Longer Guide Scope, better guiding??

[Peje]

My imaging scale on my main scope is 1.17" per pixel, my guiding on a good night is RMS 0.6" and an average night it's about RMS 0.9" and with poor seeing 1.2" and above.

Previously my imaging scale using the DSLR was 1.47" so this guiding performance was fine but now I'd like a little bit more margin. I was thinking that upgrading my ST-80 (400mm) to the ST-120 (600mm) would give me better guiding performance. A quick google search gave me lots of info but no clear cut answer, it seems the internet is divided about this.

Ironically, I'd like the opinion of people on the internet on this. Thoughts?

[Starlight 1]

No help here Peje, I do rember see a bit by Ianl on phd  that said about scopes and guiding scopes but was some time ago.

[Peje]

The logic makes sense that you are zoomed in more so should be easier to see the errors but if the errors are due to polar alignment then the errors are the same at any zoom level.

It sounds a bit like zooming in more with a rifle-scope, at some point more zoom doesnt make any difference. That being said, I'm not sure...

[Starlight 1]

I think it was in here Pete, 

PHD Guiding Basic Use and Troubleshooting 

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      By IanL, June 20, 2013    

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[Peje]

Quote

So a good rule of thumb is your imaging pixel scale should be no more than four times your guiding pixel scale.

So reading that I am well inside the recommended spec, I am currently 60% of my main scope imaging scale. Upgrading the scope would take me to only 10% difference but I wonder if that's really going to make any difference.

I could just use the £250 for some nice parfocal Baader NB filters.

[Starlight 1]

Well it was the only help I could come up with.

[Peje]

It was a great help, it seems like a very reliable answer to the question

[michael8554]

There was a discussion a while back on guidescope size involving the PHD2 gurus:

https://groups.google.com/forum/?fromgroups=#!topic/open-phd-guiding/xE5SDi5S_1I

Rule of thumb:

0.2 x guidecam pixel size x imaging focal length / imaging cam pixel size

For your Quattro, SX 674, and QHY5-II, it works out to 114mm focal length.

The days of long focus guidescopes are way over with PHD2's sub-pixel guiding.

Using their formula I came down from 300mm to 200mm for a 1280mm imaging OTA, without issues.

And small guidescopes are much lighter too, so less chance of the dreaded flexture.

Michael

[Peje]

Thanks Michael. I think in terms of hardware upgrades for guiding my better option may be a more sensitive camera but even that's probably not worth the expense.

I'm on a pier and flexure seems minimal with my CF scope so perhaps I'll just look at another way to improve my subs.

Pete

[michael8554]

Just seen your setup on your site, you seem to have everything you need to get good guiding.

Assuming you've wrung the last arc-secs out of PHD2, all I can suggest for guiding improvement would be a super-tune of your mount to improve PE, does it have belt drive ?

Think you'll need the ST-120 after all, to balance that huge Moonlight and stepper :->

Michael

[Peje]

12 hours ago, michael8554 said:

Just seen your setup on your site, you seem to have everything you need to get good guiding.

Assuming you've wrung the last arc-secs out of PHD2, all I can suggest for guiding improvement would be a super-tune of your mount to improve PE, does it have belt drive ?

Think you'll need the ST-120 after all, to balance that huge Moonlight and stepper :->

Michael

Watching some video guides on the belt mod at the minute, I'm a little nervous about it but I probably shoudn't since I used to be a mechanical engineer in a former life....

[sloz1664]

Cousin Pete

I supertuned and belt modded my NEQ6 and it was well worth it, and the guiding certainly improved. Using AstroBaby''s super tune guide and Rowan Astronomy's kit and guide you should be well covered.

Steve Lawrence 

[Peje]

13 minutes ago, sloz1664 said:

Cousin Pete

I supertuned and belt modded my NEQ6 and it was well worth it, and the guiding certainly improved. Using AstroBaby''s super tune guide and Rowan Astronomy's kit and guide you should be well covered.

Steve Lawrence 

Thank Steve, you are encouraging me Watching the guide, so far I can mod it on the pier and not even mess up my polar alignment (but I am only at part 6)

Any chance you have figures to for your RMS error before and after the mod?

[sloz1664]

I haven't got actual figures. As you have already mentioned it's all to do with the "seeing" But I would be confident to say around 0.2 - 0.3 pixels RMS better.

