SkyBound

I think it should be fine with a wedge - it doesn't care about what mechanical setup gets the RA axis pointing near to the pole as long as you can rotate about the RA axis and make adjustments of Alt/Az.

@mountainmadman

No, there is never a 'Next' button enabled in Step 3. I have made this clearer in the next version of SharpCap (2.10 beta) which gives a guide as to the quality of your PA and I think I removed the button too. It's up to you how close you want to get, but for most purposes 1 minute of arc is good enough!

cheers,

Robin

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