Advice wanted on manual guiding

[Matthew.Blake]

Given I'm a "pov" and cannot afford a guide scope AND a guide camera, I'm thinking about trying manual guiding with a little travel scope I have (yet to figure out how I'm going to properly piggy back it with my scope without buying a set of guidescope rings). This presents a number of questions:

1) Is it practical to manually guide over a 5 minute exposure?

2) What sort of magnification would I need to see changes small enough that they would affect the picture? I have a 12.5mm reticle eyepiece but the scope I would use is only 700mm focal length.

3) I've always assumed but never asked that the idea of guidescope rings is I can adjust where the guidescope is pointing to get a nice bright star in the centre of its FOV. Is this right?

[FraserClarke]

Given I'm a "pov" and cannot afford a guide scope AND a guide camera, I'm thinking about trying manual guiding with a little travel scope I have (yet to figure out how I'm going to properly piggy back it with my scope without buying a set of guidescope rings). This presents a number of questions:

1) Is it practical to manually guide over a 5 minute exposure?

Retro!

Edwin Hubble used to guide for two nights straight on the 100-inch telescope to get the data that showed the Universe is expanding. So, yes, it's perfectly possible. However it is quite a skill to learn. I've only tried manual guiding once; I made a very nice squiggle line image with the stars

2) What sort of magnification would I need to see changes small enough that they would affect the picture? I have a 12.5mm reticle eyepiece but the scope I would use is only 700mm focal length.

Well in theory this should be OK.

Magnification here is 700/12.5 = 56x. The human eye has a resolution of ~2-3 arcminutes if you've got decent vision (can you split mizar and alcor easily? they are 11-arcminutes apart). So, with 56x mag, that is about 2--3 arcseconds. With a reticule you should be able to see shifts around that level I would think.

What focal length is the imaging scope? I guess you'll end up with guide errors around the ~5" level, so you'd want to set the image scale to match that.

3) I've always assumed but never asked that the idea of guidescope rings is I can adjust where the guidescope is pointing to get a nice bright star in the centre of its FOV. Is this right?

Usually they are used to co-align the guide scope and the main scope -- but yes, you can certainly use them this way if you have enough adjustment.

You might always want to consider just imaging without guiding. For a lot of objects you can take sufficiently short images to avoid guiding, without compromising the image -- look at the images Nadeem (Deneb) takes... That's particularly true for broad-band (RGB) imaging, where the night sky is quite bright. For narrowband imaging (e.g. H-alpha), guiding is a lot more important.

[Matthew.Blake]

The imaging scope is 1000mm F/5. I've found anything above 30 seconds unguided unsatisfactory, partly because of my alignment I think. I find it difficult to get a really good alignment from the hard surface in my back garden as polaris is blocked and my drift alignments suck.

Is there a neccesity to co align the guidescope and main scope? I would have thought it beneficial to be able to use a brighter star to guide that is not necessarily within the field of view of your picture.

[FraserClarke]

Is there a neccesity to co align the guidescope and main scope? I would have thought it beneficial to be able to use a brighter star to guide that is not necessarily within the field of view of your picture.

No necessity -- just useful if you use it as a finder scope as well...

1000mm scope seems a good match to the 700mm guide scope. depending on the camera, you should be 1-2" pixels.