Drift alignment - 1st time

[malc-c]

Whilst last night wasn't ideal seeing conditions, it was the 1st clear night we've had for some weeks, so I took the advantage and tried to drift align the mount, and would welcome comments as I'm not 100% sure I did it right

Time was between 11:45pm and 01:15am BST. Having opened the observatory and powered everything up I placed the scope in the normal HOME position with weights down scope pointing towards NCP. I rotated the DEC axis until the OTA was horizontal and was looking East, and then rotated the RA until the OTA was effectively on a North/South line. The brightest star in that area that I could use with the webcam was what I think was Altair (still trying to learn the sky), so the scope was lined up on that and set tracking via EQMOD.

I opened up Sharpcap, and focused the star in a 640 x 480 window and then placed the recticle so it was centred on the intersection. I turned off the tracking and watched the star dirif, rotating the recticle so it drifted along between the two lines. Re-engaged the tracking and recentred the star and cross-hairs. I then watched to see it drift up, so I adjusted the Azimuth bolts until there was hardly any drift at all over several minutes.

I then rotated the RA and DEC so that I was pointing at the brightest star I could see that was to the West, and even though it was on the limit of observation at this time due to the observatory wall, repeated the above using Arcturus but adjusting the altitude bolts until the star stopped drifting above or below the cross-hair.

Does this sound OK and was my choice of stars suitable for the time window. Ideally everything should of been spot on meridian / equator intersections, but I'm limited by the viewing conditions and equipment at my disposal. I'm hoping that with the LP clip filter in the Canon, I'll be able to go for 3-5min subs without noticeable drifting, and hopefully no fogging with LP, would the chosen stars give me that precision ?

Comments welcome

[Digz]

I honestly couldn't say if what you have done is correct. It sounds right to me, but I struggle to understand drift alignment completely.

I do know, however, that if you are using PHD that you can use that to help drift align. I think the theory is that you switch the software from tracking to dx/dy (or something like that) and then for the RA and Dec tweak your alignment to get the graph flat. Seems a lot easier to handle to me?

[malc-c]

I honestly couldn't say if what you have done is correct. It sounds right to me, but I struggle to understand drift alignment completely.

I do know, however, that if you are using PHD that you can use that to help drift align. I think the theory is that you switch the software from tracking to dx/dy (or something like that) and then for the RA and Dec tweak your alignment to get the graph flat. Seems a lot easier to handle to me?

- it took me several replays of the video ( Andy's Shot Glass - Drift Alignment for Amateur astrophotography,ccd, Neutonians and Refractors, amateur astronomy ) to get my head around the basics... I've not delved into PHD.... sounds complicated

[lukebl]

That sounds good to me.

Can I suggest you have a go using EQalign, which uses the webcam just as you say, but a bit more scientifically (for instance it will tell you by how much you need to adjust the altitude and azimuth) and has little graphs to show how good or bad your alignment is. You can download it here. I find it very useful, and you can use it as a GOTO too.

[malc-c]

Cheers Luke

[steppenwolf]

Your work-flow sounds smack on to me - it all seems counter-intuitive but it does work.

[malc-c]

Steve, what about my choice of stars ?

[steppenwolf]

Altair was a good choice for around 11.45 and Arcturus was good and low down in the west at 01:15 so, yes, these were good choices.

[malc-c]

Thanks for the re-assurance. Just need another suitable clear night when I don't have work the next day to get some alignment points set up. I've downloaded the EQAlign app, but even choosing multi-language and English from the drop down list, 90% of the buttons and text is in Spanish

[RobH]

Sounds like you've done everything right to me.

I find straightforward drift alignment much easier than all the programs you can get to align personally!

Rob

[malc-c]

Still having a few issues. Due to work in the morning I could only spend an hour or so under the clearest night we've had for ages (Why is it that we always get excellent nights when we need to get up in the morning for work ) - anyway, I spent that hour dialing in sync points on various stars spaced NSEW (ish !) and then tested by parking the mount, powering down and then powering back up and selected one of those points and pleased to say that it found it on first attempt.

