Dithering with SW AZ-GTI

[NGC_Mike]

I recently purchased an AZ-GTI mount and star adventurer EQ base for lightweight, portable and cheap astrophotography. I’m using a SW Evoguide ED50, Starizona flattener and DSLR.

I’m using this with an EQMOD cable and have the EQ firmware installed. It works perfectly and able to achieve 2 and 3 minute subs with sub arc second guiding.

The only issue I have is that my guiding goes wrong when I attempt to dither. I have the dither set up in NINA and after it performs the dither, either the RA or DEC or both starts climbing away from zero as if they get stuck. If I don’t dither the guiding is fine and just sticks at around 1 arc second  

I’m wondering if there is a trick I’m missing in my eqmod settings or in PHD2?

 

 

Edited December 12, 2021 by NGC_Mike

[bobro]

Lovely image.

The guiding RMS values suggest that guide camera pixel size and/or guide scope focal length may not be correct in PHD2 Advanced Settings. What guide scope and camera are being used and what values have been set?

[michael8554]

Hi Mike

You'll have to post a PHD2 GuideLog, as you haven't shown an example of the Dither, or the Dither setting, and as bobro suspects, wrong settings.

You'll find the Log in the PHD2 folder, check the date in the filename.

Michael

[happy-kat]

Could I ask please, is your T ring exactly 11mm as I can't see a spacer on your rig?

I'm having to use homemade plastic washers to make my distance 55mm as my T ring is slightly thinner than 11mm.

[michael8554]

2 hours ago, happy-kat said:

Could I ask please, is your T ring exactly 11mm as I can't see a spacer on your rig?

I'm having to use homemade plastic washers to make my distance 55mm as my T ring is slightly thinner than 11mm.

Mike doesn't seem to have a Focal Reducer or Field Flattener in his setup.

You only need 55mm spacing if you have a FR or FF that has a 55mm Back Spacing requirement.

Michael

[happy-kat]

There is a starizona flattener it will be their v2 which works with a DSLR they are super slim

sorry for slightly drifting this post

 

Edited December 13, 2021 by happy-kat

[michael8554]

The T2 Adapter Spec is for 55mm Back Spacing for all brands of  DSLR.

So a Nikon has a longer Flange Distance (46.5mm) than a Canon (44mm), so has a thinner T2 Adapter.

If yours is for a Canon and is not 11mm, it's non-standard.

Michael

[happy-kat]

My canon T2 is 10mm.

For @NGC_Mike this thread on imaging with your mount may help whilst waiting for a reply on your question

https://stargazerslounge.com/topic/348979-imaging-with-an-az-gti

[NGC_Mike]

Thanks all,

I'm imaging again tonight and tried dithering again but the same problem occurred - I've attached a screen shot of PHD during a dithering.

PHD2 was happily guiding at 0.7-0.8 arc seconds then went off the chart as expected during a dither, but didn't settle back down again.

[michael8554]

12 hours ago, NGC_Mike said:

then went off the chart as expected during a dither, but didn't settle back down again.

Not enough of the graph to see the whole story, but after 15 steps every 2.5 secs = 37.5 seconds, Dec and RA do return to the zero position.

Then the problem you are asking about:

RA and Dec overshoot, but then the graph runs out.

Post a PHD2 GuideLog, screenshots just frustrate.

Michael

[NGC_Mike]

11 hours ago, michael8554 said:

Not enough of the graph to see the whole story, but after 15 steps every 2.5 secs = 37.5 seconds, Dec and RA do return to the zero position.

Then the problem you are asking about:

RA and Dec overshoot, but then the graph runs out.

Post a PHD2 GuideLog, screenshots just frustrate.

Michael

Hi Michael, I appreciate you looking at this. Log attached from last night's session.

PHD2_GuideLog_2021-12-15_204706.txt

[NGC_Mike]

I just did the one dither at around 21:28.

