Re: Side by side saddle or piggy back scope alignment
Good point. I was using total image exposure time and even there, field rotation wasn¡¯t a problem I had to worry about.?
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On Aug 21, 2020, at 12:16 PM, Arun Hegde <arun.k.hegde@...> wrote:
?In reading the article linked (authored by Frank Barrett), I am not so sure that those calculations apply to the type of guided imaging we do. In fact, you could use his calculations to prove that with reasonable care, field rotation is unlikely to be a problem at all with today's methods of polar alignment.?
Hook's equations in Frank's article are for unguided imaging and Frank appears to have extended it to guided imaging. Which is fine, but then the relevant exposure time is not the exposure time of your sub, but the exposure time of your guide camera. This is normally on the order of a few seconds, and much shorter than the exposure time of your sub.? As Frank notes, in autoguiding, the motion of the centroid of the guide star is tracked with reference to a reference pixel. Motion beyond a certain tolerance results in a correction. And that correction is applied in both RA and DEC. And as long as that correction is applied before appreciable field rotation occurs, you should be fine. As noted by Frank himself in his example:
"In other words, if our alignment error is less than about 11.25 arc minutes we should see no field rotation greater than 9 microns during a 15-minute exposure at 35 degrees declination and the given setup."
11.25 arc minutes is an absolutely horrendous standard for polar alignment! We can easily get under 2 arc minutes, and with some care, under 1 arc minute in a matter of minutes using things like SharpCap. And guide exposures are much, much smaller than 15 minutes usually 2-4 seconds.
To the point then about the mismatch between the guide scope and the OTA. With reasonable polar alignment, the issue isn't field rotation, but atmospheric refraction. If the guide cam is pointing at a vastly different area of the sky refraction will cause differences in the motion of your subject versus guide star (eg. see discussion of King Rate versus Sidereal Rate). You should be able to avoid this very easily by even visually assuring that the guide cam and OTA are atleast somewhat aligned.?
?
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Re: Side by side saddle or piggy back scope alignment
In reading the article linked (authored by Frank Barrett), I am not so sure that those calculations apply to the type of guided imaging we do. In fact, you could use his calculations to prove that with reasonable care, field rotation is unlikely to be a problem at all with today's methods of polar alignment.?
Hook's equations in Frank's article are for unguided imaging and Frank appears to have extended it to guided imaging. Which is fine, but then the relevant exposure time is not the exposure time of your sub, but the exposure time of your guide camera. This is normally on the order of a few seconds, and much shorter than the exposure time of your sub.? As Frank notes, in autoguiding, the motion of the centroid of the guide star is tracked with reference to a reference pixel. Motion beyond a certain tolerance results in a correction. And that correction is applied in both RA and DEC. And as long as that correction is applied before appreciable field rotation occurs, you should be fine. As noted by Frank himself in his example:
"In other words, if our alignment error is less than about 11.25 arc minutes we should see no field rotation greater than 9 microns during a 15-minute exposure at 35 degrees declination and the given setup."
11.25 arc minutes is an absolutely horrendous standard for polar alignment! We can easily get under 2 arc minutes, and with some care, under 1 arc minute in a matter of minutes using things like SharpCap. And guide exposures are much, much smaller than 15 minutes usually 2-4 seconds.
To the point then about the mismatch between the guide scope and the OTA. With reasonable polar alignment, the issue isn't field rotation, but atmospheric refraction. If the guide cam is pointing at a vastly different area of the sky refraction will cause differences in the motion of your subject versus guide star (eg. see discussion of King Rate versus Sidereal Rate). You should be able to avoid this very easily by even visually assuring that the guide cam and OTA are atleast somewhat aligned.?
?
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Morning Joe,
I am looking for help understanding?CCDOPS software in relationship to my Sti Camera and G11/G2. ?Specifically, what the issues with X&Y axis telling me showing me there is either drift in each axis and how to correct the errors I am seeing as the drift only gets worse and no apparent correction going on. ?
