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basic question about meridian flips

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#1 nimitz69

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Posted 26 June 2019 - 05:05 PM

So, the CEO just gave the go ahead for building an observatory in my backyard and I plan to put in a pier for my refractor.   I don’t really know much about flips since I havent had the need to do one yet but won’t a pier installation negate the need to do one since all they are for is keeping your camera from banging into your tripod when imaging near the zenith.  I guess if I Had a 20” diameter pier or something it could be an issue but I don’t ever plan to be using something like a 14” SCT so I wouldn’t expect my pier to be very Wide  So, Am I missing something?


Edited by nimitz69, 26 June 2019 - 05:08 PM.


#2 John Fitzgerald

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Posted 26 June 2019 - 05:31 PM

If you don't do a flip, and don't hit the pier, your OTA will eventually be upside down.  With a GEM, the counterweight would be sticking nearly straight up.  Flips are necessary with eq mounts.



#3 Ishtim

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Posted 26 June 2019 - 05:34 PM

Have a look at this video... he walks thru the whole flip.  The scope tracks, then the "flip" occurs at 1:42. 
https://www.youtube....h?v=-9q3Ft8TCoU


Edited by Ishtim, 26 June 2019 - 05:36 PM.


#4 spokeshave

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Posted 26 June 2019 - 05:40 PM

Even with a pier, there is typically a limited declination range for which a rig would not strike the pier. Too far north and the camera can strike the pier, and too far south and the OTA can strike the pier. Additionally, many mounts have hard stops that prohibit the mount from moving more than an hour or so past the meridian. Some mounts, however, are designed to image through the meridian (AP is a good example) but even those require that you set up the software to tell it what range of declination is safe for the gear. 

 

Tim


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#5 photoracer18

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Posted 26 June 2019 - 05:41 PM

Also keeps your scope from banging into the mount near the zenith. Depends totally what part of the sky you are trying to image in and the length of your equipment to the rear of the saddle. And an 8"-12" pier is usually enough for everything no matter how heavy or large.



#6 kyle528

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Posted 26 June 2019 - 05:44 PM

Some mounts, like my CEM25, require a flip regardless of declination, the mount simply cannot rotate 360 degrees without crashing.



#7 Alex McConahay

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Posted 26 June 2019 - 05:44 PM

>>>>>>>  but won’t a pier installation negate the need to do one since all they are for is keeping your camera from banging into your tripod when imaging near the zenith.

 

You are correct. The reason to flip is to avoid banging into something. You are incorrect in that the scope and rest of the rig can bang into something besides the pier. 

 

Some mounts cannot image past the meridian. Built in hard stops or software stops mean the mount will refuse to go past a certain "safety stop." That point is sometimes fixed at the meridian.

 

Other mounts can happily image past the meridian, and be perfectly happy for hours. 

 

SO, yes, with a pier, there is less to run into. That does not mean that flips are never necessary.

Alex



#8 kathyastro

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Posted 26 June 2019 - 05:49 PM

On a GEM, the flip is mandatory.  Something expensive is going to hit the pier if you try to image with the rig upside down.  And, since you will be in an observatory, the sill of the dome slot or wall top will be far above the scope when the rig is upside down, limiting your view of low elevation objects.

 

However, if you are using a wedged fork mount, there are no meridian flips to worry about.

 

Meridian flips are nothing scary.  You will get used to doing them.  Most image acquisition software will have provision for automating the flip, so that it occurs between frames and re-acquires the target after the flip.  You just have to set tracking limits that make sense for your equipment, and then it is all handled for you.


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#9 elmiko

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Posted 26 June 2019 - 05:57 PM

Depending on your mount also. Weight limitations, balance issues could make you guiding worse if you don't do a meridian flip. I can go maybe 30 to 45 minutes past the meridian, after that my guiding can get a little squirrelly. 

So I would do the flip you can collect more quality data. Software like Pixinsite can stack subs on both east and west side of the meridian with a click of the mouse. 

Mike



#10 kisstek

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Posted 26 June 2019 - 06:25 PM

On a GEM, the flip is mandatory. 

Playing Devil's Advocate: Even if I'm imaging at the North Pole? Then the scope and the counter weights are "orbiting" the top of the pier. There's no way for either of them to hit the pier as far as I can tell.

 

In the real world, things like built-in mechanical stops, twisted cables, ... will eventually bring things to a stop and require you to reverse things and then restart.


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#11 ButterFly

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Posted 26 June 2019 - 06:26 PM

If you don't do the flip eventually, the weights will be above the OTA.  One slip and you have weights barrelling toward the OTA.  Even if stopped before then, something has to eat that momentum.  Avoid the risk of the unlikley and stay within a few degrees past meridian.


