Alignment problems


Cantini Mauro
 

Good evening and hello to the group. A few days ago I am the new owner of an Iexos 100 pcm eight. I managed to control it via serial cable and computer with internet. I read that to be able to drive it via PHD2 it was better to use the ASCOM driver via Ascom HUB DEVICE. So I can connect it indifferently via native driver or Ascom. But in both cases leaning on a client like APT Astrophotography the mount does not point where it should point ..... even after doing a couple of plate-solves and solving them, if I point a third star it is wrong direction. I'm sure it depends on some initial setting, but which one? My site coordinates on both APT and native driver are accurate. The Ascom Device Hub ones cannot be changed. Anyone have an idea or encountered similar problems? Thanks in advance and clear skies.
Cantini Mauro - Italy


@jerrybx
 

Hi Cantini, how are you polar aligning?
Jerry
--
TELESCOPE:  Explore Scientific ED80APO, Celestron Classic C8, Mead 60mm
GUIDING:        SVBony Mini Guide Scope ZWO 120MM-MINI Guide Camera
MOUNT:           IOPtron Skyguider Pro, EXOS2-GT PMC-EIGHT
CAMERA:        Canon EOS77D, Canon Rebel T6, Various kit and prime lenses
SOFTWARE:    Microsoft Windows 10 Home 10.0.19043 Build 19043, Explore Stars (Windows10,Android Tablet) Siril, Sequator, Affinity Photo, ON1 Photo Raw, APT, Stellarium, Ascom, Poth, USB>Cat5e Hub, PHD2, Sharpcap, N.I.N.A.,  Astrroberry


 

Ive had it happen to me.  It was because my time zone settings were incorrect. I was using stellarium at the time.  I changed it to eastern time zone and then clicked now in stellarium which fixed the issue. Apparently it happened after installing the latest stellarium update


On May 15, 2022, at 1:08 PM, jerry@... wrote:

Hi Cantini, how are you polar aligning?
Jerry
--
TELESCOPE:  Explore Scientific ED80APO, Celestron Classic C8, Mead 60mm
GUIDING:        SVBony Mini Guide Scope ZWO 120MM-MINI Guide Camera
MOUNT:           IOPtron Skyguider Pro, EXOS2-GT PMC-EIGHT
CAMERA:        Canon EOS77D, Canon Rebel T6, Various kit and prime lenses
SOFTWARE:    Microsoft Windows 10 Home 10.0.19043 Build 19043, Explore Stars (Windows10,Android Tablet) Siril, Sequator, Affinity Photo, ON1 Photo Raw, APT, Stellarium, Ascom, Poth, USB>Cat5e Hub, PHD2, Sharpcap, N.I.N.A.,  Astrroberry

--
Carter Sills

Retired due to disability

Ex software programmer/Web Developer/Sql developer and     IT administrator

Astrophotagraphy Tools:

IEXOS 100 PMC-Eight equatorial mount
Orion 80mm Short tube-A Doublet Achromatic scope
ZWO 120MC- S color camera used for guiding
ZWO 178MM/MC color camera used for main camera
ZWO ASI482MC camera used for main camera
TPO 0.5X focal reducer
2 inch extension tube
Orion electronic focuser
Svbony 30mm guide scope
Svbony Dew Heater Strip

Batinov mask
Svbony light pollution filter set
Celestron portable power tank

Software: 

Sharpcap and ASI Studio for image capture and stacking
PHD2 for Guiding
Nina and Stellarium for target aquisition and platesolver
Nina Plate Solver programs:
Platesolver 2, ASTAP, All Sky Plate Solver, Astronomy. Net. 


Dennis Vijlbrief
 

Hi Cantini,

A couple of things come to mind:
1 - What Jerry is asking about: The bigger the error in you polar alignment the bigger the error will be when slewing somewhere else
2 - Your scope is not necessarily parallel to the mounts RA axis. This is called Cone error and normally you can adjust this to minimize the error. But probably it won't be 100% perfect
3 - When you platesolve, in APT it is a good idea to hit the 'sync' button. That should improve things when you move to a different target that is in the vicinity
4 - Expectations: The platesolving allows you to point with pin point accuracy to your target. But when you want to change targets there will always be a an pointing error again and you will have to plate solve again. The farther the 2nd target is from the first the bigger the error.

