View Full Version here: : A idea to improve guide star selection and reduce flex

26-07-2017, 09:24 PM
I have only guided manually and found finding a guide star a pain.

Often I would use a Barlow in an effort to detect star movement better and finding something bright really was never easy.
In addition adjusting the guide scope was a pain even though I developed a bracket that seemed to work better than rings.

So with that in mind my thoughts are these.
The guide scope is bolted to the capture scope as solid as possible to eliminate any possibility of flex.

At the end of the scope in a bracket that I will design later tonight there is a secondary type mirror that can be rotated easily to select a guide star.

I even thought that say imaging an object in the South that via this system I could select a faster moving star in the North which I would think should provide advantage.

Why won't this idea work?


26-07-2017, 09:29 PM
Use a good guide cam like a Lodestar or an ASI and there really is no issue. I use a converted 50mm finder and the Lodestar or ASI and have never once failed to find a star in the FOV. Ever. I also mount it in a 6 point ring system and no flex (system is so light weight).

Using a less sensitive cam like an Orion Starshoot, original QHY 5 or an LPGUIDE have their limitations. Frustratingly so. Let alone a converted Web cam.

26-07-2017, 10:09 PM
I hope to be using a DMK 21AU04 mono camera {640480.

I guess I have to get my head around being able to use 50mm if I am to use rings.

I expect you could use the finder scope.

Still I find ring systems fiddly.

It was great out tonight.
It is a pity I don't have any gear here other than binos.

Thanks for your thoughts Raymo.


26-07-2017, 10:10 PM
No worries Bruce :D:P;)

26-07-2017, 10:50 PM
I use 102 mm f5 as my guide scope and made the carrier plate that goes between the rings of the capture scope myself out of 12 mm aluminium . There's no sag lol.

My capture scope is a 100 mm f9 so it's smaller though not shorter than the guide scope.
I've never had a failure to find a guide star@ 2seconds per frame.

27-07-2017, 05:00 AM
Lewis please forgive me for the unforgivable I did say my head is spinning and there clear proof.


27-07-2017, 05:06 AM
Thank you Jennifer.
It was your photos with the new camera that got me going in part ..I started thinking about how I would like to get one and then getting out here with such clear dark sky...well it all started to get me moving...and I needed.something...the city.life is not good for me.

27-07-2017, 09:34 AM
With a guide scope and reasonably sensitive camera you shouldn't have any trouble finding guide stars. With an OAG at long focal length it can get a lot more difficult...

Wrt the idea of a "secondary mirror," using a guide star too far away from the centre of the imaging FOV is not a good idea. You'll get field rotation.


27-07-2017, 09:48 AM
Thanks Ric...it seemed like a good idea at the time.


27-07-2017, 09:52 AM
Just for reference Alex, the other night with my Lodestar and converted Takahashi x50 finder (I put on a helical focuser), in the relatively sparse area near Spica and in the not-so-wide FOV, I had a choice of at least 8 guide stars (that was with 0.5 sec exposure) - up the exposure to 1 sec or more, and literally take your pick. Slew to M8 for the imaging run, and I had MANY to chose from at 0.5 sec - switched to my usual 1 sec interval for guiding and the choice was endless.

With a Lodestar, I also have with regular monotony been able to view the target (GX, Neb or whatever) in the guide shots, so that confirms my targeting too! I do NOT move my guidescope around - it is ALWAYS aligned with the imaging scope, as I have no need to hunt for a star.

I remember trying to image Eta Carinae years ago with an Orion Starshoot guidecam and being frustrated trying to get a useable star.

Cam sensitivity makes a HUGE difference.

27-07-2017, 09:59 AM
Hi Alex,

The effect of refraction is to "lift" objects in altitude.

The amount of lift from refraction is a non-linear function of the star's zenith distance.

At the horizon, refraction is the greatest, causing an apparent lift of about
half a degree.

At 45 degrees zenith distance, the lift is a couple of arc minutes.

At the zenith itself, it reaches zero.

So for two stars that have different zenith distances, their apparent angular
separation is continuously changing as a function of time as the Earth rotates.

Their instantaneous tracking rates are also continuously varying and are not
the same.

So if you choose a guide star that has an appreciably different
zenith distance to the target and the exposure is long enough, your image
will have trails.

For the same reason, there is no such thing as a "perfect polar alignment".
There are only compromises. Equatorial mounts are an engineering kludge
to help minimize field rotation, not a perfect solution.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Phone +61-2-9457-9049
Fax +61-2-9457-9593

27-07-2017, 10:14 AM
Hi Lewis
I come from manual guiding and using max f/l so even a bright star was dim.

Not moving the guide scope is the best news I have heard all week.

It was a pain.

Thank you for sharing your thoughts Lewis you give me hope and that is the kindest thing one human can do for another.


27-07-2017, 10:28 AM
Hi Garry
Thanks for your wonderful explanation.

27-07-2017, 02:30 PM
Good news.
Sbig have a camera with built in guiding and its only $15,000
Where can I sell a perfectly good kidney.

27-07-2017, 04:19 PM
Unless the guide camera part is in front of the filters, it's not as sensitive as it could be. Older integrated camera + guide camera systems were like this including some from SBIG (e.g. ST8, etc). They probably would be ok for a dark site, but I found it struggled on some filters in the city.

27-07-2017, 04:29 PM
The SBIG ST-8, ST-10 and ST-2000 can be had for around $800 to $1200, and they are dual chip (imaging plus guide chips) depending on owner/seller :) If you get an OSc (One Shot Colour) version, then the dual chip works quite well since it has no filters in front of it.

The ST-8 was my favourite of these old models.