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iPolar or guidescope upgrade?

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#1 pixelsbyjim

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Posted 20 February 2020 - 10:57 AM

I'm currently using a modified Dslr and 300mm lens on my Skyguider pro and don't plan to upgrade that for at least another year. My question is which option will I get the most benefit from, adding the iPolar or an autoguiding setup?

#2 bobzeq25

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Posted 20 February 2020 - 11:15 AM

Depends.  How good are you at doing polar alignment now?  Good enough, autoguide.  Pretty sloppy, iPolar.

 

Yes, those are not quantitative.  <smile>  With my iOptron Skytracker, I could rate my polar alignment by how well Polaris stayed on the circle.  I started by putting it on the circle.  If, after the session, it was still on the circle (just having moved around it), my PA was excellent.


Edited by bobzeq25, 20 February 2020 - 11:18 AM.


#3 Hesiod

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Posted 20 February 2020 - 11:15 AM

In general term, the latter would give you more options (including the possibility of more accurate polar alignment) but, before anything else, should really identify where the main issue lies.



#4 pixelsbyjim

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Posted 20 February 2020 - 11:22 AM

My polar alignment is good enough to get me 90 second subs from my light polluted backyard (without a LP filter currently)

#5 Hesiod

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Posted 20 February 2020 - 11:30 AM

Do you need to take longer subs?

If, as I presume, your subs are already rightly exposed, there is not point in purchasing accessories which would add nothing to your setup's effectiveness


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#6 pixelsbyjim

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Posted 20 February 2020 - 11:46 AM

I am considering narrowband imaging which would need considerably longer subs.

#7 Hesiod

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Posted 20 February 2020 - 12:44 PM

In such case get the guider, even if, in your boots, I would rather start by replacing the mount



#8 t-ara-fan

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Posted 20 February 2020 - 04:24 PM

My polar alignment is good enough to get me 90 second subs from my light polluted backyard (without a LP filter currently)

At what focal length?

 

Tracker alignment can be rated by focal length * exposure in seconds = mm seconds.

 

If I can get 60 seconds at 200mm FL (12,000 mm*seconds), I know with the same setup I can get 120 seconds at 100mm and also 240 seconds at 50mm.   


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#9 pixelsbyjim

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Posted 20 February 2020 - 04:52 PM

Focal length is 300mm on a crop sensor, so 480mm equivalent

#10 Hesiod

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Posted 20 February 2020 - 05:38 PM

Focal length is 300mm on a crop sensor, so 480mm equivalent

Focal length is 300mm, period.

The "crop" thing refers only to the field of view, which is "cropped" if compared to that of the "full frame" standard.

Cropping the field of view has no consequences on sampling, and therefore on the accuracy required to have pinpoint stars, or on the image scale (if two cameras have pixels of the same size, the target will have the same size regardless of the size of the sensor; but the one with the larger sensor will have more room around the subject)



#11 t-ara-fan

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Posted 21 February 2020 - 01:18 PM

Focal length is 300mm, period.

The "crop" thing refers only to the field of view, which is "cropped" if compared to that of the "full frame" standard.

Cropping the field of view has no consequences on sampling, and therefore on the accuracy required to have pinpoint stars, or on the image scale (if two cameras have pixels of the same size, the target will have the same size regardless of the size of the sensor; but the one with the larger sensor will have more room around the subject)

True. 

 

However in real life crop cameras often have smaller pixels than full frame cameras.  So the crop factor does come into play when looking at trailing and star size in pixels.  


Edited by t-ara-fan, 21 February 2020 - 01:18 PM.


#12 Hesiod

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Posted 21 February 2020 - 02:55 PM

In the real world cameras from different brands have different pixel sizes so, to be accurate, it is to the sampling that we should refer.
And this has only a vaguely remote relationship with the misleading concept of "crop": to make an example, nikon 810 has 4.8 micron pixel, the canon 6dMkII 5.7 ones, the popular d5300 has tiny 3.9 pixels, the eos80d is around 3.7 micron and my own eos1100d* has large 5.3 micron pixels, so the "canonic" 1.6x factor would not help much.
Therefore I think it is more accurate either to stay with 'optical" focal length (which allows to compare different lenses with the same sensor) or speak in term of sampling if willing to compare two "systems" (sensor+lens)


*older models have usually larger pixel so a "cropped" old camera likely has a looser sampling than a modern full frame


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