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#12 Garyth64

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Posted 26 June 2019 - 06:48 PM

It all depends on how past the meridian you keep observing or keep taking photos.  There's nothing wrong with having the CW up high, as long as it is secure.

 

As you can see, I deliberately set up both scopes where they were past the meridian.  It offered a comfortable eye position for viewing.  I only observed for an hour or so.  (looking at Jupiter a couple of years ago.

 

two scopes.jpg

 

Yes, if I had observed for about 2 hours, the scope would hit the mount.


Edited by Garyth64, 26 June 2019 - 06:50 PM.

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#13 DaveB

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Posted 26 June 2019 - 08:35 PM

It depends on your mount. A fork mount doesn't need a meridian flip, for example, but I will assume that you will image with a GEM.

 

With my old MI-250, I couldn't go more than a couple minutes past the meridian without the dec motor hitting the RA housing. It was the only design shortcoming of that mount IMHO. With my current mount (AP1100), if I'm imaging close to 0 deg declination, then I can probably go a full 360 degrees without hitting anything. But if I go more than 20 degrees north or south in dec, then my tube will hit my pier anywhere from 30-60 minutes after passing the meridian.

 

There are unique designs like the Avalon M-Uno mount that avoid meridian flips, but with telescope control software like Sequence Generator Pro, NINA, or Voyager, a meridian flip is nothing to worry about at all.



#14 TrustyChords

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Posted 26 June 2019 - 09:28 PM

So after the flip, do you continue to image with the camera rotated 180 degrees and rotate the images in software? You probably don’t want rotate the camera right?

#15 Alex McConahay

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Posted 26 June 2019 - 09:57 PM

>>>>> So after the flip, do you continue to image with the camera rotated 180 degrees and rotate the images in software? You probably don’t want rotate the camera right?

 

Yeah, it is standard operating procedure for any stacking software to "register" the images. It is just something that happens in the processing. This makes all the stars point where they belong. No big deal at all. 

 

Alex


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#16 kisstek

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Posted 26 June 2019 - 10:10 PM

So after the flip, do you continue to image with the camera rotated 180 degrees and rotate the images in software? You probably don’t want rotate the camera right?

I use the FastRotate process in PI to fix the orientation. I don't know if PI's StarAlignment process can handle things 180 degrees out of sync. If it can, then I'm just wasting time doing manually what it'll do automatically.


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#17 Alex McConahay

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Posted 26 June 2019 - 10:17 PM

>>>>> then I'm just wasting time doing manually what it'll do automatically.

 

Yes, you are wasting your time. 

Alex


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#18 kisstek

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Posted 26 June 2019 - 10:24 PM

>>>>> then I'm just wasting time doing manually what it'll do automatically.

 

Yes, you are wasting your time. 

Alex

Great! blush.gif At least I've gotten a lot of practice separating out images using Blink! smile.gif I guess that explains why I haven't seen anything about correcting the rotation after the flip. I always assumed it was because all of the real astrophotographers had rotators.



#19 DaveB

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Posted 26 June 2019 - 10:33 PM

I use the FastRotate process in PI to fix the orientation. I don't know if PI's StarAlignment process can handle things 180 degrees out of sync. If it can, then I'm just wasting time doing manually what it'll do automatically.

StarAlignment does handle 180 degree flips (any number of degrees, really), no need to do the FastRotate. (I didn't even know that PI had FastRotate process...)

 

Note that it will align to the image that you set as the reference image, so if you happen to have one or a few images that weren't centered properly, don't use them as the reference. With plate solving, it shouldn't be an issue, but I've had cases where I decided to reframe the target after my initial image, so in that case I just avoided using the first sub as the ference.


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#20 kisstek

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Posted 26 June 2019 - 10:58 PM

StarAlignment does handle 180 degree flips (any number of degrees, really), no need to do the FastRotate. (I didn't even know that PI had FastRotate process...)

 

Note that it will align to the image that you set as the reference image, so if you happen to have one or a few images that weren't centered properly, don't use them as the reference. With plate solving, it shouldn't be an issue, but I've had cases where I decided to reframe the target after my initial image, so in that case I just avoided using the first sub as the ference.

I have the plate solver in APT setup to align within 75 pixels. (My mount can't aim reliably less than 60-65 pixels.) But that's usually not a problem. Guiding allowing the target to walk down and out of the image is an occasional "issue". The lack of repeatability Squirrel! Squirrel! ... is probably my biggest headache with imaging.



#21 Alex McConahay

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Posted 26 June 2019 - 11:22 PM

>>>>>  (My mount can't aim reliably less than 60-65 pixels.)

 

Does that mean that if you tell it to go to a target, it will miss by 60-65 pixels (which is not all that bad in most cases). 

