Which mounts get below .5 arcsec error guided?

[uhb1966]

Hi all,

i just read on mounts that

- the usual seeing limits us to about 2 arcseconds anyway, and

- only expensive mounts can get down to or below consistent  .5 arcsec even if guided.

Out of curiousity, which mounts would that be? I am getting with my eq-g / eq6 around 2 arcsec with stretches as low as .7 and as high as 3.0 ...

Edited October 27, 2023 by uhb1966

[Stuart1971]

38 minutes ago, uhb1966 said:

Hi all,

i just read on mounts that

- the usual seeing limits us to about 2 arcseconds anyway, and

- only expensive mounts can get down to or below consistent  .5 arcsec even if guided.

Out of curiousity, which mounts would that be? I am getting with my eq-g / eq6 around 2 arcsec with stretches as low as .7 and as high as 3.0 ...

With some work on the mount, you should get 0.5 RMS easily with the mount you have….maybe some tweaking on the bear engagement, and if you don’t have already the belt mod would be good….👍🏻

[ONIKKINEN]

In good seeing and no wind my AZ-EQ6 is hovering right around 0.5-0.7, but in worse seeing it is probably closer to 1''. If its windy anything goes, the worst night where i didn't immediately pack up my stuff and head back home defeated landed me in almost 3'' rms for the night. That was in 12m/s winds though, which is probably something i wont try again.

If you are frequently having 2'' guiding with your EQ6 when conditions aren't completely busted, then i think you have some issues in the mount itself. The fact that you are having stretches of better and worse guiding suggests that something is not ok in the innards and something can be improved. Time for a strip down and new bearings?

[vlaiv]

I had my HEQ5 guide as low as 0.36" RMS.

Better thing to ask would be - which mounts guide below 0.5" RMS on regular basis.

Here are a few that can do it:

Mesu 200

E.fric

10Micron mounts

Astro-Physics mounts (not sure if all, but I'm certain that some will do that)

ASA direct drive mounts

 

[tooth_dr]

10 minutes ago, vlaiv said:

I had my HEQ5 guide as low as 0.36" RMS.

Better thing to ask would be - which mounts guide below 0.5" RMS on regular basis.

Here are a few that can do it:

Mesu 200

E.fric

10Micron mounts

Astro-Physics mounts (not sure if all, but I'm certain that some will do that)

ASA direct drive mounts

 

Without wishing to jinx myself, my mesu e200 guides at 0.4” RMS or less regularly using an OAG (with say 30kg load and 20kg CWs).  But yes these are expensive mounts. 

[The Lazy Astronomer]

To further what onikkinen said, my eq6r consistently guides in the range of 0.5 - 0.7 rms. In good conditions, and when pointing high in the sky, I've seen as low as 0.4 rms.

One thing to note is it didn't guide this reliably out of the box - dec especially suffered from quite significant backlash, which would often see me guiding more in the range of 0.8 - 1 rms. Fortunately it was possible to tune most of the backlash out, plus I took the opportunity to give the gears and bearings a bit of a clean and regrease, which probably helped some too.

[vlaiv]

5 minutes ago, The Lazy Astronomer said:

One thing to note is it didn't guide this reliably out of the box

Good point. I forgot to say that. My HEQ5 is very far from stock mount. Had it tuned, replaced all bearings for SKF ones, belt modded, changed saddle plate clamp, put it on Berlebach planet tripod

[andrew s]

My Paramount ME II gave about 0.9 arc sec peak to peak after training out the periodic error.

Regards Andrew 

[tomato]

My Mesu Mk1 guides around 0.6 arc sec total RMS but it has a big dual rig on it which is tricky to balance and all up weight is in excess of 100 kg.

[Zermelo]

46 minutes ago, vlaiv said:

Good point. I forgot to say that. My HEQ5 is very far from stock mount. Had it tuned, replaced all bearings for SKF ones, belt modded, changed saddle plate clamp, put it on Berlebach planet tripod

https://en.m.wikipedia.org/wiki/Ship_of_Theseus

[tooth_dr]

21 minutes ago, Zermelo said:

https://en.m.wikipedia.org/wiki/Ship_of_Theseus

Triggers broom more like

[Budgie1]

2 hours ago, vlaiv said:

Good point. I forgot to say that. My HEQ5 is very far from stock mount. Had it tuned, replaced all bearings for SKF ones, belt modded, changed saddle plate clamp, put it on Berlebach planet tripod

Likewise, out of the box my HEQ5 had a lot of play in the motor gears, which was quite easy to tune out and has now been replaced with the belt mod. Over this summer I took the mount apart, cleaned it and replaced all the bearings with better quality ones. After reassembly I adjusted the RA & DEC backlash and then rechecked it again once the mount was back on the pier and had a scope on top of it.