I am just getting back into astro?photography after a couple of years away from it due to health issues, so I have?forgotten how to use this?software the STi or my ST5c. ?By the way, I am using a?modified Cannon Xsi?camera, so am able to take images via Images Plus and @ 5 minutes that do not show any visual movement, but in the 10 minutes there is?slight visual drift. ?In essence how ?do you turn on corrections of the mount if possible?
Any reply & suggestions most appreciated,
Doug Askew?
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Re: Side by side saddle or piggy back scope alignment
Chuck -- If you are wondering about the effect of misalignment of the guidescope on astrophotography, it can cause star trailing.? This is due to ¡°field rotation¡± and it could occur if the polar alignment isn¡¯t perfect, and the guidescope and OTA don¡¯t point to the same spot in the sky. There is a good calculator available for this () and an article that explains it in some detail.? For exposures of a few minutes or less and decent polar alignment (as with Polemaster, Sharpcap Pro, or drift alignment), and reasonably close alignment of the OTA with the guidescope, it¡¯s not likely to be a problem.? I aligned my OTA and guidescope on a tower about 3 miles away and I haven¡¯t seen a problem in my images, but YMMV.? ? Joe ?
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From: [email protected] < [email protected]> On Behalf Of E. Brett Waller Sent: Friday, August 21, 2020 2:30 AM To: [email protected]Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment ? Chuck, Two scopes with the centerline of their optical axes exactly parallel will point to the same location on the sky. So if you shim them to be parallel. their fields of view will be concentric. ? Brett Thanks for your reply. Yes, by concentric I meant having the same view in the sky. Since I put a losmandy dovetail on the top and bottom of my c11 (it has hyperstar capability) and put an 80 mm refractor f5 on top of that it was apparetly their view in the sky was close but not totally concentric and the refractor doesn't have adjustable rings. So i was wondering do I shim the dovetail using a laser to get the two perfectly parallel or not. I understood in general their view of the sky would be nearly the same but would that matter and in what circumstances.? But I found they were slightly off of each other.? Since I don't do AP yet but have been playing with a camera and I am a complete babe in the woods with AP I was looking for input.?
Chuck
-----Original Message-----
From: E. Brett Waller <cedargreenobservatory@...>
To: Losmandy_users <[email protected]>
Sent: Tue, Aug 11, 2020 01:25 PM
Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment
Chuck, If I am reading your post correctly, you are wondering whether the scopes?should be parallel?or should point to the same place in the sky (I assume that is what you mean by having the views concentric). If they are parallel, they will point to the same place in the sky, despite the fact they are separated by several inches. This would NOT be true for a terrestrial object where the distance between two parallel scopes would offset the field of view by the same spacing as between the two optical axes.? For an astronomical object such as the moon, your best case resolution is going to be on the order of a kilometer or so, and you would never notice such a small offset. In similar fashion, you would be hard pressed to see the difference in the fields of view on a terrestrial object at a distance?of several miles. In those cases, the lines of sight for the two telescopes?are effectively parallel. Thus, two telescopes which are parallel will point to the same location on?the sky. I hope I haven't misread your original question, if so I apologize. At any rate, I hope you find my reply useful. ? On Tue, Aug 11, 2020 at 12:02 AM crocco1250 via <crocco1250=[email protected]> wrote: This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.
Thanks
Chuck
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Re: Side by side saddle or piggy back scope alignment
Chuck,
Two scopes with the centerline of their optical axes exactly parallel will point to the same location on the sky. So if you shim them to be parallel. their fields of view will be concentric.
Brett?
