PHD Guiding Basic Use and Troubleshooting

[Midnight_lightning]

On 20/06/2013 at 17:12, IanL said:

So I image with a Canon EOS 500D with pixels that are 4.7µm square and a Skywatcher Evostar 80ED and 0.85x Reducer with an effective focal length of 510mm:

(4.7µm / 510mm) x 206.3 = 1.9 arcseconds per pixel

I guide with a QHY5 with pixels that are 5.2µm square and an Orion ST80 with an effective focal length of 400mm:

(5.6µm / 400mm) x 206.3 = 2.67 arcseconds per pixel

If you don’t feel like doing the maths, use my Imaging Toolbox to do the hard work for you. The upshot is that I my imaging resolution is about one and a half times my guiding resolution (2.67 / 1.9 = 1.41). That is well within the 4 x rule we established above.

Excellent article. 

You say above that your Imaging Resolution is 1.5 times guiding resolution but your guiding resolution is actually higher than your imaging resolution, so I would have said your imaging resolution is 0.7 times that of your guiding resolution.

I don't mean to be pedantic but I am struggling to get my head around the whole topic of PA and Guiding and I don't know whether this is important. For example does the imaging scale just need to be within 4x the guiding scale, either way, or does the imaging scale always have to be less than the guiding scale AND by no more than four times?

Thanks

 

 

 

 

[IanL]

47 minutes ago, Midnight_lightning said:

Excellent article. 

You say above that your Imaging Resolution is 1.5 times guiding resolution but your guiding resolution is actually higher than your imaging resolution, so I would have said your imaging resolution is 0.7 times that of your guiding resolution.

I don't mean to be pedantic but I am struggling to get my head around the whole topic of PA and Guiding and I don't know whether this is important. For example does the imaging scale just need to be within 4x the guiding scale, either way, or does the imaging scale always have to be less than the guiding scale AND by no more than four times?

Thanks

 a) There was a small error (which I corrected in a follow up post), the guiding calculation should have read "(5.2µm / 400mm) x 206.3 = 2.67 arcseconds per pixel", i.e. result was correct but mistyped the camera pixel size. Not relevant to the question but just thought I'd clear it up.

b) "1.5 times" was perhaps a poor choice of words. The imaging resolution is 1.5 times better than the guiding resolution, i.e. there are 1.5 times more arcseconds of sky on each guider pixel than on each imaging pixel. So pedantically you are correct, but hopefully the point is clear that your guiding resolution needs to be reasonably proportionate to your imaging resolution.

Your guider could have a better resolution than your imager, e.g. piggybacking a widefield camera/lens on a long focal length SCT which is guiding would be OK. Going the other way the four times statement is a rule of thumb, your mileage may vary at worse guider:imager scale ratios but if I was buying equipment it wouldn't make sense to go beyond that rule of thumb.

[jpoulette]

Again, I'm late to the party but thank you for posting this - it is excellent for people like myself who have been alt-az gazers for decades but just now getting into imaging.

The pixel scaling was especially well explained - and it was the first time I've encountered the "4x rule".

My scaling is as follows:

Guider: (3.75um / 240mm) x 206.3 = 3.22 arcseconds / pixel

Imager: (2.40um / 400mm) x 206.3 = 1.23 arcseconds / pixel

3.22 / 1.23 = 2.6 (so, the imaging resolution is ~2.6x better than the guider) It looks like I just came in under the wire !

One question: So, if I wanted to try, say, a 2x Barlow in front of the imager - that would push it beyond the 4x rule. What would the immediate (visible) consequences be?

Thank you again! 😀

Jim

[Stefan73]

21 minutes ago, jpoulette said:

One question: So, if I wanted to try, say, a 2x Barlow in front of the imager - that would push it beyond the 4x rule. What would the immediate (visible) consequences be?

I think it probably depends on how good your guiding is but ultimately elongated stars will be the result.  The bigger the ratio of guiding resolution to imaging resolution the better your guiding errors will need to be in order to have good subs.  On a bad night's guiding you might not even get away with 4x.

[jfrijhoff]

Just starting my first steps in guiding (when the clouds clear up, so most likely not before summer 🤣), this is great!