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It will result in a lower SNR for the guide star since the light will be spread over more pixels, which might make the guiding a bit more erratic as the centoid will wander around more. If you do see problems then try increasing the guide exposure length. You will get larger stars in the image due to the seeing anyway, so you won't be losing out by doing so and should help to avoid trailing due to wild swings in guiding.

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post-1155-0-45002200-1416222848_thumb.jpHi many thanks for the reply on this.

The question arises because i changed my guide camera from the QHY5 to the QHY5L-ii mono, it appears to me that the smaller pixels has upset my guiding a bit!

The graph looks horrible and the RMS appears to have worsened.

However on the other hand, i did a 30 min sub on the Rosette nebula (attached) the other night and whilst it appears o.k...if you dig deeper it does have some oval stars.

I'm not sure if i'm being too picky??

Ray

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Well you asked about seeing not changing the guide camera! Same difference though, smaller pixels may give a worse SNR. If you are using PHD guiding maybe try the Noise Reduction setting at 2x2 or 3x3 to see if that improves matters. You might just have had a night of bad seeing though.

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Yes i did Ian, simply because in my reading up of guiding issues; i came across a statement that said;

 you are wasting your time if you are trying to guide at a resolution higher than your seeing!!

of course..in lowering my Pixel size, i have indeed lowered the guiding scale to well below the local seeing....what i am trying to assert is...

1. Is this fact having a big effect on my guiding

2. Is the fact that my guide scale is identical to my image scale simply making my graph look awful

i did try the noise reduction but it did not appear to make much of a difference and i accept the points about increasing the exposure time and Min motion parameter, I'll look to check that of once the  weather clears.

In the meantime i feel reasonably confident that it is indeed the graph that looks worse than the guiding is.

hope that all makes sense.

Ray

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OK I think I see now. Are you using PHD or PHD 2? If the former then the graph and the RMS error are shown in terms of guide camera pixels. So if you half the size of the pixels then things will look twice as bad but won't be assuming the original guider wasn't under sampling. You can manually convert the pixels to arc seconds as explained in the PHD guiding sticky in the Imaging Tips and Tricks section. Or you if are using PHD 2 you can enter the camera pixel size and guider focal length and have the graph and RMS error displayed in arc seconds automatically; again pixels are used if you don't.

I suspect if compare the error between the two cameras in arcseconds not pixels it may be similar.

Guiding at more than the seeing will allow(I.e. oversampling) is not a problem other than the potential reduction in guide star SNR as previously explained.

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Hi Ian, yes its hard to translate a point of view sometimes, i always use PHD2..i have also used Maxim.

but under both suites i see the same guiding graph roughness that appears to suggest that the guiding is bad.

I see now that my MIn motion is probably to small and i need to raise it to allow for the pronounced movement of seeing.

i can obviously deal with the low SNR...1. by reducing the Gain (i had initially raised it to 50%) to 10% or less and 2. increasing the exposure.

i don't expect that this will reduce the intensity of the graph but it will mean that i'm making less corrections and that has been an issue...in fact corrections on every exposure.

This cloud cover is irritating and makes it so hard to test.

Ray

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  • 2 weeks later...

When you guide at finer resolution you do see a worse looking graph. Most people using an OAG for the first time at long focal length are horrified by the Himalayan guide trace they see! I certainly was, but when you work it out there is no real difference in guiding accuracy. If you want a nicer guide trace bin the guider 4x4 - but it won't help your guiding!  :grin:

In bad seeing a longer guide sub certainly helps enormously because it averages out the seeing. The EQ8 should be good for 4 to 6 escond subs, no?

That's a very crisp Rosette. I wouldn't be worrying if that's what you're getting.

Olly

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Cheers Olly, as always an honest and succinct answer, yes i can use the longer guide subs and you are correct; for a 30min sub the rosette looks good...I'm a pedantic [removed word] though and i try to get the fwhm down as low as i can.

just giving Metaguide a good test for that reason.

Ray 

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When you guide at finer resolution you do see a worse looking graph. Most people using an OAG for the first time at long focal length are horrified by the Himalayan guide trace they see! I certainly was, but when you work it out there is no real difference in guiding accuracy. If you want a nicer guide trace bin the guider 4x4 - but it won't help your guiding!  :grin:

Thats a good point Olly. It's easy to become obsessive about the guide graph. I sorta look at it like this "if the graph is a bit up and down then it shows that the guiding is working". You can make the graph look flat by altering the PHD settings, but that doesn't mean that the image is sharp. For me, the proof of the pudding is in the eating...if the stars are sharp, then the guiding is good.

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Thats a good point Olly. It's easy to become obsessive about the guide graph. I sorta look at it like this "if the graph is a bit up and down then it shows that the guiding is working". You can make the graph look flat by altering the PHD settings, but that doesn't mean that the image is sharp. For me, the proof of the pudding is in the eating...if the stars are sharp, then the guiding is good.

Agreed. I note that you said 'sharp' when many would have said 'round' and this is a good distinction because a truly random error will give round but soft stars. We saw this while refining the PHD parameters for the Mesu in the early days. Initially it was moving too much but in an evenly distributed way.

Olly

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