Steve

[MartinB]

Another key to good guiding is having a good guide star.  The star needs to be as bright as possible, up to the point where the response of the chip ceases to be non linear.  Having a shorter length imaging scope greatly improves your chances of finding such a star.  You've already discovered that, even with a short focal length scope, your guiding is often "seeing limited".  There's no way of getting around this, short of active optics, but turning down the guiding aggression can stop things getting a whole lot worse.

[Stub Mandrel]

My understanding is that the ability of guide cameras/programs to  'centre' a star is much smaller than the p[pixel scale, say 1/10 of a pixel. I suppose it's like a tap running water into a pair of buckets - you can work out the position of the tap to better than a bucket-width by looking at which one collects the most water

 

[MartinB]

8 minutes ago, Stub Mandrel said:

My understanding is that the ability of guide cameras/programs to  'centre' a star is much smaller than the p[pixel scale, say 1/10 of a pixel. I suppose it's like a tap running water into a pair of buckets - you can work out the position of the tap to better than a bucket-width by looking at which one collects the most water

 

Exactly.  Rather than a pin point the light levels from a star form a distribuation curve which allows the calculation of a "centroid" hence sub arc second guiding from a short focal length scope

[Jessun]

I don't quite see the practical correlation between the guide scope and imaging scope, looking only at the pixel scale. It works well for mathematicians perhaps.

With sub pixel guiding the troubles lie with the other components that sit between the scopes, ie the mount and the scope fittings. A top end mount don't need guiding at all.

On ‎20‎/‎11‎/‎2017 at 23:59, michael8554 said:

And small guidescopes are much lighter too, so less chance of the dreaded flexture.

How would you know that the guide scope is the one flexing? It's differential flexure that is the issue. Two identical OTAs might come up with LESS differential flexure than that of a small and large OTA combo.

/Jesper

[michael8554]

28 minutes ago, Jessun said:

Two identical OTAs might come up with LESS differential flexure

Quite so, they may well both droop the same amount.

Michael

[Jessun]

Well, I was perhaps to hasty in my comment assuming a side by side arrangement, but piggy-backing gives you the point back Michael :-)

/Jesper

[Stub Mandrel]

I use a small guidescope mounted directly on the main scope's mounting rings to get as little opportunity for flexure as possible.

[Starlight 1]

Hope you work it out Peje, after getting the  azeq-6 mount I not have any luck at all. 

[ollypenrice]

My practical experience is that our Lodestar/ST80/PHD2 happily guides our TEC140 imaging at 0.9"PP.

22 hours ago, MartinB said:

Exactly.  Rather than a pin point the light levels from a star form a distribuation curve which allows the calculation of a "centroid" hence sub arc second guiding from a short focal length scope

Martin, this seems reasonable to me but when I ask PHD2 to auto-select a guidestar it rarely picks the brightest. I don't understand what's going on here. If I suspect we might lose clarity in the sky I over-ride the choice and pick a brighter star.

Olly

[Peje]

1 minute ago, ollypenrice said:

My practical experience is that our Lodestar/ST80/PHD2 happily guides our TEC140 imaging at 0.9"PP.

Martin, this seems reasonable to me but when I ask PHD2 to auto-select a guidestar it rarely picks the brightest. I don't understand what's going on here. If I suspect we might lose clarity in the sky I over-ride the choice and pick a brighter star.

Olly

What sort of RMS figure are you getting in arcsecs?

[vlaiv]

1 hour ago, Peje said:

What sort of RMS figure are you getting in arcsecs?

I believe you will not be able to go reliably below 0.5" RMS no matter how much of hyper tuning and belt modding and you do to your mount, or using different guide setups. It is the stepper motor resolution and electronics that prevents this on HEQ5 / EQ6 family of mounts. With belt mod, and hyper tuning (replacing all bearings with high quality ones and lubing & tightening everything up properly) you can expect guiding performance to be 0.5-0.6" on really good nights, with occasional drop below 0.5" on exceptional nights (but still in range 0.4-0.5" RMS and not below that).

On topic, yes, one can guide OK with even small guide scopes - I was using 60mm F/4 - 240mm FL guide scope with 3.75um pixels and it works ok, but once I switched to OAG I have no plans going back.