However, when dialing in the sync points I had to correct using the number pad to get each star centred, and then when I pointed the scope at M31 and used an 8mm eyepiece the cluster drifted out of view even with the RA driving. I think the error was because having tracked on Altair the instructions suggest I should of locked the dec axis and rotate the RA to the west - to center on Arcturus I had to release the DEC axis.... would this be why there is some drift, or is it some other issue such as motor slip (scope appears balanced) or EQMOD not driving the scope right ?

[steppenwolf]

I think the error was because having tracked on Altair the instructions suggest I should of locked the dec axis and rotate the RA to the west - to center on Arcturus I had to release the DEC axis.... would this be why there is some drift, or is it some other issue such as motor slip (scope appears balanced) or EQMOD not driving the scope right ?

The Polar alignment is a totally separate exercise to finding and tracking and you have done that correctly, including the unlocking of the DEC axis.

If you didn't have a GoTo mount, you would still drift align as you have done and then you would manually move the mount in RA and DEC to acquire M31, then you'd lock the clutches and you'd expect the mount to track M31 just fine.

I think your current issue is something else like not tracking at sidereal rate, not tracking at all, terrible balance or clutch slip.

This all assumes that I have understood your last paragraph correctly!

[malc-c]

The Polar alignment is a totally separate exercise to finding and tracking and you have done that correctly, including the unlocking of the DEC axis.

If you didn't have a GoTo mount, you would still drift align as you have done and then you would manually move the mount in RA and DEC to acquire M31, then you'd lock the clutches and you'd expect the mount to track M31 just fine.

I think your current issue is something else like not tracking at sidereal rate, not tracking at all, terrible balance or clutch slip.

This all assumes that I have understood your last paragraph correctly!

Steve, thanks for the additional comments.

I did notice that whilst slewing the motors were sounding strange, as if there was some harmonics, or the frequency of the pulse rate wasn't quite right. Up until now I've always used the handset to align and select objects rather than EQMOD and the DIY USB EQDIR cable as per their website. I'm guessing that the actual sysnscan box controls the pulse train to the stepper motors, and the PC is simply sending the codes to the unit to slew, track, etc?

I'm fairly sure that the OTA was balanced, but will go and re-check this again.

[RogerTheDodger]

IIRC eqmod only sets four slew rates on the little pull-down by the NSEW bit, and the 4th one isn't the maximum slew rate which is what makes mine sound like that. I have added a 5th option with the full speed from the setup tool.

[malc-c]

IIRC eqmod only sets four slew rates on the little pull-down by the NSEW bit, and the 4th one isn't the maximum slew rate which is what makes mine sound like that. I have added a 5th option with the full speed from the setup tool.

On setting 4 the digits in the boxes display 800. I assume that this equates to the same 800x that the handset runs the mount at. The motors sounds the same, normally just the other night it sounded as if there was a 3rd motor running at the same time ....

[RogerTheDodger]

Maybe I dropped the speed on setting 4 at some point then, I certainly had to add a 5th a couple of weeks ago... 800 sounds right though. You haven't set a slew speed limit on the right inadvertantly?

[chrisshillito]

IIRC eqmod only sets four slew rates on the little pull-down by the NSEW bit, and the 4th one isn't the maximum slew rate which is what makes mine sound like that. I have added a 5th option with the full speed from the setup tool.

By default the 4th preset speed should be x800 but of course folks can choose whatever speeds they prefer for those presets. We chose only 4 preset rates by default because, when asked, most folks seemed to think that in practice three was all they ever used - so we gave them one more for luck! You can of course define up to a maximum of 10 preset speeds if you wish - 10 being chosen simply to be one better than the synscan which has only 9 )

If you're using a gamepad then be aware that the defaults button assignments use the four end buttons to change the slew speed not the slew preset (you can have them change/select presets if you wish).

The motors/gears do sound odd at some rates - it's not a problem control wise but I suspect most folks will experiment to find preset rates that not only give them the control they need but also don't wake the neighbours up!

