Do I need a barlow for autoguiding? guiderscope =200mm FL main scope =1000mm FL

[pixueto]

Dear all,

I've been successfully autoguiding with a webcam and a stellarvue F50 finder (which takes eyepieces). The finder has a 200mm FL so I have been using a 2X barlow with the webcam to increase the focal length in the guiding scope (finder). This has represented a real challenge as the chip in the webcam is small and I really struggle to find and centre a bright enough star in the FOV that I can select in PHD.

Now, I've been reading that 200mm FL is good enough for the sensitivity of the camera and software. That would make my life much easier but Is that really so? Would it be any different if I use a dedicated autoguider?

Thanks for your help

[dugpatrick]

200mm should be fine. I have used 175mm guiding with my 1000mm telescope and it work. I prefer to use a ST80, which is 400mm, but I think you should be fine with 200. If you have a good S/N ratio then PHD can detect partial pixel movement, and this helps guiding with a short focal length guidescope.

Doug

[Gina]

A 2x Barlow will reduce your light by 2 stops ie. one quarter of what you get without. I may be wrong but I'd think you would do much better without the Barlow in your guide system. You'll cover 4x the area of sky and get a 4x as bright image.

[stem1989]

I think gina is correct. You can never have too many guide stars!

[Jannis]

I'm using an old 135mm f/3.5 olympus lens for my SPC900, and it guides my 1000mm scope very well.

[russellhq]

This is the way I look at it:

First, work out the ratio of your guide scope to your imaging scope. Do this with the following formula:

[(pixel size of guide camera)*(Focal Length Imaging Scope)]/[(pixel size of imaging camera)*(focal length guide scope)]

This tells you for each pixel your guide star moves how many pixels a star moves on your imaging camera.

For example, on my set-up it works out as [(6*1200)]/[(5.1*185)] = 7.6

Now, if I assume my seeing limits me to 2 arcsec/pixel then I must be able to keep my guiding within (2 / 7.6) = 0.26 pixels on my guide scope.

If you read off your graph from PHD, that will give you an idea of your guiding error.

[pixueto]

This is the way I look at it:

First, work out the ratio of your guide scope to your imaging scope. Do this with the following formula:

[(pixel size of guide camera)*(Focal Length Imaging Scope)]/[(pixel size of imaging camera)*(focal length guide scope)]

This tells you for each pixel your guide star moves how many pixels a star moves on your imaging camera.

For example, on my set-up it works out as [(6*1200)]/[(5.1*185)] = 7.6

Now, if I assume my seeing limits me to 2 arcsec/pixel then I must be able to keep my guiding within (2 / 7.6) = 0.26 pixels on my guide scope.

If you read off your graph from PHD, that will give you an idea of your guiding error.

Could you please elaborate a bit on that? I don't quite follow the explanation entirely. I'll be using an orion starshooter and a canon 450D which gives me aratio of 5

Many thanks

[pixueto]

The problem is that even with the barlow, I still find myself throwing some subs. I'm concerned that with a 200mm FL things will become worse.

[russellhq]

The problem is that even with the barlow, I still find myself throwing some subs. I'm concerned that with a 200mm FL things will become worse.

Give it a go. I guide with 185mm FL guider and 1200mm FL imaging scope and seem to get on OK.

[Jannis]

The problem is that even with the barlow, I still find myself throwing some subs. I'm concerned that with a 200mm FL things will become worse.

That's odd. When i guided my 1000mm even with a 50mm lens i rarly had to throw any subs. I Guess it depends on wind, vibrations, polar alignmend, guiding settings, balancing scope and gear slack as well - but i can easily take a 15 min exposure now without any startrails at all with my 153mm guide-lens mounted where the finder-scope usually sits.

I am, however, struggeling with the gears on my mount wich i first thought was a guiding problem. I've noticed sometimes, in one axel (dec i think it was), the motor gears will not start to move at all at 2x slew rate before after 0,5 sec. 0,5 sec might not sound so bad, but in my case it makes it so that my scope doesn't correct at all when PHD tells it to - and i get odd images (slight startrails, then suddenly overcorrected when the gear starts turning).

[Sammyb]

That's odd. When i guided my 1000mm even with a 50mm lens i rarly had to throw any subs. I Guess it depends on wind, vibrations, polar alignmend, guiding settings, balancing scope and gear slack as well - but i can easily take a 15 min exposure now without any startrails at all with my 153mm guide-lens mounted where the finder-scope usually sits.

I think that is probably more to do with the difference in mount EQ5 vs HEQ5

[Jannis]

That could also be of course, but i don't think the mount itself would require longer focal lenght on the guide-scope. Though, before my guiding, i struggeld to get 45-60 sec with no trails.