[david_taurus83]

The AZGTi is not designed to be used as an EQ mount for AP though it does a pretty good job with the EQ firmware anyway. The internal gearbox has a lot of play between the 3 little nylon gears on the motor output shaft so there is a lot of backlash that can't be avoided. In PHD turn off the option to dither in Dec axis and dither in RA only. Its capable of 5 minute subs with good polar alignment.

[michael8554]

Hi David

From the 15th Dec GuideLog:

There's no Calibration in the log, the last Cal was 10th Dec - is this a permanent setup ?

Doesn't look like it from the images, so you should take a few minutes each night to re-Calibrate.

PHD2 detects a mount setting of RA Guide Speed = 7.5 a-s/s, Dec Guide Speed = 7.5 a-s/s, 

But the Calculated rates from Cal were only Norm rates RA = 3.9"/s  Dec = 4.3"/s;   almost half the mount setting.

I suspect you have too low a PulseGuide Setting in EQMOD.

And Cal is best near Dec = 0 and near south, not at Dec 35.6,.

Your DSLR / 50ED image scale is about 4arcsecs/pixel, which some might consider to be Under Sampling.

But it also means huge moves are required of the mount to get a typical 12 pixel Dither, about 4 x 12 = 48 arcsecs !

At 21:27 there was a 35 arcsec RA Dither, PHD2 made three huge corrections that Settled the Dither, but RA overshot 10arcsecs, and it took three more corrections to finally Settle.

Total Settle time was about 90 seconds.

You have MultiStar Guiding selected, that means you can speed up exposures from 2.5 secs to as fast as 1 sec max,  MultiStar will compensate for "Chasing the Seeing".

That together with higher Guide Rate would mean faster PHD2 corrections, which could improve the Settle times.

But a smaller pixel imaging camera would help 😆

Dec Backlash was measures at 16.5 secs, you might want to adjust that !

PA error at up to 9arcmins was always over the 5 arcmins max that PHD2 considers to be guideable.

But if you can't improve Dec, run with that order of PAE.

Start guiding, let it settle down, then switch Dec to OFF, and note the direction of Dec drift due to the PAE.

Then set Dec to correct in the opposite direction.

With luck Dec may never be pushed over the line, where corrections would first have to overcome all that Backlash.

If you want Dithers, then leave Dec on Auto, but some Dithers may take a while to Settle :-<

There was a focus warning in the GA run, it changed from HFD =4.3 to HFD = 7.2 on your last run, something loose ?

Michael

 

[bobro]

Excuse me if I an missing something, but the guide log shows a guide scope focal length of 225mm, whereas the guide scope on the equipment setup looks closer to one with a 150mm focal length as it is much shorter than the 50ED FL of 242 mm. Something unusual about the guide scope?

[michael8554]

Good point bobro.

The 50ED imaging scope is 242mm focal length , and the guidescope looks significantly shorter.

That error would explain the guide rates and much more.

Michael

 

 

[rnobleeddy]

I just got one of these and whilst I got good guiding on the target I was capturing, the DEC backlash was pretty disastrous. 

There are some guides on how to tune this, but if your focal length in PHD2 is correct, I'd just continue with RA dithering only.

[NGC_Mike]

On 17/12/2021 at 15:09, michael8554 said:

Hi David

From the 15th Dec GuideLog:

There's no Calibration in the log, the last Cal was 10th Dec - is this a permanent setup ?

Doesn't look like it from the images, so you should take a few minutes each night to re-Calibrate.

PHD2 detects a mount setting of RA Guide Speed = 7.5 a-s/s, Dec Guide Speed = 7.5 a-s/s, 

But the Calculated rates from Cal were only Norm rates RA = 3.9"/s  Dec = 4.3"/s;   almost half the mount setting.

I suspect you have too low a PulseGuide Setting in EQMOD.

And Cal is best near Dec = 0 and near south, not at Dec 35.6,.

Your DSLR / 50ED image scale is about 4arcsecs/pixel, which some might consider to be Under Sampling.

But it also means huge moves are required of the mount to get a typical 12 pixel Dither, about 4 x 12 = 48 arcsecs !