Brett
Thanks for your reply. Yes, by concentric I meant having the same view in the sky. Since I put a losmandy dovetail on the top and bottom of my c11 (it has hyperstar capability) and put an 80 mm refractor f5 on top of that it was apparetly their view in the sky was close but not totally concentric and the refractor doesn't have adjustable rings. So i was wondering do I shim the dovetail using a laser to get the two perfectly parallel or not. I understood in general their view of the sky would be nearly the same but would that matter and in what circumstances.? But I found they were slightly off of each other.? Since I don't do AP yet but have been playing with a camera and I am a complete babe in the woods with AP I was looking for input.? Chuck -----Original Message----- From: E. Brett Waller < cedargreenobservatory@...> To: Losmandy_users < [email protected]> Sent: Tue, Aug 11, 2020 01:25 PM Subject: Re: [Losmandy_users_io] Side by side saddle or piggy back scope alignment Chuck,
If I am reading your post correctly, you are wondering whether the scopes?should be parallel?or should point to the same place in the sky (I assume that is what you mean by having the views concentric). If they are parallel, they will point to the same place in the sky, despite the fact they are separated by several inches. This would NOT be true for a terrestrial object where the distance between two parallel scopes would offset the field of view by the same spacing as between the two optical axes.? For an astronomical object such as the moon, your best case resolution is going to be on the order of a kilometer or so, and you would never notice such a small offset. In similar fashion, you would be hard pressed to see the difference in the fields of view on a terrestrial object at a distance?of several miles. In those cases, the lines of sight for the two telescopes?are effectively parallel. Thus, two telescopes which are parallel will point to the same location on?the sky.
I hope I haven't misread your original question, if so I apologize. At any rate, I hope you find my reply useful.
Best regards,
Brett On Tue, Aug 11, 2020 at 12:02 AM crocco1250 via <crocco1250= [email protected]> wrote: This may not be the right place for this question but a recent post talked about side by side saddles and it raised a question. If you had a side by side or piggy back, say a c11 and 80 mm refractor, do you want them perfectly parallel or have the views more or less concentric? What are the pros and cons? If someone has a site I would appreciate more info.
Thanks
Chuck
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Re: Counterweight down setup??
Never thought about "Upside Down".
But I did get the RA Extension to make my "chunks" more friendly to carry around. I have my GM811G configured into 1. Tripod; 2. RA/12" extension; 3. Dec/counterweight; and 4. My telescope/imaging rig.
The RAEXT allows me to separate the RA/Dec into lighter movable assemblies.
And interesting perk to the RA extension is to image through the meridian, do to the clearance advantage.
I've recently been doing that a few times. I use an Atik Infinity camera (OSC). And I can get on my object in the Eastern side of the Southern Meridian, and image deep into the Western side Non-Stop.
As in No Meridian Flip. And I've gotten files stacked 1300 images x 13,000 seconds long (10 second subs). And they almost take on a 3D depth.
Until now, with the RA extension, Meridian flips were always a PITA for me.
Now I can totally ignore them.
Like Michael, I've readjusted my limits to allow me to go further without crashing. (Different objects may require different settings. But they can be done on the fly while imaging, even)
--
SonnyE
(I suggest viewed in full screen)
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Re: Counterweight down setup??
That is possible...you just set up the mount normally at CWD, but you must enable, in a Gemini, the RA safety limit to surpass 90 degrees.? I do set my mount to pass the 90 degree RA limit for certain types of imaging, like Solar where I want to track the sun through the meridian while imaging the solar surface.??
As long as you have physically confirmed that your scope and dovetail are not going to crash into the RA motor or tripod legs etc, at any part of the tracking path.?
?I can get away with that for solar imaging as the scope is small, and?pointing near the celestial equator, and I use a GM8 DEC extension and a tripod vertical extension for example.? ??
To do that with a Gemini, you have to set the "RA safety limits" setting to be past 90 (degrees). Attached is a PDF about where to find those settings using the Gemini.net applet.? The settings are hidden in the "Advanced" panel of the Applet.
-----
Using an RA extender on a G11 helps keep the dovetail away from the RA motor end.??
On my GM8, I created a DEC extender to keep the dovetail from hitting the RA motor end, and that also allowed me to do this more easily.? (Contact me if you want one if these extenders, as I have a few spares. Here is a picture of that.)