Chris.

[RogerTheDodger]

I have tried that to find a quieter speed, that's probably when I modified the preset. One thing I did seem to notice Chris, is that when I used the max slew limiter that the DEC speed seemed to be affected more than the RA... that's more of an impression than a measured effect though.

[chrisshillito]

I have tried that to find a quieter speed, that's probably when I modified the preset. One thing I did seem to notice Chris, is that when I used the max slew limiter that the DEC speed seemed to be affected more than the RA... that's more of an impression than a measured effect though.

If by more affected you mean "noisier" then that may just reflect the fact that your DEC gear trains needs some adjustment.

The motor controllers for DEC and RA are identical so there really shouldn't be any reason for them to move at different angular speeds when issues with identical commands. Of course if your watching the mounts position change on a planetarium the apparent speed of movement in RA will appear to change with Declination but the angular speed remains the same.

Chris.

[RogerTheDodger]

That's what I thought, but I was repeatedly testing from parked to the same point, at full speed if time to reach point is A, then at half speed it should be A x 2 to reach it. It seemed to be a lot longer in the DEC axis. I'll try again this weekend when I'm home and see if it was my imagination

[malc-c]

Steve, thanks for the additional comments.

I did notice that whilst slewing the motors were sounding strange, as if there was some harmonics, or the frequency of the pulse rate wasn't quite right. Up until now I've always used the handset to align and select objects rather than EQMOD and the DIY USB EQDIR cable as per their website. I'm guessing that the actual sysnscan box controls the pulse train to the stepper motors, and the PC is simply sending the codes to the unit to slew, track, etc?

I'm fairly sure that the OTA was balanced, but will go and re-check this again.

OK, managed to grab an hour tonight before the clouds rolled in and tried something.

Using EQMOD to control the scope I cleared all previous points and set two new ones on Vega and Arcturus, then selected M13 in CdC and slewed to it... but nothing was visible. So I selected Vega on Cdc and slewed to that... it was close, but not centred. OK I thought, my drift alignment must be off. So I closed everything down, and powered down the mount, swapped the EQDIR lead out and plugged in the handset.

I chose the 3 star alignment with the handset chosing Vega, Arcturus and Dubhe, and apart from Dubhe the alignment was fairly close and only required a small manual tweak to center the star in an 8mm eyepiece. To my amazement it was successful - 1st time I've managed a 3 point alignment, so the mount must be fairly spot on for polar alignment (?). I then selected M13 via the handset and it was bang in the middle of a wide field 32mm eyepiece. Selecting Vega again also resulted in a spot on alignment...

I was then going to repeat this with the same three stars using the PC / EQMOD to drive the scope... but the clouds and the fact that I have work in the morning put pay to that.

I still have the SW RS232 serial lead that plugs into the handset so might give that ago to see if the control issue is EQMOD related.

[malc-c]

No one have any comments...

On the face of the tests I've made (be that limited) it does seem as if the miss-alignment is down to EQ-MOD, as it only occurred when using that to drive the scope. I'm not able to work out how EQMOD communicates with the mount, or what it uses for timing, or what the handset does that EQ doesn't. Any ideas as I really would like to use the wonderful features EQMOD has to offer

[RobH]

Interesting finding.

It would be good if a few others with EQMod could do an A/B type of test too.

Rob

[malc-c]

Rob,

With so many people using EQMod it would point to something wrong with my set up, but I can't replicate it when using the handset, hence the question about what goes on behind the scenes with EQMod. I'm going to do some more experiments at the weekend (if clear) to verify that the mount is aligned correctly, and see if I can replicate the A-B misalignment.

[chrisshillito]

Malcolm,

A two point EQMOD model will use a nearst point algorithm for pointing correction - only one point is therefore ever active in providing the correction so you can think of this a being equivaleny of a 1-star alignment in the handset (well kind of multiple 1-star if you know what I mean). It is therefore no surprise to me that a handset using a 3-star model will produce better results in some areas of the sky. At the moment you are comparing apples and pears.

Chris.