Barlow A Finder Guider

[Mav359]

I'm just sat in board at the moment playing with stellarium and stumbling around the forum and i just had a thought whilst playing with the reticules & sensors....

Could you put a Barlow lens in front of a CCD being used as a finder guider?

I have a orion finder with a QHY5II-L in it and it all works fine, but i was thinking if you put say a 5X barlow in front of the CCD would the greater magnification mean that it would see movement it wouldn't have seen without it, would the finder guider be then working on even more subtile corrections increasing guiding accuracy?

I have absolutley no need to do this, my 9.25 with its hyperstar is a wider FOV then the finder guider anyway so its doing a great job and i haven't had any issues or reason to change it but i'm just curious as to whether the increase in magnification would help

As i said, im board

 

[wuthton]

Yes it would increase your "magnification" but your focal ratio/speed would be decimated. This would lead to a limited selection of guide stars and a miserable SNR. 

Barlows have no place in guiding. 

[ollypenrice]

In reality, since none of us has the good fortune to be able to afford an F7 (or faster) refractor with a 2 metre focal length, there is no need to use a guidescope of extended FL. If using a reflector, as you really will be doing at longer FL, you should use an OAG which can 'see' mirror movement which would be invisible to a guidescope.

Remember that an autoguider does not guide on a star, it guides on the calculated 'centroid' of a star, and the mighty Craig Stark of PHD fame says that tight focus is not the best for calculating centroids.

In my opinion an amateur refractor FL can be guided by a 400mm FL guidescope in bin 2 quite happily. Reflectors will always be best guided by OAG.

Olly

[Mav359]

21 minutes ago, ollypenrice said:

In reality, since none of us has the good fortune to be able to afford an F7 (or faster) refractor with a 2 metre focal length, there is no need to use a guidescope of extended FL. If using a reflector, as you really will be doing at longer FL, you should use an OAG which can 'see' mirror movement which would be invisible to a guidescope.

Remember that an autoguider does not guide on a star, it guides on the calculated 'centroid' of a star, and the mighty Craig Stark of PHD fame says that tight focus is not the best for calculating centroids.

In my opinion an amateur refractor FL can be guided by a 400mm FL guidescope in bin 2 quite happily. Reflectors will always be best guided by OAG.

Olly

Yeah but this is an SCT with a Hyperstar, OAG aren't an option because the guider would stilck out over the corrector plate

The only real reason it came to mind was if i return the scope to its factory setup the FL is 2350mm @ F10.  The guidescope is good upto a 1500mm FL as far as i am aware so i was thinking that by adding the barlow i could continue to use the finder guider, i suppose at that point an OAG could come into play.

As i said from the start it was really only a thought, the only reason i'd revert back is to do planetary with the scope and that doesn't require guiding anyway.

Cheers chaps, that is my lesson for the day

 

[theastrodragon]

Depends on the ratio between the two systems.

Practical tests and experience seem to show that under typical UK seeing, you want the guider to resolve no less than 4x the size of the main scope.

So for a main scope with 1.5"/pixel, your guide should be no less than 6"/pixel. If it's less than this, a barlow on the guider may well give better guiding, but there are other issues like the guiding software in use, for example. Of course, the barlow will reduce the fov, but if your using a short focus guidescope in the first place this is likely not an issue. A 5x powermate seems a bit overkill, mind.

One advantage of using a short guidescope with a barlow is physical size; the shorter and lighter the guidescope, the less differential flexure.