All the best, Michael
Is it possible to perform a mount setup with the counterweight up so you can track an object from east to west and avoid a meridian flip?
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Counterweight down setup??
Is it possible to perform a mount setup with the counterweight up so you can track an object from east to west and avoid a meridian flip?
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Re: PEMPro Results Analysis Question
On Wed, Aug 19, 2020 at 02:03 PM, Arun Hegde wrote:
A bit curious how much value there is in a PEC exercise unless you have a really large and high frequency PE; you could detect if you have this by running GA in PHD2. I would think it depends on your skies? Under SE Wisconsin skies, without PEC, I consistently get an average of 0.65" RMS with numbers sometimes as low as 0.45" RMS.?
If your PE is fairly slow changing and smooth, then the value of PEC over guiding is reduced as the guider can correct before the error becomes large. That said, with good PEC you can reduce the frequency of corrections resulting in less chasing of seeing and less need to find a high quality guide star. At shorter focal lengths you may even get away with no guiding. And then, any error that an autoguider corrects is most likely already has happened, so it already has lowered the resolution a little. In your case, it doesn't sound like it'll be a huge improvement, but even an improvement of 0.2" on FWHM of 2" is not a bad thing, and may even become noticeable, especially at smaller pixel scales/longer focal length. Regards, ? ? -Paul
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Re: PEMPro Results Analysis Question
Hi Brian - When the monsoons let up I will be rerunning PEMPro.
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Re: PEMPro Results Analysis Question
A bit curious how much value there is in a PEC exercise unless you have a really large and high frequency PE; you could detect if you have this by running GA in PHD2. I would think it depends on your skies? Under SE Wisconsin skies, without PEC, I consistently get an average of 0.65" RMS with numbers sometimes as low as 0.45" RMS. This is purely by good PA, balancing, and cable management. Under my skies, that means I am probably seeing limited rather than guiding limited. Generally, if you? have 2" skies, improving guiding much below about 0.8" wouldn't result in meaningful improvement in IQ. If you're moving to a larger scope and your guiding becomes worse, I would think that's not a problem solvable using PEC but rather a limitation of the mount itself or the setup of the equipment (for example the inability of the mount to quickly enough compensate for the disturbance amplified by a large moment arm with a large scope). Just trying to see if my understanding is correct. In any case, you would want to be absolutely sure you're taken care of all the other fundamentals (weight limits of the mount, balance, clutch tightness, accurate PA? etc.) before looking to use PEC.
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Re: PEMPro Results Analysis Question
not yet, but it's coming
I'd be happy to look at your results Jim, which I think i offered before ;)
If I recall correctly, your initial data gathering seemed suspect, with no clear periodic error at the fundamental or multiples of that. If you don't have good data to start with, you won't be able to do a good PEC, no matter how you fit it.
Brian
On Wed, Aug 19, 2020 at 10:34 AM Jim Waters < jimwaters@...> wrote: Is there a good tutorial on how to interpret the PEC curves and FFT waveform analysis results?
-- Brian?
Brian Valente portfolio
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PEMPro Results Analysis Question
Is there a good tutorial on how to interpret the PEC curves and FFT waveform analysis results?
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Re: G11 Mount Not Tracking Correctly
Hey everyone, first post here.
