Author Topic: CEM60-EC : Testing out the Encoders  (Read 338 times)

stunfalriave

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CEM60-EC : Testing out the Encoders
« on: December 28, 2017, 02:06:43 AM »
Last night, I began the night autoguiding on NGC981 for practice, as I’m learning my new CEM60-EC. I was using my EdgeHD 8 non-reduced at F10 with a 1.6X APS-C 7D Mark II sensor (equates to 3248mm of FOV). The seeing conditions was bad and the moon was out. I’m limited, at current, with a 400mm F5.6 guide scope, on axis guider (I’ve ordered a Orion TOAG, yet to arrive). I couldn’t get decent guiding results no matter what I tried, probably due to the limitation of my on axis guide scope and bad seeing conditions. About 45 minutes into the session, I thought that this would be a great time to test out the EC encoders. So, I filtered the RA guiding within the HC and I shut down PHD2.
What I saw was pretty impressive. I had configured my session through SGP and I began with 2 minute subs. Perfect round stars. I kept it here for a little over an hour. I then changed the exposure time to 3 minute subs. Again, perfect round stars. I kept it here for about an hour and a half. I then changed again to 4 minute subs. Again, perfect round stars. After another hour, I moved to 5 minute subs – once again, perfect round stars.
With an image scale of 1.45 arc-seconds, I have to say that I was very pleased. Here is a rough stretch / quick edit of the results; pleasantly surprised to get as much detail as I did with the moon out/bad seeing.

AB LINK

Moving forward, I think I’ll ditch the Autoguider and stick with the encoders! For all of you out there with the EC versions, what are you seeing? I'm looking forward to trying 10 minute subs next time



rankkozical

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Re: CEM60-EC : Testing out the Encoders
« Reply #1 on: December 28, 2017, 06:29:46 PM »
Actually the effect of APSC is to reduce the FoV (compared to a larger sensor), but it doesn't change the EFL, which would still be ~2000mm in your case. However the results are very good. I do detect a slight elongation but this may have been PA error or atmospheric refraction.

Joe Mallard

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Re: CEM60-EC : Testing out the Encoders
« Reply #2 on: January 04, 2018, 12:28:18 AM »
Quote
Actually the effect of APSC is to reduce the FoV (compared to a larger sensor), but it doesn't change the EFL, which would still be ~2000mm in your case. However the results are very good. I do detect a slight elongation but this may have been PA error or atmospheric refraction.

Thanks Duncan. You are correct that it narrows FOV. You are also correct that the focal length remains at 2030mm. What does changed is a narrower FOV, changing the Full Frame FOV of 2030mm to APSC-C FOV of 3248mm as i stated above, which puts more demand on accuracy as your image scale changes. I don't recall ever stating that it changed the FL. It's hard to see elongation to my eye, but if it's there, it's good enough for me

Jeffrey Hunter

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Re: CEM60-EC : Testing out the Encoders
« Reply #3 on: January 04, 2018, 06:23:44 PM »
Nice. Stars look perfectly round to my eyes. Looking forward to seeing what you get with 10 minute subs.Derek

unamprodce

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Re: CEM60-EC : Testing out the Encoders
« Reply #4 on: January 08, 2018, 01:14:05 PM »
Quote
Quote

Actually the effect of APSC is to reduce the FoV (compared to a larger sensor), but it doesn't change the EFL, which would still be ~2000mm in your case. However the results are very good. I do detect a slight elongation but this may have been PA error or atmospheric refraction.

Thanks Duncan. You are correct that it narrows FOV. You are also correct that the focal length remains at 2030mm. What does changed is a narrower FOV, changing the Full Frame FOV of 2030mm to APSC-C FOV of 3248mm as i stated above, which puts more demand on accuracy as your image scale changes. I don't recall ever stating that it changed the FL. It's hard to see elongation to my eye, but if it's there, it's good enough for me
A smaller FOV will require greater pointing accuracy to place a given object into the frame but not necessarily greater guiding accuracy.

The guiding accuracy required for round stars and to determine image scale is based upon the pixel size of the imaging camera and is unrelated to FF or APSC sized sensors. A FF camera with small pixels will require greater guiding accuracy than an APSC camera with larger pixels and vise versa.