At 21:27 there was a 35 arcsec RA Dither, PHD2 made three huge corrections that Settled the Dither, but RA overshot 10arcsecs, and it took three more corrections to finally Settle.

Total Settle time was about 90 seconds.

You have MultiStar Guiding selected, that means you can speed up exposures from 2.5 secs to as fast as 1 sec max,  MultiStar will compensate for "Chasing the Seeing".

That together with higher Guide Rate would mean faster PHD2 corrections, which could improve the Settle times.

But a smaller pixel imaging camera would help 😆

Dec Backlash was measures at 16.5 secs, you might want to adjust that !

PA error at up to 9arcmins was always over the 5 arcmins max that PHD2 considers to be guideable.

But if you can't improve Dec, run with that order of PAE.

Start guiding, let it settle down, then switch Dec to OFF, and note the direction of Dec drift due to the PAE.

Then set Dec to correct in the opposite direction.

With luck Dec may never be pushed over the line, where corrections would first have to overcome all that Backlash.

If you want Dithers, then leave Dec on Auto, but some Dithers may take a while to Settle :-<

There was a focus warning in the GA run, it changed from HFD =4.3 to HFD = 7.2 on your last run, something loose ?

Michael

 

Thank you for this detailed dive into my PHD log - very much appreciated.

I'll have to go through point by point the next time I have everything on again. I have 3 young kids and 2 jobs and amount of time I can allocate to AP is extremely limited, but I get out as much as I can...

I was using PHD to guide my old lager setup, which was a 6-inch newt on a HEQ5, so some of my old calibration data might still be lurking about in the settings. I thought shift-click on the guide button would re-calibrate, but I've clearly not thought this through too much. Do I need to save after re-calibrating or does PHD2 do it for you?

I'll check out what my pulse guiding settings are on EQMOD - I suspect they are the same as I was using on the HEQ5.

I know my DSLR pixel size is 4.3, which is way too big for the Evoguide ED50, but it's what I have for now until I can get a dedicated astro cam like the 183MC.

When you mention to adjust the backlash (16.5 seconds) - is that something I should do in PHD2?

Not sure what you mean re PAE, though I'll do some research as my knowledge is pretty lacking. 

I'm getting the inkling that the AZ-GTI doesn't do DEC dithering very well and my try without.

Thanks again for your help.

 

[NGC_Mike]

On 18/12/2021 at 12:57, bobro said:

Excuse me if I an missing something, but the guide log shows a guide scope focal length of 225mm, whereas the guide scope on the equipment setup looks closer to one with a 150mm focal length as it is much shorter than the 50ED FL of 242 mm. Something unusual about the guide scope?

Good spot - I did see that in my settings but it was greyed out at the time, and I have no clue how to fix it. It may have been un-editable at the time as PHD2 was actually guiding, so I'll have play beforehand. The guide-scope is a svbony 30mm F4, so 120mm FL.

[michael8554]

11 hours ago, NGC_Mike said:

some of my old calibration data might still be lurking about in the settings. I thought shift-click on the guide button would re-calibrate,

Yes, but a new Cal doesn't reset your HEQ5 Equipment Profile.

You should start over with a new Equipment Profile with the correct focal lengths etc, call it AZGTI if you like.

PHD2 will save and reuse any Cal until you shft/clk to start a new one.

Which you should do once a night if your setup isn't permanent.

11 hours ago, NGC_Mike said:

When you mention to adjust the backlash (16.5 seconds) - is that something I should do in PHD2?

Ideally that should be an adjustment of the AZGTI Dec worm, but I'm not sure if that's possible with the AZGTI.

11 hours ago, NGC_Mike said:

Not sure what you mean re PAE, though I'll do some research as my knowledge is pretty lacking. 

Polar Alignment Error.  As I suggested, if Dec Backlash can't be improved, run with a higher than usual 5 to 10 arcmins drift, to allow guiding to negate the Dec Backlash.

Michael