I have used the G11 mount before and had success with it, but this past weekend, it seems like no matter what I tried, I could not get the alignment correct. ?I purchased the GPS attachment thinking that would make things more accurate and simple, but that didn't seem to help. ?After setting up my mount, I aligned it due north with a compass and waited for sunset to align with Polaris using the polar scope. ?The app I have on my phone to match the reticle never seems to be in the same position when looking through the reticle of the polar scope, but I've never had difficulty aligning before. ?So after aligning Polaris between the correct lines in the reticle, I fired up the mount and used quick start. ?I verified the GPS data with another source and verified the time zone UTC -5, or Central Daylight Time, and ensured the time was also correct, and it was. ?However when using the two bright stars to perfect the alignment, (typically I use Vega and Deneb), the GoTo was way off. ?It would slew but point in totally different directions for either star. ?After using the buttons to slew to the correct position, it still would not go back to the correct spot and then trying to slew back to Polaris did not even work. ?I put everything back in the correct position with CWD and all that, all my cameras pointing at Polaris and tried again using Vega and Arcturus, and then Arcturus and Dubhe and never got good results. ?All my images ended up with trailing and I was using short focal lengths with relatively short shutter times of 3-4 minutes. ?I have previously done 8 minute exposures and had no trailing whatsoever at 24mm focal length on a 35mm full frame. ?For this past weekend I had three cameras mounted and balanced, with an 85mm lens, a 24mm lens, and a medium format camera with 23mm lens (18mm full frame equivalent).
I wanted to also do a series of Andromeda with my 600mm lens, but that would have been out of the question since I was getting trails at short focal lengths.
Any recommendations for how to fix this? ?Honestly, even if the GoTo doesn't take me to the exact position, I'm fine with using the buttons to slew to where I want to go, but I at least need it to track properly.?
Hi Doug,
Sorry to hear of your problems with the G11. First a GPS will only help the Gemini get its initial positional GOTO closer. Once polar aligned and even a single star model is built the Gemini knows where it is in that part of the sky.
A few questions, which Gemini are you using? Did you install a battery you tested @ 3.0V or higher? Do you know that Gemini GPS coordinates are entered using DMS format? If your GPS was a recent purchase do you know if it has the firmware fix for GPS rollover? If you are using well know navigational stars for Gemini alignment and your GOTOs are off and you cannot slew and return to a once centered location you have a few potential issues.
The troubleshooting checklist I use is: test battery, wipe battery clip contacts and install battery, do a full factory reset, verify GPS data you used is correct and entered using DSM format, enter UTC and only use UTC - no need for local offset, verify time format is entered using correct Gemini format is different, verify polar alignment. If you do this checklist 99.9% of all issues are resolved.?
--?
Chip Louie - Chief Daydreamer Imagination Hardware
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Re: G11 Mount Not Tracking Correctly
In addition to verifying that you are not in lunar or solar, make sure that your dec wire is in fact to dec and RA to RA.? Sounds stoopid but I have confused the wires more than once.
Sometimes the wires are OK but one is not completely plugged in.
I would only use GPS if you can't see Polaris.
Greg, What does the GPS have to do with seeing Polaris?? -- Chip Louie - Chief Daydreamer Imagination Hardware
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Re: Valid Guide Rates for PEC on Gemini Level 4
I avoid messing with these deep dark secret settings. ;^)
I just run the PEC, and have it start for me automatically, with the Gemini Telescope virtual hand controller I open on my display.
But then, I only use my single telescope/imaging rig, and have things set for repeat accuracy with each assembly.
I believe PEC is a good thing to implement. But I don't find it to be overly critical for my Fly-on-an-Elephants-Back equipment load.
I'm more concerned with the hobble-wobble in my telescope and focusser. ;^)
Simple Life = Simple Strife's.
--
SonnyE
(I suggest viewed in full screen)
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Re: G11 Mount Not Tracking Correctly
I always like posts like this. I can come along after the usual prospectors and usually pick up a few nuggets.
I've never extended the legs on my tripod. Never had the need. But I think Michael's Point 4 has merit for me. So I will check that my leg clamps are tight.
That nugget was not missed by me, Michael. ;^)
I've almost always checked my tripods level and plumb as I begin my set up for the evening. (Which hasn't been lately due to heat, smoke, and clouds.)
My tripod remains set up on it's leveling pads I made for it, covered but ready to assemble my "chunks" and begin a session.
If I was to be accused of anything, it would probably be of tightening a smidgen too tight. I often notice this when breaking down and I look around for the gorilla who tightened my clamp on my D-bar.
