Tracking error different in RA + DEC?

[rnobleeddy]

Tried a few different targets last night as part of testing a new scope. I realized that my EQ6 has different tracking performance depending on where in the sky it's pointing. That makes sense. However, when I started to look at some images I finally joined the dots and realized this is what's causing me to sometimes get oval stars. 

I can't remember what order this was in in terms of height in the sky (although I imagine it's easy enough to guess), but let's say I was seeing tracking of 0.3/0.5" in PHD2 in RA/DEC in one position, 0.4/0.4" in another, and 0.5/0.3" in another. Exposing for 5 minutes, I can see oval stars for either of the  0.3/0.5" combos, but perfectly round stars for the subs tracking at  0.4/0.4". 

Based on my knowledge of what's going on here, I have no reason to expect to get equal tracking, because the errors are caused by different things.

Is this something I have to live with, or do your mounts tend to track at the same rate? The error is pretty much regular noise rather than occaisional large jumps - so I don't see how shorter exposures would help?

It's an old EQ6 but tends to track pretty well (always below 1" RMS, and as low as 0.6"). No belt mod yet, but I have one that I'm waiting to install. I realize this may help DEC tracking, but it's not always going to help the imbalance.

 

 

[vlaiv]

As you say yourself - RA performance depends on where the mount is pointing.

RA error will be largest near equator - at DEC values close to 0°. Once you start moving "upper" to DEC values towards 90° - your RA error will stay the same in terms of arc seconds but it will not be the same in term of pixels on sensor. That makes sense if you think about it - if you point the scope at Polaris - you won't really be tracking as Polaris stays put - you'll be rotating your FOV at very slow pace - one full rotation in 24h.

For this reason it is best to calibrate your PHD at equator - close to DEC 0° - regardless where in the sky you'll be guiding.

Second thing that I'd like to point out would be that if you have 0.3" / 0.5", 0.4" / 0.4" and 0.5" / 0.3" RA and DEC guide values and you can notice star elongation in your images - well your guide setup is not measuring things correctly.

You are probably using very coarse guide setup - like very small guide scope with short focal length. Are you sure you entered guider parameters correctly in PHD2 (focal length and pixel size)?

Say you have 0.3" vs 0.5" error, you use 6" scope and have good seeing conditions of 1.5" FWHM. That is 12% elongation or 1.12 (one axis is x1.12 "longer" than other). Here is what that looks like:

Do your stars look elongated like that or more? With smaller aperture and worse seeing - they will be even more round as guiding performance will contribute less.

[rnobleeddy]

2 hours ago, vlaiv said:

As you say yourself - RA performance depends on where the mount is pointing.

RA error will be largest near equator - at DEC values close to 0°. Once you start moving "upper" to DEC values towards 90° - your RA error will stay the same in terms of arc seconds but it will not be the same in term of pixels on sensor. That makes sense if you think about it - if you point the scope at Polaris - you won't really be tracking as Polaris stays put - you'll be rotating your FOV at very slow pace - one full rotation in 24h.

For this reason it is best to calibrate your PHD at equator - close to DEC 0° - regardless where in the sky you'll be guiding.

Second thing that I'd like to point out would be that if you have 0.3" / 0.5", 0.4" / 0.4" and 0.5" / 0.3" RA and DEC guide values and you can notice star elongation in your images - well your guide setup is not measuring things correctly.

You are probably using very coarse guide setup - like very small guide scope with short focal length. Are you sure you entered guider parameters correctly in PHD2 (focal length and pixel size)?

Say you have 0.3" vs 0.5" error, you use 6" scope and have good seeing conditions of 1.5" FWHM. That is 12% elongation or 1.12 (one axis is x1.12 "longer" than other). Here is what that looks like:

Do your stars look elongated like that or more? With smaller aperture and worse seeing - they will be even more round as guiding performance will contribute less.

Thanks. I think I've jumped to causation when I really just observed correlation! 

I'm using a 9x50mm finders converted to a guidescope with 120MC/224MC - two separate setups with similar guide scopes give the same guide numbers with the correct settings in PHD2, although the internet suggests perhaps one of my 9x50mm guidescopes is 15mm shorter than the other, so maybe 10% out.

I'll dig out my maths textbook, but I naively assumed double the error meant double the star size. After some googling I realise this is wrong. At worst, perhaps I had 0.25" is one axis and 0.5" in the other.

I've looked back through the subs and realised:

- some of the worst subs in the position I thought was bad appear to have double stars that look like "jumps" in the mount, so I might have some differential flexure/movement in the main OTA that's not in the guide scope. It's a brand new setup with a short focal length (280mm with reducer) and the OTA dovetail mount I got to may not be optimal.

- however, even the best subs have some distortion in the corners but are good in the centre, so I think my reducer is too close. I hadn't realised the flattener spacing was so sensitive, or at least, so far away from the spec, because I measured very closely! It's textbook "too close" according to https://stargazerslounge.com/topic/340444-flattener-spacing-does-it-work/ and the software I use just showed me the top left corner, so the straight line stars looked like a tracking issue.

 

So I think I probably have a few different issues going on and I jumped to the wrong conclusion - thanks for the guidance. I'll just wait another month for a clear sky to try again

 

Edited August 29, 2021 by rnobleeddy

[skybadger]

I wouldn't be happy with the advice to calibrate PhD at dec0 myself, the mount imperfections on either side of the pier are greater than the calibration imperfections . I know I get much better guiding if I do a quick recalibrate near the target, as long as the target isn't too high in dec, if it is, pick a target with a lower Dec but on the same side of the pier. 

[vlaiv]

1 minute ago, skybadger said:

I wouldn't be happy with the advice to calibrate PhD at dec0 myself, the mount imperfections on either side of the pier are greater than the calibration imperfections . I know I get much better guiding if I do a quick recalibrate near the target, as long as the target isn't too high in dec, if it is, pick a target with a lower Dec but on the same side of the pier. 

High DEC lowers your calibration precision significantly. You can also choose side of pier when calibrating near DEC 0° as well.

Not sure what you mean by mount imperfections on either side of pier and how it relates to good calibration of guide system?

[skybadger]

Things like the difference in balance due to asymmetry in mounting and less than perfect 3d balance causing different loadings on the gears dependent on its position in the sky. I agree that perfect mounts would take best calibration from Dec 0 due to the cos factor but , for me at least, I get better calibration nearer the target. 

[alacant]

23 hours ago, rnobleeddy said:

0.3/0.5" in PHD2 in RA/DEC in one position, 0.4/0.4" in another, and 0.5/0.3" in

Hi

If you are correctly balanced, that should not happen. Remember that there are two corrections needed to balance DEC, compared to just one for RA. 

How are you balancing each axis?

Cheers

[ollypenrice]

As well as the size of the circle described by the star, we also need to think about the degredations of the atmosphere. If the celestial equator happens to be very low from your location it may be better to calibrate higher in more stable seeing.

Olly