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Orion 60mm and color Lodestar X2 combination

astrophotography equipment
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#1 jcridings

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Posted 16 September 2020 - 11:11 AM

I put this combo onto my OTA, a 120mm refractor, instead of using the OAG. More stars, easier to focus, etc. 

I think I'm getting some gravity assisted flex with this combo because of the overall length. 

I'm wondering if anybody else has had experience with this combination and maybe experienced the same.

 

Additional info...

I can take very good unguided images at 5 min exposure time. Nice tight, round stars.

My guide logs indicate the guider is working much harder than it should. Something like I'd expect from a bad, but not unreasonable, polar alignment. RA and Dec RMS values reasonably close. But the images themselves have stars that go from eggs to actual streaks. I've seen eccentricity values in PI of 0.70 up to 0.90. In comparison, my unguided values are 0.45 to 0.5 with better FWHM values. 

 

I'm thinking of buying a different camera that doesn't extend so far back, but wanted some opinions, first.

 

Thanks.

-Jeff



#2 rkinnett

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Posted 16 September 2020 - 12:31 PM

Could be many things.  You haven't provided enough info to rule out some of the more likely ones.  It would help if you describe your guiding configuration in more detail, and your imaging parameters as well.  PHD2 screenshots are handy for troubleshooting because we can see your settings, RA/DEC error over time, and RA/DEC reactions.  What exposure time did you use in PHD2?  A common mistake is to use low exposure time, 2 sec or less, in which case you end up chasing seeing.  Are you compensating for backlash?  I'm guessing you have very low PE if you're able to expose 5 mins unguided, but backlash is an effect that impacts autoguiding but not unguided tracking (generally).  What is your effective imaging focal length and which camera are you using for that?  The Lodestar X2 has exceptionally large pixels.  In order to achieve usable image scale ratio, you may need a longer focal length guide scope.  I see two common 120mm refractors on the market, one f/5 (600mm focal length) and another f/8.3 (1000mm).  I would bet you have the f/5 since you're able to shoot unguided with it.  If you have the f/8.3 then you would need an ST80 or longer focal length guide scope to use the X2.  I'm guessing your primary imager pixel size is in the neighborhood of 4.3 um.  If you have smaller pixels, then this guide scope/camera will be an even worse match.  Using a color guide camera exacerbates all of these problems.  PHD2 finds the center of a star by fitting a gaussian curve across the intensity distribution across neighboring pixels around your guide star.  In your case, with such large pixels, your light profile might span across just 2-4 pixels, but you won't get a usable intensity profile because of the Bayer pattern on your sensor.

 

You can use this chart to check suitability of various guide scopes given your primary imager and guide camera pixel sizes:

https://docs.google....lw58fGYa98/copy

 

If you can put a finger on your guide camera and push it somewhat gently from side to side without any noticeable rattling or flex then it's unlikely that's a significant contributor.  Make sure your guide scope focuser is locked down, and find a way to secure your guide cam usb cable.  A mono guide camera with smaller pixels may be a more worthwhile investment than a different guide scope.  Also look into backlash compensation in PHD2.

 

EDIT:  another thing you could try is defocusing your guide scope to spread the star light across more pixels.  This might work well with the X2 which is very sensitive, even through the Bayer filtering.  Look at the star profile in PHD2 and try to get it to look like a smooth hill rather than a spike or tall/narrow volcano.

 

By the way, if you can reliably shoot 5 min exposures unguided, why do you want to autoguide?


Edited by rkinnett, 16 September 2020 - 12:36 PM.


#3 michael8554

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Posted 16 September 2020 - 12:49 PM

"I put this combo onto my OTA, a 120mm refractor, instead of using the OAG. More stars, easier to focus, etc."

 

The LodeStar is perfect for guiding long focal length OTAs via an OAG because of its large pixels, so it's not a good match to a 60mm guidescope.

 

What's the problem focusing the OAG ?

 

It's a fiddly but once-only operation.

 

Once the guidecam and imaging cam are focused you don't take the LodeStar off the OAG.

 

So you only need to focus the main image camera for each session, the guidecam will then be in focus too.

 

In my case at 1280mm I always get a guidestar with the LodeStar


Edited by michael8554, 16 September 2020 - 12:51 PM.



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