Longer Guide Scope, better guiding??

[ollypenrice]

2 hours ago, Peje said:

What sort of RMS figure are you getting in arcsecs?

About a third of an arcsecond. That's with a Mesu 200 which has an excellent natural tracking accuracy, allowing us to use 4 second subs to average out the seeing on the guide star. It's roller drive so there is no backlash. We can easily improve the quoted RMS by using shorter subs but that's a false figure because it's based on the apparent rather than the real position of the guide star. ('Chasing the seeing.')

Olly

[Stub Mandrel]

1 hour ago, ollypenrice said:

('Chasing the seeing.')

Reminds me of 'chasing tenths' (of a thousandth of an inch) when some machinists claim to be working to a resolution better than they can measure in a non-temperature controlled workshop 

For precision you need to know what is the limiting factor in any setup and if the seeing, pixel size or scope resolution are bigger than your guiding accuracy, furtehr improvements to guiding accuracy are not going to make a significant difference to the end result. You can put a telescopic sight on a shotgun but...

[MartinB]

4 hours 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

I guess PHD is looking at the saturation level of the guide star.  A saturated guidestar is a very poor choice since an accurate centroid can no longer be calculated.  Also if the brightness is moving beyond the linear response of the chip, accuracy will be reduced.  So I think it is a case of brighter being better until brighter gets worse!!  

[Peje]

8 hours ago, Starlight 1 said:

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

You aren't finding the mount good or the weather isn't playing ball?

[Peje]

5 hours ago, vlaiv said:

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.

I do recall someone quoting the 0.5 being the max and even having some convincing math to back it up.

OAG definitely interests me, my small sensor would likely lend itself well to it. I heard it was painful to get right though, any thoughts on that? Do you need a specific camera or would the qhy5l-ii style camera and the oag fitting for a filter wheel suffice?

[michael8554]

Hi Pete

I think there's a danger of getting into a viscious circle, or even worse, spiral.

As vlaiv has suggested, there is a minimum achievable arc-sec.

Going to an Off-Axis setup might make it easier to consistently reach the best for your location.

Or not if you often have trouble finding guidestars.

And what is your average seeing in Lisburn? Is it worth chasing the seeing?

Michael

[Peje]

2 hours ago, michael8554 said:

Hi Pete

I think there's a danger of getting into a viscious circle, or even worse, spiral.

As vlaiv has suggested, there is a minimum achievable arc-sec.

Going to an Off-Axis setup might make it easier to consistently reach the best for your location.

Or not if you often have trouble finding guidestars.

And what is your average seeing in Lisburn? Is it worth chasing the seeing?

Michael

I don't think I have ever struggled to find a guide star. Although I do think I see less now that I used to but that could be because I'm shooting dimmer targets than when I first started a couple of years ago.

The seeing here is rarely great, I'd say it is normally less than what I'd be happy with.

[vlaiv]

13 hours ago, Peje said:

I do recall someone quoting the 0.5 being the max and even having some convincing math to back it up.

OAG definitely interests me, my small sensor would likely lend itself well to it. I heard it was painful to get right though, any thoughts on that? Do you need a specific camera or would the qhy5l-ii style camera and the oag fitting for a filter wheel suffice?

I decided that OAG is the way to go for be based on practicality. I was chasing elusive sub 0.5" guiding on HEQ5 for quite some time, and one of the steps was making sure that guider accuracy is up to the task - hence OAG was at the time sensible thing to try out. But when I started using it I noticed it is much easier to use in my case.

I have two imaging scopes and both are "back heavy" when loaded with imaging gear - RC8" and TS80mm apo, so mounting 60mm guider in standard finder position just meant balancing issues for me. When I was using F/6 8" dob - it was front heavy with guider in finder position (It was actually SW dob OTA and dob mounting plates prevented me from having enough freedom to adjust OTA position in mounting rings for eq). Also OAG is quite a bit lighter than guide scope.

Since I don't have permanent imaging setup, I spend roughly about 2 hours for setup each time, and using OAG means long usb2 cable is replaced with really short and convenient usb2 cable that goes from ASI185 (that is the camera that I use for guiding now - I also have QHYIILc but I switched to ASI185 - a bit larger chip and drivers are better) to USB hub on ASI1600 - so less cable snag there, also I can assemble optical train indoors before attaching it to OTA. So overall less fuss to setup.

I have not once had trouble finding guide star with it. I use 3-4s guide exposure, but even with 1s, there is always suitable guide star. I even did not need to rotate it to find guide stars. On RC8" I use binning to lower the noise, because guide precision is just there. In regards to guide precision - I did not gain much there except being sure that resolution is not causing any problems with guiding, and being sure that my RMS values are good and not skewed by low guider resolution (I know that it is not proper measure of quality of guiding, but I was doing some calculations in terms of contributions of seeing, guiding and airy disk size on resolution and overall FWHM of stars and detail in general).

So in general terms of guide quality improvement, only thing that OAG brought over guide scope was differential flex being removed. It was not causing me any problems at the time since I use short exposures (order of 60s), but over the course of evening with guide scope I would notice drift of about 10-15" (which can even be good if one sees it as natural dithering ). With OAG this simply does not happen - last imaging run peak displacement of frames was under 0.5" - so almost no alignment was needed.

I'm going to stick with OAG simply because, for me, it is much simpler to use.

[Peje]

Well the seeing isnt great tonight, I'm using 3.5 sec interval and averaging 0.75" so I guess I really shouldnt complain.

I do have noticable backlash in my RA so I may tune that out since it doesn't involve stripping the mount to pieces.

[michael8554]

22 hours ago, Peje said:

I don't think I have ever struggled to find a guide star.

I was suggesting you might have problems finding a guidestar with an OAG due to their tiny FOV, not with your guidescope setup.

There are reports here on SGL of difficulties, but vlaiv reports he's never had problems finding guidestars with an OAG.

Not many people think of binning as vlaiv does.

Michael

[vlaiv]

22 minutes ago, michael8554 said:

I was suggesting you might have problems finding a guidestar with an OAG due to their tiny FOV, not with your guidescope setup.

There are reports here on SGL of difficulties, but vlaiv reports he's never had problems finding guidestars with an OAG.

Not many people think of binning as vlaiv does.

Michael

I bin guide camera because I don't really sacrifice any precision at 1624mm FL and 3.75um pixel size - bin x2 on this setup gets me just below 1"/pixel guide resolution which would translate to less than 0.1" centroid precision (with assumed 1/10 pixel centroid precision, although I've seen figures of 1/16 being quoted as attainable). Other reason is that I use OSC camera, and with binning no Debayer is necessary so less noise due to that and of course SNR increases when binning anyway.

I even used IR pass filter a couple of times, and was still able to find guide stars without trouble. ASI185 does not have IR cut filter and is sensitive to IR quite a bit so I experimented to see if IR guiding would help with what I at that time believed was seeing induced problems when guiding. It did help a tiny bit but I still was not able to go below 0.5"