polar alignment spot on but star trails?

[VilleM]

"Cheap" mounts, like SW, arent designed to run 600s subs unguided.. Too much periodic error and imperfections in gears.
Just use PHD for PA and guiding and your mount performs way better than unguided one.

-V

[alcol620]

Hi Olly

Outside on a not perfect night here in the UK.

Took a 300s image without guiding and stretching the image in Pixinsight it looks as if a star drifted by about 10 pixels.

I am using a Atik 383 camera on a ed80 scope. The 383 has a pixel size of 5.4um.

Is this what you are eluding to in your post. As far as I know by carrying out a drift alignment, I have pretty accurate PA.

As you suggest I will check tomorrow in daylight (subject to the fog lifting) if the polar scope is collimated with the mount (I thought it was, but I will double check). Can I get rid of some of the PE and still guide with PHD, I thought I read somewhere that there could be a conflict here?

Regards

Alec

 

 

[wimvb]

With your camera (5.4 um) and scope (fl = 600 mm), you have about 1.9 "/pixel (= pixelsize * 206 / focal_length)

10 pixels = 19 "

The period of the wormgear on a SW AZ-EQ6 is about 8 minutes, so with 5 minutes you got little more than half a period, which in best case went from one peak to the other peak (+ to -), and in worst case from 0 to + to 0 again. Your PE should be between 19 and 38 arc seconds (peak to peak). Which is about the value Olly mentioned.

You can do periodic error correction, which shouldn't affect guiding. Otoh, if the PE guides out easily, there may not be a benefit to this.

 

Cheers,

[alcol620]

Thanks Wim

Making a bit more sense now, although it all seems so complicated. As PHD seems to get rid of all trailing that I can see by eye, then all in the world is fine. It is just that I was interested why despite having good polar alignment stars still had tails. Obviously other factors come into play. The next opportunity I get I will check the setting of the polarscope in the mount to make sure it is collimated.

happy new year Alec

[alcol620]

A follow up question: Had a look this morning to check the collimation of my polar scope. Problem - or am I being stupid - probably!

I have the AZ EQ6 on a fixed pier and have no aerials or the like anywhere within the mounts polar scope's line of vision. Presumably my only options are i) take the mount off the pier and reposition it to view some distant object - which I don't want to do or ii) after dark use Polaris as the distant object to check collimation of the polar scope?

Feedback appreciated

Alec

[Davey-T]

I would always use Polaris having tried the distant object route, be careful adjusting the polar scope it's a really fiddly job, quite easy to make it worse, undo the little screws too far and drop them in the dark, don't try to rush it.

Dave

[alcol620]

Thanks Dave, much appreciated. I'm hoping when I check that it is collimated anyway.

Alec

[alcol620]

Hi folks

 

Checked the collimation of the polar scope and all looks fine so no problem there. in due course will double check polar alignment using drift alignment. Running PHD now on Andromeda and getting the following data as it guides:  RA rms = 0.18  0.85"   DEC rms 0.19  0.88"   Total 0.26   1.24"   RA Osc  0.39

Could someone explain these numbers to me.  Guidescope focal length 162mm pixel size 3.75um  4.78 arc sec/pixel. Main scope ED80 and Atik 383

Thanks

Alec

[wimvb]

I believe:

RA: 0.85 " / 4.78 " = 0.18 pixels

So the first number is the "average" (root mean square) deviation in pixels, the second number the rms deviation in arc seconds. Same for DEC

Since the error is much less than one pixel, I'd say guiding is working very well.

You can calculate how much error there is in the imaging scope:

rms_error (") / arcsecs_pixel

where arcsecs_pixel is your imaging resolution in arcseconds per pixel

rms_error is the RA or DEC rms error in arcseconds (0.85, 0.88)

Cheers,

[alcol620]

Thanks Wim, most grateful for you taking time to explain. Seems that PHD is doing a good job in the face of poor mount PE?

[ollypenrice]

Even with perfect PA your unguided tracking errors seem to be typical of a mount like this. I didn't check Wim's sums but he is (very!) likely to better at maths than i am... If you pay several times the price of an EQ6 you can buy a mount with maybe 3 arcseconds PE rather than maybe 30 arcseconds. This sounds like a huge gain but, since we need to guide anyway (or use very, very costly encoder-managed mounts) does it really matter? At modest pixel scales I really don't think it does. 

If you want an observatory class PA just do a drift alignment. This method is software free, financially free and cannot lie. But if, under guiding, you can do subs as long as you want to do without seeing field rotation in the corners then it ain't broke so don't fix it!

Olly

[alcol620]

Thanks Olly for clarification. It's good to know what is going on and what is important. I shall continue in the knowledge that despite the short comings of the mount, there is nothing I need do as long as PHD keeps doing the business and the skies stay clear.

Thanks everyone for some very helpful feedback

Happy New Year Alec

[iapa]

On 24/12/2016 at 15:10, alcol620 said:

I thought I read somewhere that there is some sort of conflict when correcting for PE and running PHD guiding?

Possibly a contentious question.

In the Imaging - Tips, Tricks and Techniques forum I found this PHD Guiding Basic Use and Troubleshooting topic which I have taken an extract form.

Other views are that recording PEC along and guiding whilst playing back the correction is the way to go.

Suck it an see if probably the best solution. Try both and see what works for you. Note that EQMod will work only on Skywatcher mounts.

Periodic Error Correction (PEC) is a feature of some mounts and/or their drivers. All mounts will suffer from Periodic Errors in tracking in the RA axis, which relate to the rotation of the RA gears (gear periods, hence the name). As the gears rotate, machining imperfections in the gears cause the tracking to drift East and West of the correct position in a repeating cycle. The PE of a high end mount might be a few arcseconds, but on a low end mount (sub £2,000) a PE of 10 arcseconds is considered ‘good’, and figures much higher than that are not uncommon.

PEC records those errors by monitoring guiding corrections for at least one full cycle of RA worm gear. Subsequently the mount (or driver) plays back those errors to correct the tracking before it drifts off target. When used properly, PEC should be superior to guiding alone since it prevents most of the tracking errors before they happen, not after they are detected.

Whether you should use both PEC and guiding together is an ecumenical question! If used properly, then PEC should give superior performance to guiding alone, but I am definitely in the camp that thinks it is rather a lot for a beginner to take in conjunction with learning the ropes of guiding.

If you are using an HEQ5, NEQ6 or related mount with the EQMOD drivers on your laptop and you also want to use EQMOD’s PEC features, you should definitely use ASCOM pulse guiding. The drivers have been written to properly combine the PEC corrections with pulse guide commands from PHD (or any other software that produces pulse guide commands). This avoids conflicts between PEC and PHD which could otherwise over or under-correct guide errors, whereas using an ST4 cable with EQMOD PEC may result in such problems.