Cheers
Dennis———————


Mount: exos-2 pmc8
Telescope: skywatcher 150P
Camera: Nikon D5500
Software: APT, NINA, Gimp, PS Align Pro, Windows11
Guiding: ZWO mini 120, svbony sv165, phd2, no ST4
Filters: Astronomik Ha 12nm, Optolong L-Extreme


BobH
 

Welcome to the group! I've seen the other comments so far, those are all good ideas. Assuming the config is correct, I'd emphasize two things:
1) Have a good polar alignment. Small errors wont matter until you get to very long exposure photography. If you are using an automated tool to guide your adjustments, when it says "Good" or "Excellent", you are fine. But large errors will introduce ongoing offsets as you move from target to target.
2) After slewing to your first target, plate-solving and centering it, absolutely you want to "Synch". Different software does that different ways, but all the main packages support this function. Once synched, if your original polar-alignment is decent, after the first synch, subsequent moves to new targets should be very close. Of course, when you get to each new target, solve and synch to it, to keep errors from accumulating.

On my EXOS2, I tend to get polar-alignment in the single-digit arc-seconds (<10" combined error), as I often take long exposures. With that alignment, after a first solve-n-synch operation, subsequent moves to new targets are generally less than 1/10 of a degree off - usually within a few hundredths of a degree.

As an experiment (not to debug, but to help your intuition), set up your mount, polar align it, then return the mount to the "parked" state. So your telescope is now pointing in the general vicinity of the pole, and the counterweight bar is ideally pointing straight down. Reboot the PMC8. Release the RA clutch, manually rotate the RA axis about 5-10 degree away from this parked/polar-home position, then re-secure the clutch. This introduces a rather dramatic intentional error in initial position - the scope is not pointing anywhere near where the software assumes it to be. In these next steps, don't use any keypad functions like N/S/E/W L/R/U/D to move the scope. In your software, select a target object, and have your software slew to it - you will of course be far off, because of that initial large RA error you introduced. Use your plate-solving to center the target, and synch to it. This will erase that error. Now park your mount. You should find that the counterweight bar if now pointing straight down, the error is gone. If it is not, then there is a problem, most likely a procedural mistake in the slew/solve/synch process.
--
Cheers, Bob
(A recently retired CTO, returning to the hobby after a 30 year hiatus...)

Mounts: EXOS-II/PMC-8 (new); SkyWatcher Star Tracker; Meade LXD55; Celestron EQ Fork
Optics: RedCat-51; 1980's Celestron C-11; Meade AR5; Various DSLR lenses
Cameras: ASI071MC & ASI183MC (DSO); ASI178MC (Lunar/Planetary); ASI290MM (guiding); Sony A700 DSLR (desperation)
Misc: Pegasus Pocket Power Adv (power mgmt, USB hub, dew control); ZWO EAF; ZWO 1-Filter Drawer;
PCs: Dell Latitude Laptop, Win10Pro (ASCOM, near-mount control); Pi4b, Linux/Stellarmate (INDI, on-mount control);
         Home-built Workstation, Win10Pro (remote control, processing); Galaxy S6 Tablet
Capture Software: PHD2; SharpCap4 Pro /w ASTAP; NINA; Stellarium; ASCOM 6.5.1; Stellarmate/EKOS/INDI;
Processing SW: PixInsight; Deep Sky Stacker; GIMP; F4W2HDU; Autostakkert; PIPP; RegiStax6
Wish-list: 5-inch class APO; Losmandy/PMC8 G-11; RASA-11?
Favorites: Vast glowing clouds of gas and dust


Cantini Mauro
 

Hi guys and thanks for the valuable advice. Let's say that in part I had anticipated the vs. hints, by resetting the initial shift by pointing the Polaris (incorrectly pointed) and then loosening the two axes to actually put it where she should be positioned. Then doing two or three plate-solves and synchronizing with APT this problem was over the last night when I tried the iexos 100 in the field. But and I apologize if for this you have to open a new thread, I had problems with the next step: that is the guide with PHD2 ..... I state that I use the mount with serial cable, internet, ASCOM device HUB and APT. I connect PHD2 with Orion SSAG - On Camera - Hub Device. Calibration starts, but after a while PHD2 tells me that it manages to make a few movements, perhaps due to excessive backlash or incorrect parking at the pole, or even that there are connection and mounting problems with the ST4 cable. I can assure you that the balance is sufficient, as well as the pole alignment .... Reading here on the forum I read that I should correctly set the driving speed values ​​on the driver, as well as on PHD2. Could you be kind enough to show me the procedure better? Thanks in advance and clear skies as always !!


BobH
 

Before discussing PHD, a comment about alignment. Apologies if I am misunderstanding you. Generally, it is very unreliable to try to achieve good polar alignment by slewing to Polaris by name/coordinates, then adjusting the Alt/Az bolts to center Polaris in the field-of-view. Close calculation of RA/DEC coordinates when pointing near the pole is difficult and error prone, and you can get into loops that diverge rather than converge to the correct solution. One of the most accurate ways to polar align (SharpCap & ASIAir use this technique, I don't know about others) is to plate-solve a picture of the polar region, have you rotate the scope in RA by 90 degrees, then plate-solve again. By measuring precisely how the known stars moved between those two exposures (90 degrees need not be precise for this to still work), the software can mathematically derive the precise position of the true pole (NCP), and your offset from it - then instruct you how much you need to move in Alt & Az. It keeps taking and solving pictures, so you get updated instructions as you get closer to the correct position. This process always converges to a precise solution, though it does require that you can see the NCP region.

