Finder-Guiders - Friend or Foe?

[OzDave]

For quite some time I had been using a 50mm Skywatcher finder as my guide scope with the help of an adapter from Modern Astronomy to allow my Imaging Source camera to attach. With this setup I had been managing to get what I thought were very nice flat guiding graphs out of PHD. I'd get RMS errors of < 0.2 pixels, but oddly the images coming out of the main camera never really had great looking stars. They were always slightly misshapen somehow. I had been blaming this on wind and other factors.

I had tried guiding via OAG in the past but could never seem to get the OAG par-focal and then had trouble finding guide stars. So I abandoned that route and went down the finder-guider route. I now class this as a bad idea due to lack of experience.

My QSI 683 has a built-in OAG and connecting my Imaging Source camera and getting it par-focal was a snap. The first night I tried it, I was imaging within Cygnus so there were plenty of guide stars to choose from. PHD required a little bit of adjustment because the guiding was now more sensitive. The guiding graphs did not look pretty at all. Nowhere near flat and smooth. I thought it was going to be a disaster, but I kept at it. The result was perfect subs with excellent round stars for the first time ever.

So I think the problem with finder-guiders is that they just don't have enough focal length. My 50mm finder has a focal length of 162mm (I measured it). Compared to an OAG on a 500mm FL scope, that is roughly 3 times difference. Such a short focal length doesn't allow PHD to see much star movement. The guide star has to move A LOT in order for the guide camera to see much. Now I know PHD does sub-pixel guiding and yes it does detect these small movements, but the results speak for themselves.

I think an analogy would be watching to the TV with the volume turned down too low. You can kind of hear and understand what is going on, but things improve a lot when you turn up the volume to an appropriate level. This is what guiding at a longer focal length does. Tinier guide star movements can be detected more easily and converted into appropriate guiding corrections.

So if you're using a finder-guider and not really getting brilliant imaging results, you might want to consider changing your setup. It has worked wonders for me.

Regards,

David

[nmoushon]

What were you trying to achieve, sub length wise, with the 50mm? Are you in heavy LP skies?

[OzDave]

I was trying to get 10-20minute subs with the finder-guider depending on the target. My skies are not light polluted (yet). The glow of the milky way is easily visible with the naked eye.

Maybe that was pushing it for a finder guider and they are ok for shorter subs. All I know is that for longer subs, which I really need to be doing to get the results that I want from my imaging, the finder-guider just wasn't cutting it, but I didn't realise for quite a while. Now I've made the switch, I'll not be going back.

I thought I would tell my experience in case it helps someone else with similar issues.

[ollypenrice]

I haven't used a finder guider because the ST80, F5 and 400mm FL, is so cheap that what is to be gained by running the risk of FL issues?

However, 500mm imaging FL is very short and should be easily guidable with a finder guider. What I don't like on finder guiders is the use of adjustable tube rings. I wouldn't touch these with a ten foot pole. A guide scope needs to be bolted down hard and I would rather use Araldite (and do!) than rely on tightening screws for extenders, etc.

Sub length is not relevant. If you have short term guiding errors you will have them short term, by definition. They won't get any worse between five minutes or ten minutes or half an hour. Such long term errors arise from polar misalignment or possibly from dynamic balance - or cable drag/snatch etc.

Olly

[OzDave]

However, 500mm imaging FL is very short and should be easily guidable with a finder guider.

That's exactly what I thought. And to a certain extent it is true. However, the results I was getting just weren't good. Always oval stars. Moving to an OAG has totally cured this. So either it is the extra focal length making it easier to produce the guide corrections necessary, or I had some kind of consistent flexure issue with the finder-guider. Personally I find the flexure explanation somewhat remote. It is obvious from the guiding graphs that the extra focal length makes a massive difference. It amplifies the guide star errors so that they are more easily detectable and correctable.

I will not rule out that I just didn't have PHD well tweaked for the finder-guider. That is a possibility. I have learnt a lot about how to tweak the settings in doing this switch. But having found a formula that works, I'm not going to go back to the finder-guide to see what I might have been doing wrong.

[JamesF]

I read somewhere that it's perfectly possible to guide up to 1500mm focal length with a finder-guider, but having tried it (because I don't currently have any way to piggy-back my ST80 on the 127 Mak) I have myself been wondering if part of the problem isn't that the stars available are just too small to allow really good guiding, especially at the sub-pixel level.

I'd make up an alternative finder with a 50mm binocular objective but I think they're only about 200mm focal length so that's not a huge improvement.

Try sticking the guide camera in a barlow, perhaps?

James

[RikM]

I use a finder-guider to guide 750mm and 1200mm focal lengths with up to 20min subs. I found that optimising the settings made a big difference when using PHD. One night I had some issues where PHD just wouldn't calibrate, so I tried using Maxim. Much faster to calibrate, nice guide graph and much sharper stars than normal. I haven't looked back since. I bought an ST80 to use as a guide scope, but I haven't felt the need to since getting the settings / application sorted.

[davew]

Hi David,

You haven't said how you attached your finder guider to your scope. The main clue to your problems could have been mentioned in post #4 by ollypenrice.

