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Guiding an ultra-wide lens

Accessories Astrophotography CMOS Imaging Lens Making Software
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#1 Memo Tron

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Posted 18 June 2021 - 09:59 AM

Hi,

 

I´ve been looking for some guidance on this topic, but not finding too much, sorry if the question is a bit obvious for most of you.

 

I´ve been enjoying astrophotography for almost a year now, imaging mainly with a C9.15" and an 80mm apo, even using my Askar FMA 180 guide scope as an imaging one from time to time, always using a ZWO ASI 533MC as an imaging camera. Overall, no major problems with my guiding (I use APT and PH2D). I recently got a Rokinon 85mm and a 14mm to explore wider field of views.

 

Last night, and for the first time, I tried the 85mm lens with 533MC to shoot some wider fields of view, everything seems to work well, but I just couldn´t get a good guiding for this set up (I used a small 32mm William Optics guide scope). I usually get around 0.5 to 0.8 arc seconds when guiding (which I believe is not that bad), but yesterday I was AT ~1.3 to 1.6 arcseconds, no better than that. I shot 5 min subs (with L-Extreme filter) and got overall good shots, at least rounded stars...

 

I realized that the focal length of the guide scope is 120mm, which is quite larger than the 85mm lens that I´m using for imaging. Is there any way to guide an 85mm lens, or even a 14mm? Should I used a shorter FL guide scope (is there any that shorter??) Am I missing any basic piece in PHD2 configuration?

 

Many thanks in advance for the advices!



#2 happylimpet

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Posted 18 June 2021 - 10:07 AM

No harm in your guiding being better than necessary, which is what youve got here. Carry on!



#3 Phillip Creed

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Posted 18 June 2021 - 10:41 AM

I personally wouldn't bother trying to guide an 85mm lens in the first place.  Pixel resolution with the ZWO533 (3.75-um pixels) is 9"/pixel with an 85mm lens.  That hides a lot of issues.  If you're using a CEM70, the native periodic error is 7 arc-seconds peak-to-peak (+/- 3.5").

Heck, I image with a 135mm Samyang unguided on a Star Adventurer, and it's got a LOT more native PE.  I'm at 6"/pixel resolution using my D5300, and I only toss 5-10% of the subs with a good polar alignment.

But even with 1.3" - 1.6" guiding, I gotta think that's more than sufficient given the constraints.  Is the guiding error enough to show up in the subs if you zoom in real close?  If not, I'd just leave it as-is.  Numbers are one thing, but pictures are all that matters in the end.

Clear Skies,

Phil



#4 Memo Tron

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Posted 18 June 2021 - 10:59 AM

I personally wouldn't bother trying to guide an 85mm lens in the first place.  Pixel resolution with the ZWO533 (3.75-um pixels) is 9"/pixel with an 85mm lens.  That hides a lot of issues.  If you're using a CEM70, the native periodic error is 7 arc-seconds peak-to-peak (+/- 3.5").

Heck, I image with a 135mm Samyang unguided on a Star Adventurer, and it's got a LOT more native PE.  I'm at 6"/pixel resolution using my D5300, and I only toss 5-10% of the subs with a good polar alignment.

But even with 1.3" - 1.6" guiding, I gotta think that's more than sufficient given the constraints.  Is the guiding error enough to show up in the subs if you zoom in real close?  If not, I'd just leave it as-is.  Numbers are one thing, but pictures are all that matters in the end.

Clear Skies,

Phil

 

 

No harm in your guiding being better than necessary, which is what youve got here. Carry on!

 

Thanks happylimpet & Phillip, looks like I´m doing it the right way... As mentioned, the shots from yesterday looks fine, no trailing starts at all, I just wanted to be sure of not been missing a key piece in guiding at this wide fields of view.

 

Regards




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