Re PHD2. When you are first starting up all your software, you never connect applications like PHD2, APT, Stellarium, SharpCap (any ASCOM applications) directly to the mount's driver - that driver can only support a single active connection at a time. You need to run POTH, and configure it to connect to the PMC8's ASCOM driver (one time). Then, every session, run POTH, let it connect (to the PMC8). Then PHD2, APT, whatever you are running, they each connect to POTH. POTH provides the necessary logic to allow multiple applications to access the mount at the same time.

Next, you probably shouldn't use "ST4" guiding. Done correctly, it will work, sort of. But it is an ancient legacy, long ago deprecated. Remove the unneeded cable between your guide camera's ST4 port and the PMC8 - your PC will better deliver guiding commands directly to the PMC over the serial/usb cable. So PHD2 should be configured in its Connect Equipment screen with the Mount set to "POTH Hub (ASCOM)", not to "On Camera" (which is the setting for ST4 guiding). The system will then use the more flexible "Pulse Guiding", issuing the Pulse commands directly to the PMC8.

I'd leave the Guide Pulse duration settings in the PMC ASCOM driver at their installation defaults. The designers chose those values most commonly appropriate for your mount. Once you are settled in, you may find minor adjustments that will improve guiding accuracy for you conditions, but that is a far more advanced undertaking. In general, PHD2 defaults are also a good place to start. Before experiencing how all the features interact, it is easy to alter PHD2 parameters to settings that are unstable or at odds with one another, so make only minor changes, and observe their impact.

You do not want the mount "Balanced", as a general rule. What you want is a correct level of intentional unbalance. For starters, adjust your DEC axis balance so it is distinctly "camera end" heavy. This will help reduce the effects of DEC backlash, which is commonly large in these mounts (all low to mid-priced mounts!). Set PHD2 to "Resist Switch" as the DEC guiding algorithm. In RA, set the imbalance so the east side is heavy. If your scope is on the east side of the pier, looking to the western sky, then the counterweight (which in now on the west side of the pier) should be higher than the true balance point, closer to the axis. If the scope is on the west side, weight on the east, the weight should be lower, below its true-balance point. This protocol keeps the gears maximally engaged, and used properly will eliminate almost all effects of backlash. Couple this with either Hysteresis or Predicative Pec algorithms in PHD2. If you don't want to move your weight around when you cross the meridian, then set it east-heavy when in your preferred side of the sky, and let it be west-heavy on the off-side. East-heavy is best. West-heavy is OK. Near-balance is trouble, as the axis can bounce around to the extent allowed by the backlash. Experiment a bit - ideal imbalance can make a huge difference in guiding accuracy. For my EXOS2, untuned, as-is from the factory, with large backlash in both axis, I regularly guide at 0.62" - 0.78" RMS for extended periods of time - and the iExos-100 is rumored to be better behaved than my mount.

Enjoy!
--
Cheers, Bob
(A recently retired CTO, returning to the hobby after a 30 year hiatus...)

Mounts: EXOS-II/PMC-8 (new); SkyWatcher Star Tracker; Meade LXD55; Celestron EQ Fork
Optics: RedCat-51; 1980's Celestron C-11; Meade AR5; Various DSLR lenses
Cameras: ASI071MC & ASI183MC (DSO); ASI178MC (Lunar/Planetary); ASI290MM (guiding); Sony A700 DSLR (desperation)
Misc: Pegasus Pocket Power Adv (power mgmt, USB hub, dew control); ZWO EAF; ZWO 1-Filter Drawer;
PCs: Dell Latitude Laptop, Win10Pro (ASCOM, near-mount control); Pi4b, Linux/Stellarmate (INDI, on-mount control);
         Home-built Workstation, Win10Pro (remote control, processing); Galaxy S6 Tablet
Capture Software: PHD2; SharpCap4 Pro /w ASTAP; NINA; Stellarium; ASCOM 6.5.1; Stellarmate/EKOS/INDI;
Processing SW: PixInsight; Deep Sky Stacker; GIMP; F4W2HDU; Autostakkert; PIPP; RegiStax6
Wish-list: 5-inch class APO; Losmandy/PMC8 G-11; RASA-11?
Favorites: Vast glowing clouds of gas and dust