Guide scope, Short VS Long

[steve000]

Hi Guys,

I would like first hand experiences on using guide scopes specifically your experience with short tubes up to 500mm and long tubes up to 1000mm

The aperture should be only around 70-80mm. The guide scope designs are similar to the cheap skywatchers typically the short tubes 70mm x 500mm and the 80mm x 500mm as well as the larger ones like the 70mm x 700 and 900mm. This design.

I ask this because I am planning on purchasing one for first use imaging the eclipse then use as a guide scope. The camera for the guide scope will be a SPC880 and probably an autoguider like the little orions but basically as webcam design.

Non first hand experience will be taken into account but id prefer someone who has actually used one or more sizes and how they felt.

Key points id appreciate covered are ease of finding guide stars, tracking and precision, and how they go with lower quality cameras.

If the points on this topic are comprehensive with Mike's permission I would like to turn it into an article.

Im sure this has been covered around the place, id like to collect up to date info and hopefully some fresh opinions.

Thanks heaps

Steve

In addition to this, opinions on using finders or finder size scopes as guide scopes.

[traveller]

Hi Steve,
do you have a budget? you can pick up a 80mm cheap achro and use that as a guidescope with your webcam, should set you back $200-300 depending what accessories you will need.
But by the time you add up all the bits and pieces, you might consider one of these. (http://www.bintel.com.au/Astrophotography/Autoguiders/Orion-Awesome-Autoguider-Package/404/productview.aspx)
The program that came with the package is easy to use.
My two cents.
Bo

[steve000]

So you suggest a shorty.. What are your experiences using it?

[marki]

I have used quite a few guiding options over the years and will try to list some with my own insights of their effectiveness. All of these were used bar the last with a QHY5 (same as the orion guider).

A meade series 5000 80mm F6 apo.

This gave good results but was a little heavy for the task. The FOV was great and I never had difficulty finding a guide star. It did show signs of flexure unless I bolted it down in a fixed position (no adjustable guide scope rings). To overcome the inability to point the scope I used the orion XY guide star finder. I liked this alot but again weight is a killer and the setup was over the top.

William optics ZS 66mm F5.3 doublet.

This also worked well giving a wide FOV and finding guide stars was never a problem. Again this needed to be nailed to the main rig to avoid flexure so I also used the XY star finder on this. Problem was the focuser had a bad habit of coming loose sometimes delivering interesting guiding corrections. Again over kill.

Stellarview F60M3 finder scope.

I liked this setup as the finder had an adjustable focuser to which meant easy focusing and it only weighed about 380g. Wide FOV meant guide stars were easy to find. Only problem was the mounting system allowed lots of flex to creep in and if fixed solidly to the main rig, it did not have enough back focus for me to use my XY guide star finder. Still a good option.

Orion off axis guider

Not good as it had way too much slop in it and I put it aside quickly.

TS Optics OAG9

This is a great little unit as it only takes 9mm of back focus out. I got my roundest stars using this unit but it does have some flaws. The prism is very small meaning the QHY5 was not really sensitive enough unless the guide star was bright and it only offers a very limited FOV. This is compounded by the very restricted movement of the prism which means you have to re-orient your framing to suit a good guide star, not ideal. As I said this unit delivered my best guiding results to date but is a bit of a pain to use and can be very frustrating if your imaging time is limited and you do not have a permenent setup.

SBIG ST-i with lens kit.

Although I have used the ST-i with the OAG and stellar view (excellent sensitive guider) I am yet to use it with the lens kit due to weather and time restraints. It is a small compact unit with the kit fitted offering a wide FOV (about 1 brick wide by two deep (pointed it at a wall)) so I don't expect finding a guide star will be a problem. It is easy to focus and comes with a very rigid mounting system so hopefully no flex. Best of all it is very light and compact so will be easy to mount any where on the rig to aid in balancing.

The ability of software to precisely calculate the centroid means a fast scope is best as the mount will still correct and you get the widest FOV to find a guide star. Most guide camera's use tiny little pixels so you are unlikely to loose much with a fast scope. I have used the above to guide up to F8.


Mark

[swannies1983]

I image with my 8" Newt, using an old 60mm (focal length 700mm) tasco refractor as a guide scope and an unmodded toucam as a guide camera. So far, there hasn't been a time that I haven't been able to find a guide star. I would say the limiting magnitude of my skies would be around 4.5.

In terms of tracking, have a look through my pics. I can get out to about 15mins (if I push it), with round stars. I have to manually guide in DEC due to issues with my mount. I also tend to get star trails if the object I'm imaging is close to the meridian. This is due to slop in the focuser of my tasco.

[DJ N]

Hi there Steve,

Similarly to Mark, I have just recently gone from using an ED80 as a guide scope to using the TS OAG9. To be absolutely honest, I wish I had done this sooner. Obviously no issue of flexure now (not that I seemed to have it using the ED80) but the biggest thing is that I have gone from a payload of around 14kg to around 10kg (and hence 3 counterweights to 2 on the EQ6).

To date, I have had absolutely no problem getting a guide star (ED120, QHY8L imaging CCD and a DMK21 as the guide camera).

Hope this helps.

Cheers,

Daniel

[steve000]

This is good info guys, keep it comming

[Gem]

I use a cheap (70 pounds in London) 80mm f/5 refractor. Works fine with my QHY5 guider... however, it needs an extender tube since it is designed to have a prism in its normal configuration. Hence, it has a bit of flex at times. Otherwise, great guide scope and at a good price.

[2stroke]

lol got a 70mm meade ds series, haven't test yet but will let you know in time how it goes. I think its going to come down to your wallet, wish i had the coin for a oag9 as everyone raves about them. I've seen a few finders converted into guiders as well, this cold be a very cheap option if your hard up money wise. Planing to do a ccd changer over to the BW with my 900nc for guiding after reading about the results.

[Irish stargazer]

I bolt a WO Megrez 72 onto my C11 and have imaged with and guided them both interchangeably with a DMK camera. Only done this a few times. I would like to get a more sensitive camera though as my DMK is the color version.

[Terry B]

I have used 2 scopes as guide scopes both with the same side by side mount and the same guide camera- a QHY guider.
The first was a 120mm skywatcher acro with a focal length of 600mm. The second is my 127mm APO refractor with a fl of 950mm.
The 127mm is heavier but both were connected the same way. I can always get a guide star with a 0.5sec exposure with either scope. I found though that the longer FL APO refractor does give me better guiding.
Differential flexure was similar for both.
Whilst the guider can do sub pixel guiding it has less problems at the longer FL.

[scagman]

Hi Steve,

Don't know if this is valid or not for you, but do you need to take into account the pixel size on the guide scope/camera combo compared to the imaging scope/camera combo? eg If you're imaging at long focal lengths and guiding at short FL's you may get movment in the long FL thats not seen buy the short FL. Does this make sense? Not sure if I have explained it right or not.

Someone will now what I'm taking about and shoot me down if I'm why off track and hopefully correct me.

Regards.

PS I clicked on your link to your web page in you sig and it points to http://www.iceinspace.com.au/forum/www.starrynightobservatory.com.au.

[marki]

Much of this is taken care of by the software which calculates the movement of the centre of the guide star and generally irons out any movement in a well setup rig.

Mark

[steve000]

Thanks for everyones replies, I originally wanted a 700mm but have gone a 500mm.. Once I have some experience with it I'll do up an artical. thanks heaps.

steve