For example the APSC 7D2 has 4.1 micron pixels while a FF 6D has 6.5 micron pixels so the APSC 7D2 will require greater guiding accuracy. A FF 5DS has 4.1 micron pixels while the APSC 40D has 5.6 micron pixels and so the FF 5DS requires greater guiding accuracy.

Anyways the only way I could detect the elongation was by measuring the FWHM of your stars in the X and Y axis, but the variation between the two was still very small.

Mark Richmond

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Re: CEM60-EC : Testing out the Encoders
« Reply #5 on: January 08, 2018, 07:20:49 PM »
How did you calculate 1.45 arc-sec per pixel image scale? My numbers for the 7DII give .42 arc-sec per pixel at 2000mm FL.

At that image scale, if you see the SDE error of the CEM60-EC expressed as about 4" error, you'll get about 9 pixels of elongation over the period of the worm. So from my rough look at the image I'd say you've got about that, and it looks pretty good. I could live with that.

Max Sandell

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Re: CEM60-EC : Testing out the Encoders
« Reply #6 on: January 13, 2018, 05:15:59 AM »
Quote
How did you calculate 1.45 arc-sec per pixel image scale? My numbers for the 7DII give .42 arc-sec per pixel at 2000mm FL.

At that image scale, if you see the SDE error of the CEM60-EC expressed as about 4" error, you'll get about 9 pixels of elongation over the period of the worm. So from my rough look at the image I'd say you've got about that, and it looks pretty good. I could live with that.

Hi Chuckwagon; I believe you are correct. I think i glanced at the pixel scale that is calculated from Astrobin and referenced that.

David Schwartz

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Re: CEM60-EC : Testing out the Encoders
« Reply #7 on: January 13, 2018, 03:35:03 PM »
Quote
Quote

How did you calculate 1.45 arc-sec per pixel image scale? My numbers for the 7DII give .42 arc-sec per pixel at 2000mm FL.

At that image scale, if you see the SDE error of the CEM60-EC expressed as about 4" error, you'll get about 9 pixels of elongation over the period of the worm. So from my rough look at the image I'd say you've got about that, and it looks pretty good. I could live with that.

Hi Chuckwagon; I believe you are correct. I think i glanced at the pixel scale that is calculated from Astrobin and referenced that.
Ah, that makes sense, since the Astroibn image is only 1400x955 and the 7DII native resolution is 5472 x 3648. I assume you scaled it down to post on Astrobin. I scaled it up to get a rough idea of how much pixel elongation there was. Either way, the pic looks nice.

Noe Subeydhi

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Re: CEM60-EC : Testing out the Encoders
« Reply #8 on: January 14, 2018, 02:14:02 AM »
Charles,

How long is the worm period?

Will

brigtigeartgib

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Re: CEM60-EC : Testing out the Encoders
« Reply #9 on: January 21, 2018, 09:23:23 AM »
Jammey,

Nice image!

I have gone 10 minutes with my EC. I had very slight elongation of stars but you really had to be pixel peeping to notice it. Of course... you must nail your polar alignment for any chance of success.

Will

acbacema

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Re: CEM60-EC : Testing out the Encoders
« Reply #10 on: January 22, 2018, 09:55:28 PM »
Quote
Charles,

How long is the worm period?

Will

5 minutes I believe. It's been discussed elsewhere and that was what was stated.

brunenrizap

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Re: CEM60-EC : Testing out the Encoders
« Reply #11 on: January 23, 2018, 11:29:22 AM »
Quote
What does changed is a narrower FOV, changing the Full Frame FOV of 2030mm to APSC-C FOV of 3248mm as i stated above, which puts more demand on accuracy as your image scale changes.

Your telescope is 2032mm focal length and F/10*. Changing the size of the sensor does not affect that in *any* way. I realize that terrestrial photographers like to talk about "crop factor" and talk about it as if it changes the focal length or focal ratio, but even in terrestrial photography, this is incorrect thinking as the nature of the optics is *completely independent* of the size of the sensor. I suspect that the reason that this verbiage persists in terrestrial photography because there are some effects on perspective and depth of field that result from positioning the camera differently to compensate for the smaller size of the field and it's easier to use imprecise terminology than it is to convey the true differences. In astrophotography, though, we are always imaging a 2 dimensional field, and there are no such benefits to be gained, and no reason to use the same (technically incorrect) terminology.

