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APT/dithering/phd2

Anthonyexmouth

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david_taurus83

david_taurus83

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How big is the dither? In APT the dither scale is 1 to 5, with 1 being the smallest and 5 being the biggest. I don't know what the scale is based on. In PHD settings, there is another scale. This goes from 1 to 100! Again, not quite sure what this is but given that it goes up to 100 it could be pixels. FYI, my APT scale is 3 and my PHD scale is 8 and my dithers are set to a random pattern but they regularly dither by 10/15 pixels.

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FaDG

FaDG

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I always use APT with PHD2 and seldom have issues.

First checks: 

You could try to reduce the amount of dithering, if I remember correctly it can be set from 1 to 5

Try to e large PHD2 guiding box: if dithering causes the star to leave it, guiding Will be lost

Check PHD log to see if the star is Lost during guiding or otherwise. 

Fabio

We posted at the same time! I was late!! ?

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david_taurus83

david_taurus83

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Ok, I've just read through some of the bumf. The scale in APT is a maximum pulse issued to the mount for a dither. So 1 would be 100ms max and 5 would be 500ms max. The max value would not always be issued to keep things random. We see the results of this as a "distance" moved from the guide star in the form of no. of pixels. The PHD scale of 1 to 100 is a multiplier of the image applications dither command. The default is 1.0 in PHD. So in your case Anthony, your APT scale is one. So APT issue a random 55ms dither command. If the PHD scale is 1.0, the mount will move 55ms. If your PHD scale is 10.0, the mount will move 550ms.

 

This probably doesnt help with your issue but its an idea of how the dithering scale works.

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FaDG

FaDG

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No mate,  seems great! But the proof of the pudding... How are your stars turning Out? 

What is your imaging resolution? 

I'd just try to increase the SNR: from my experience i tend to get best results above 40 and around 50, without saturation.

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FaDG

FaDG

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2 minutes ago, Anthonyexmouth said:

Camera resolution?

Camera and focal length. You have less than 1 arcsec RMS, so if working at about 1.3 arcsec/pixel or more you should be fine. 

For less than that, you might have slight bloating but still round stars, as RA and DEC errors are very close to one another

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Anthonyexmouth

Anthonyexmouth

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5 minutes ago, FaDG said:

Camera and focal length. You have less than 1 arcsec RMS, so if working at about 1.3 arcsec/pixel or more you should be fine. 

For less than that, you might have slight bloating but still round stars, as RA and DEC errors are very close to one another

600mm and whatever my 760d is. fingers too cold to type, going to get coffee. 

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david_taurus83

david_taurus83

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19 minutes ago, Anthonyexmouth said:

Stars look good on the good images. 

Camera resolution?

You might have seen throughout the forum people talking about image scale in the form of arc seconds per pixel. This is the area of sky that is covered by a single pixel. Assuming your using the kit detailed in your signature, your Canon 760D on the ED80 with 0.8 reducer? Your image scale would be 1.6 arc seconds per pixel. The formula for this is:  Resolution Formula:   (   Pixel Size : 3.72um   /   Telescope Focal Length : 480mm   )   X 206.265  = 1.598" per pixel.

 

You should be aiming for your guiding RMS error to be around half of this. A little bit over shouldnt matter too much.

 

A good guide here:  http://astronomy.tools/calculators/ccd_suitability

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david_taurus83

david_taurus83

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In that case your working at 1.28" per pixel. As FaDG mentions above, you could start to see slightly bigger stars as your guiding errors can effectively add to your average FWHM values. Also, even though the RMS average in PHD can look ok, the peak errors can give elongated stars. I wouldnt worry too much about it. That single sub above doesnt look too bad considering your not using a flattener. Mine still look like that lol

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Doug64

Doug64

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11 minutes ago, Anthonyexmouth said:

Not yet, FLO started selling the pre modded mounts a few weeks after I bought mine. It's on the list to do. Having my new pier has made a big difference

20190217_163454.thumb.jpg.9a579d4dcce4c8569443c33a1917d8bc.jpg

Hi,

Sorry to jump in on your thread but what did you make the pier base out of.

Doug

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FaDG

FaDG

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7 hours ago, Anthonyexmouth said:

What would be an RMS on my heq5 that I should be happy with? 

Whatever your seeing allows! ? ? ? 

Relax, the ED80 is a very forgiving scope and the HEQ5 is a great mount: I have both and it's a very easy setup, but @david_taurus83 is right: You'll notice that belt modfing the HEQ5 Will really turn it into a different mount... I couldn't believe the improvement  I got on mine! On good nights it can reach 0.4 arcsecs: but unfortunately these nights are few and far between! 

But honestly, i think that 0,97 should give you pretty good results already, your sub above seems fine to me. Just take care not to lose the star.

Fabio

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