I always handle my mount like it was made of glass. Not because I think I will hurt it, but out of my respect for how well made and beautiful I think it is.
And not banging it around keeps things as set from the previous sessions.
But for those who do extend their tripods legs, making sure they are tight is a very prudent point.
To me, .001" (0.0254 mm) on Mother Earth, can amount to a clear miss in Deep Space Objects. 8^0
--
SonnyE
(I suggest viewed in full screen)
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Re: Valid Guide Rates for PEC on Gemini Level 4
Eric, The issue with PEMPRO is not the difference between East and West moves, but I don't want to get off topic. At the time you were using PEMPro there was an issue introduced when adding the two-cyle PEC programming for the Gemini, but that has been long since fixed. Hopefully someone else can measure their residual PE/drift at different guide rates using the same PEC curve
so we have more than one sample. PEMPro has been able to program the Gemini controllers since 2005, so I've seen hundreds of curves where drift was introduced by using the wrong guide rate. Hopefully a few people will report if the amount of drift changes depending on guide rate. It could well be that the latest firmware has changed the behavior. That said, the amount of drift change when changing guide-rate will depend on the PEC curve. The lower the periodic error of the mount, the less drift will be introduced when changing guide rate. So, one arc-sec/minute could be right for your mount and a small change in guide rate. It could be a lot more under different circumstances. -Ray Gralak Author of APCC (Astro-Physics Command Center): Author of PEMPro V3: Author of Astro-Physics V2 ASCOM Driver: -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Cyclone
Sent: Tuesday, August 18, 2020 8:25 AM
To: [email protected]
Subject: Re: [Losmandy_users_io] Valid Guide Rates for PEC on Gemini Level 4
Ray,
You seemed pretty stubborn about me being wrong about the move distances being different between
East/West
The issue with PEMPRO is not the difference between East and West moves, but I don't want to get off topic.
According to your PDF when you had mismatched guide and PEC rates you observed drift.
The report does not say that. The mount reports its state as slewing periodically and momentarily, but there is
no change in drift. I assume that because the PEC divisors are different than the guide divisors, the mount
reports its state as slewing at each PEC correction, but there does not seem to be any other side effect.
Have you tried measuring drift with mismatched guide/pec rates?
I did measure the drift with the same PEC curve and different guide rates and the drift in RA does not change
(approximately 1 arcsec/min).
Hopefully someone else can measure their residual PE/drift at different guide rates using the same PEC curve
so we have more than one sample.
Eric
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Re: Valid Guide Rates for PEC on Gemini Level 4
On Tue, Aug 18, 2020 at 11:20 AM, Michael Herman wrote:
I still think it's a mis translation or a mis statement.
I think the statement should be:
?
...the PEC data is recorded at the present Tracking speed....
Michael, there are two steps and you keep talking only about the first one. Step one is recording PE data, the other is programming it into Gemini. Recording data into Gemini requires knowing the guide rate, otherwise Gemini doesn't know the size of the corrections you are sending.
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Re: Valid Guide Rates for PEC on Gemini Level 4
Ray,
You seemed pretty stubborn about me being wrong about the move distances being different between East/West
The issue with PEMPRO is not the difference between East and West moves, but I don't want to get off topic.
According to your PDF when you had mismatched guide and PEC rates you observed drift.
The report does not say that. The mount reports its state as slewing periodically and momentarily, but there is no change in drift. I assume that because the PEC divisors are different than the guide divisors, the mount reports its state as slewing at each PEC correction, but there does not seem to be any other side effect.
Have you tried measuring drift with mismatched guide/pec rates?
I did measure the drift with the same PEC curve and different guide rates and the drift in RA does not change (approximately 1 arcsec/min). Hopefully someone else can measure their residual PE/drift at different guide rates using the same PEC curve so we have more than one sample. Eric
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Doug Askew
Michael Herman
Paul Kanevsky
Chip Louie