I'm now seeing 0.6 to 0.9 RMS regularly, although I really need to recalibrate PHD2 as I've changed scope & camera since it was last done. 

[blinky]

What I always wonder though - as the op says, in the UK we usually get around 2arcseconds of seeing, so are we actually gaining anything by guiding at something like 0.5?

[tooth_dr]

16 minutes ago, blinky said:

What I always wonder though - as the op says, in the UK we usually get around 2arcseconds of seeing, so are we actually gaining anything by guiding at something like 0.5?

Probably a stupid question, but if the seeing is 2”, how can we guide at 1/2”?

[blinky]

Now with that I have a theory.....  We are taking guide images of say 2 seconds, so the seeing is smoothed out and all we are left with is guide errors - which might actually answer my last question as to why we want to guide as low as possible!

[vlaiv]

2 hours ago, blinky said:

What I always wonder though - as the op says, in the UK we usually get around 2arcseconds of seeing, so are we actually gaining anything by guiding at something like 0.5?

Seeing is expressed in FWHM and guiding in RMS so two are not directly comparable.

What you can do is convert seeing FWHM into RMS and then see how much larger it is than guide RMS.

For Gaussian distribution, conversion factor is x2.355, so FWHM of a Gaussian curve is x2.355 times larger than RMS or sigma of that curve.

2" / 2.355 = ~0.85

or in another words - having guide RMS of 0.85" in ideal conditions (no atmosphere) will produce same blur as having 2" seeing and absolutely accurate mount that you don't need to guide.

I'm always advocating for as low guide RMS as possible - but there is good rule of thumb which says that you need to have it at least half of imaging resolution. This rule of thumb is particularly well suited for 2"/px and 1" RMS case.

It's also worth noting that impact is not as large with small scopes as it is with larger apertures. Aperture again acts to blur the image and we can also "convert" it to RMS.

4" of aperture is roughly equal to ~0.95" RMS.

2 hours ago, tooth_dr said:

Probably a stupid question, but if the seeing is 2”, how can we guide at 1/2”?

When we say that seeing is 2" FWHM - that simply means that with very large telescope (so that aperture does not play a part in star image blur) with perfect mount / or rather in 2 second exposure so that mount issues don't come into equation star image is blurred to the level that it has 2" FWHM in its profile.

That has nothing to do with how well we guide. That is average of star images over two second exposure. Some of it is due to star bouncing, but some of it is just because of other types of distortion (star bouncing around is just "first order" wavefront error - or tilt, but star image is distorted by whole wavefront error and that wavefront error changes roughly every 5ms so in two second exposure we have average of about 400 different distorted star images).

Guiding RMS simply means average error in mount position compared to true star position.

Sure, when there is seeing we have a bit of trouble determining true star position because star jumps around - but:

1. in longer exposure this jumping around tends to average out and star centroid is very close to real star position

2. We can always use longer guide exposure to be more precise about star position (if our mount has smooth enough error)

3. Advances in guiding algorithms now allow to guide on multiple stars - which again adds another layer of "averaging" things out thus getting more accurate star position

 

[uhb1966]

On 28/10/2023 at 12:54, tomato said:

My Mesu Mk1 guides around 0.6 arc sec total RMS but it has a big dual rig on it which is tricky to balance and all up weight is in excess of 100 kg.

OMG what artillery have you mounted? A big bertha probably

[uhb1966]

On 28/10/2023 at 11:42, vlaiv said:

I had my HEQ5 guide as low as 0.36" RMS.

Better thing to ask would be - which mounts guide below 0.5" RMS on regular basis.

Here are a few that can do it:

Mesu 200

E.fric

10Micron mounts

Astro-Physics mounts (not sure if all, but I'm certain that some will do that)

ASA direct drive mounts

 

You#re absolutely right; i added the word consistent for that but i didnt put it in the heading---

[tomato]

34 minutes ago, uhb1966 said:

OMG what artillery have you mounted? A big bertha probably

A couple of Esprit 150s and associated kit.🙂

[AndrewRrrrrr]

my belt-modded and regreased etc neq6 nearly always guides below 0.5"  regular periods in the 0.3's. best ever was 0.22" which I had to do a screen grab of after my jaw had dropped..... 

guiding022.tiff

[900SL]

My Vixen SXD2 guides consistently between 0.4 - 0.9" depending on seeing.

If this gives up the ghost (and that seems unlikely, given the build quality) I'd consider buying an SXP2 belt drive mount as successor. I'm impressed with the Vixen mounts

[Varavall]

My Skywatcher HEQ5 Pro with belt mod and all the bearings replaced with SKL ones, regreased with lithium-PTFE grease, gives me 0.3-0.6 RMS with my 8" Newtonian and the photographic paraphenalia. When doing the upgrade I discovered that the internal attention to detail was lacking in that some components were not screwed down properly and others overtightened. So it's well worth stripping the mounts down to get best performance.