Also, your image scale is dependent on focal length and the size of individual pixels *and nothing else*. On a given telescope, a larger sensor and smaller sensor with the same size individual pixels have the *exact same* image scale. At the pixel level, the accuracy that you describe has nothing to do with the overall size of the sensor.

-Wade

*Because your telescope focuses by moving the primary mirror, which changes the distance between the primary and secondary mirrors, the focal length (and therefore the focal ratio) change every time that you turn the focus knob. If your camera sensor is precisely 133.35mm from the back of the telescope, you will be at 2032mm (within some manufacturing variance). If it's closer than that, your focal length will be less and if it's longer than that, your focal length will be more. The best way to know for sure is to plate solve an image and see the effective focal length.

ceisilipan

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Re: CEM60-EC : Testing out the Encoders
« Reply #12 on: January 31, 2018, 12:39:18 AM »
Quote
Quote

What does changed is a narrower FOV, changing the Full Frame FOV of 2030mm to APSC-C FOV of 3248mm as i stated above, which puts more demand on accuracy as your image scale changes.

Your telescope is 2032mm focal length and F/10*. Changing the size of the sensor does not affect that in *any* way. I realize that terrestrial photographers like to talk about "crop factor" and talk about it as if it changes the focal length or focal ratio, but even in terrestrial photography, this is incorrect thinking as the nature of the optics is *completely independent* of the size of the sensor. I suspect that the reason that this verbiage persists in terrestrial photography because there are some effects on perspective and depth of field that result from positioning the camera differently to compensate for the smaller size of the field and it's easier to use imprecise terminology than it is to convey the true differences. In astrophotography, though, we are always imaging a 2 dimensional field, and there are no such benefits to be gained, and no reason to use the same (technically incorrect) terminology.

Also, your image scale is dependent on focal length and the size of individual pixels *and nothing else*. On a given telescope, a larger sensor and smaller sensor with the same size individual pixels have the *exact same* image scale. At the pixel level, the accuracy that you describe has nothing to do with the overall size of the sensor.

-Wade

*Because your telescope focuses by moving the primary mirror, which changes the distance between the primary and secondary mirrors, the focal length (and therefore the focal ratio) change every time that you turn the focus knob. If your camera sensor is precisely 133.35mm from the back of the telescope, you will be at 2032mm (within some manufacturing variance). If it's closer than that, your focal length will be less and if it's longer than that, your focal length will be more. The best way to know for sure is to plate solve an image and see the effective focal length.
Yes, you're correct. I should have simply left my statement at FOV, as i didn't intend for the conversation to be diverted to terminology differences. Thanks Wade.

Marlon Hilzer

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Re: CEM60-EC : Testing out the Encoders
« Reply #13 on: January 31, 2018, 05:19:19 AM »
Quote
Jammey,

Nice image!

I have gone 10 minutes with my EC. I had very slight elongation of stars but you really had to be pixel peeping to notice it. Of course... you must nail your polar alignment for any chance of success.

Will

That's fantastic Will, 10 minute subs would be very nice while not having to fiddle with guiding. I'm really trying to nail PA with polemaster. If i can get to 10 minute subs with satisfactory results, I'll leave my PA in place.

stimtinpaso

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Re: CEM60-EC : Testing out the Encoders
« Reply #14 on: January 31, 2018, 08:06:49 AM »
Quote
Jammey,

Nice image!

I have gone 10 minutes with my EC. I had very slight elongation of stars but you really had to be pixel peeping to notice it. Of course... you must nail your polar alignment for any chance of success.
Hi Will,
Do you think that a stellarvue 102 APO (714mm FL , 570mm FL with reducer) on a CEM25-EC using an APS-C DSLR could go 4 minutes unguided and get round stars? Assuming good PA via polemaster.
I ask because I an contemplating the above rig as a "grab and go" set up.

Thanks,
Gene