 

Or 

 

That if it is kinda near a target (say 100 pixels off), it will correct anywhere from 40 to 160 when told to go 100 pixels to center (which does not sound at all right).

 

I ask because my first slew may be a couple of hundred off, then my next is maybe 40, and my next fifteen, and finally my last is within eight or so--the limit I ask it to hit. 

Alex



#22 kisstek

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Posted 26 June 2019 - 11:30 PM

>>>>>  (My mount can't aim reliably less than 60-65 pixels.)

 

Does that mean that if you tell it to go to a target, it will miss by 60-65 pixels (which is not all that bad in most cases). 

 

Or 

 

That if it is kinda near a target (say 100 pixels off), it will correct anywhere from 40 to 160 when told to go 100 pixels to center (which does not sound at all right).

 

I ask because my first slew may be a couple of hundred off, then my next is maybe 40, and my next fifteen, and finally my last is within eight or so--the limit I ask it to hit. 

Alex

Starts a couple thousand pixels off. First slew brings it to within 100-150. Next slew gets it to 60-65. Next slew gets it to within 65-70 pixels. Next slew gets it to within 60-65. After five attempts to get within the default 50 pixels, it gives up. Since the mount backs off and reapproaches on every attempt, the horrendous backlash on the AVX prevents it from centering very closely.

 

Since my sensor is something like 4500 pixels wide, there's not really much difference between dead on and within 60 pixels.

 

Hopefully the CEM60 will be a lot better than the AVX. If it ever comes.



#23 WadeH237

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Posted 27 June 2019 - 06:15 AM

On a GEM, the flip is mandatory.

This is not universally true.

 

The only reason to do a flip is to prevent a collision.  Some mounts need to flip because they will collide with themselves.  My AVX, for example would crash the axis housings into each other.  Other mounts, like my Astro-Physics mounts can rotate both axes 360 degrees without conflict.

 

So with the Astro-Physics mounts, there are two considerations:  Will the scope and imaging package collide with anything?  And what about cable wrap?

 

Cable wrap is not really a consideration because to track from the eastern horizon to the western horizon requires only 180 degrees.  If you start with the counterweights straight down and let it track for 12 hours, it will end up with the counterweights pointed straight up.  There is no reason to track further than that, unless you want to image the ground.

 

So that just leaves the possibility of the telescope and imaging package being clear for the entire 12 hours.  This is up to the user to figure out.  And if things are clear, an Astro-Physics mount will allow you to track for the full 12 hours without a flip.  And you can start the session two ways.  You can either start with the scope on top, as I mentioned above, or you can start with the scope upside down and let it track into being right side up.

 

All that said, meridian flips are no big deal.  Just about every modern imaging package will take care of it.  And to remove the mystery, here's what "take care of it" actually means:

 

A goto GEM will track at least up to the meridian, and possibly farther.  When it gets as far as you want to go, you just need to do a goto to the current target.  The mount will see that the object is past the meridian and approach the target from the other side (this is the flip).  Your software just needs to know when to issue that goto.

 

In most cases, the goto can happen any time after the target object has transited.  In some cases, the mount is able to use a reference other than the actual meridian to know when to approach from the other side.  That scenario is a bit more complicated because the imaging software needs and mount need to agree on that reference.

 

It's not any more complicated than that.


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#24 Alex McConahay

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Posted 27 June 2019 - 08:27 AM

Yeah, as WadeH237 says, we cannot speculate for every mount/imaging rig combination out there that it is necessary or not. It depends on what you have whether you will need a flip, and when you will need it. 

 

But those who say it is not big deal are also correct. 

 

I don't know that it is true, or just an internet rumor, but apparently UPS routes in big cities are built so that ALL turns are RIGHT turns. There are no left turns. Ok.....they know what works for them. But that does not mean all of us have to abandon right turns. 

 

Alex


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#25 Alex McConahay

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Posted 27 June 2019 - 08:30 AM

Starts a couple thousand pixels off. First slew brings it to within 100-150. Next slew gets it to 60-65. Next slew gets it to within 65-70 pixels. Next slew gets it to within 60-65. After five attempts to get within the default 50 pixels, it gives up. Since the mount backs off and reapproaches on every attempt, the horrendous backlash on the AVX prevents it from centering very closely.

 

Since my sensor is something like 4500 pixels wide, there's not really much difference between dead on and within 60 pixels.

 

Hopefully the CEM60 will be a lot better than the AVX. If it ever comes.

 

I have had that happen to me. I cannot remember what was wrong. 

 

On subsequent moves (when you are in the 60-65 range) is the mount in fact moving on the next slew? That is, is it the SAME 60-65 off (and the two positions are the same wrong place), or has it moved the 60-65, but to some other wrong place?

 

Alex




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