I use a 100mm f2.8 geriatric camera lens and a DMK double bolted to a plate very firmly using a socket set, not an Allen key ! If I could find an easy way of using more bolts I would. It will guide very well with a 630mm FL scope.

Dave.

[OzDave]

You haven't said how you attached your finder guider to your scope. The main clue to your problems could have been mentioned in post #4 by ollypenrice.

The finder is a standard 50mm skywatcher finder, with the eyepiece removed and an adapter put in put to allow connection of the DMK. Thus the finder attaches in the normal way in the scope's finder dovetail bracket. The finder itself is mounted in Skywatcher's finder holder with the plastic thumb screws. So I guess you can say it isn't bolted or glued down.

I don't really know how much flexure could build up there. I guess it is one possible explanation.

[orion69]

Usually, main problem with finder/guider or any type of guiding scope is not FL, it's flexure which is always present. Sometimes, if guiding scope is well connected to main scope and/or FL of main scope is short, flexure could be unnoticeable but it's always there. Solution is OAG guiding which completely eliminates differential flexure. In my opinion investing in OAG (in any system, especially mirror system) is one of the best investments you can make.

[orion69]

I don't really know how much flexure could build up there.

Well, you could be very surprised...

[ollypenrice]

The finder is a standard 50mm skywatcher finder, with the eyepiece removed and an adapter put in put to allow connection of the DMK. Thus the finder attaches in the normal way in the scope's finder dovetail bracket. The finder itself is mounted in Skywatcher's finder holder with the plastic thumb screws. So I guess you can say it isn't bolted or glued down.

I don't really know how much flexure could build up there. I guess it is one possible explanation.

The standard finderscope attachment system screams 'Flexure' in my ear at the top of its voice and is one reason why I wouldn't want to use the system. You need only a tiny amount of flexure, especially when the FLs are very different. I wouldn't touch adjustable guide rings on a conventional guide scope so I certainly wouldn't want them on a tiny short one. Bolted down hard, though, I would expect it to work.

On the other hand you can go out tonight having had perfect guiding on the same object for the last four nights and it will suddenly be dire. Why is this? Because this whole business is in the hands of unseen witches.

Olly

PS There was a prototype GEM on show a while back from the Astrotrac people. It had a guidescope built in under the saddle plate. What a good idea! What happened to that?

[laser_jock99]

..... Why is this? Because this whole business is in the hands of unseen witches.

Olly

PS There was a prototype GEM on show a while back from the Astrotrac people. It had a guidescope built in under the saddle plate. What a good idea! What happened to that?

LOL

I'm working on similar idea to that- an extended length, double sided Lossmandy plate. The idea being you can mount the main scope on one side and a smaller scope on the 'back/bottom' as it were. More to follow later......

[orion69]

There was a prototype GEM on show a while back from the Astrotrac people. It had a guidescope built in under the saddle plate. What a good idea! What happened to that?

They probably realized that it wasn't so good idea after all... It just wouldn't work with mirrors at all, and also with most refractors. It isn't always guider scope source of flexure, it can also be main refractor, focuser, spacer, rings, you name it.

I find it very interesting how many people don't want to accept OAG as a far superior solution just because they fail to achieve focus in first few attempts or they just heard it's "really difficult"...

[ollypenrice]

They probably realized that it wasn't so good idea after all... It just wouldn't work with mirrors at all, and also with most refractors. It isn't always guider scope source of flexure, it can also be main refractor, focuser, spacer, rings, you name it.

I find it very interesting how many people don't want to accept OAG as a far superior solution just because they fail to achieve focus in first few attempts or they just heard it's "really difficult"...

I use two guidescopes and one OAG. The OAG is on a big ODK. The guidescopes guide refractors. They all work but the OAG can be a difficult. On some targets (recently M74) there can be a shortage of guide stars. If you have very stable setups which you don't change or move then there is a lot to be said for OAGs. I have to have some flexibility in the system because I change the smaller refractors around and this is far easier using a guidescope.

I have no doubt that the built in guidescope would be an excellent idea but would not be suitable for reflectors with mirror flop. For those using refractors I think it would be great, compact and free from flexure.

By the way, the worst (and really the only) flexure I have had was with a Starlight Xpress OAG which had a turret pivoting on the clamp bolt. I had to brace the top of the camera.

Olly

[earth titan]

Try sticking the guide camera in a barlow, perhaps?

James

Now thats something I hadn't considered. Now I'm wondering as I'm dying to have a go with the C9.25 and 314L (against all my better judgement) just for giggles.

[JamesF]

Now thats something I hadn't considered. Now I'm wondering as I'm dying to have a go with the C9.25 and 314L (against all my better judgement) just for giggles.

To be absolutely honest it hadn't occurred to me until just before I wrote it. It might work, especially if you don't go overboard with the barlow. I've not measured, but I'm guessing the Skywatcher 8x50 finder has a focal length of about 180mm. With a 1.4x barlow that would work out at around f/5, so no worse in terms of illumination than an ST80. A 2x would take it up to about f/7, but as long as the sensor was good enough that might well still work out as regards finding guide stars.

James