Barlow a short guide scope?

[kalasinman]

OK, not happy with the FOV limitations of my OAG, but love the guide accuracy. Tried a 50mm f/3.5 guide scope I recently bought for a wide field rig. Great FOV , but very dodgy guiding. Imaging scope 750mm , guide scope 160mm. So why not 3x Barlow the guide scope? That would bring it near 500mm FL and still be somewhat undersampled, but not extremely so.

I considered a 400mm guide scope, but total near $300 with shipping and hardware. It's almost 2.5 k also and the 50 is .5k. 3x Barlow from GSO is $40 plus shipping.

Could use the 50mm for both rigs, as I have only one mount.

On balance, attractive strategy. Am I missing something?

[Xplode]

At 750mm focal length you should be ok with guiding at 160mm since you will guide at subpixel accuracy, i have hade an accuracy of around 0.15pixels in PHD2 which means  guiding would be ok to around 1500mm focal length main scope with a 200mm guidescope.

Could it be that there is some flex or some other problem making the guiding crappy?

[kalasinman]

At 750mm focal length you should be ok with guiding at 160mm since you will guide at subpixel accuracy, i have hade an accuracy of around 0.15pixels in PHD2 which means  guiding would be ok to around 1500mm focal length main scope with a 200mm guidescope.

Could it be that there is some flex or some other problem making the guiding crappy?

It is difficult to work with results in pixels as the pixel scale must be figured in order to understand results. This is why I use arc-seconds.

Of course there could be many factors contributing to poor guiding. My post, however is addressed specifically to the effect of introducing a #X Barlow into the system as a way of improving guiding.

[Xplode]

By biggest concern about using a  3x barlow is i think you will struggle a lot with finding guidestars, you will have a larger FOV than with the OAG, but since the scope is so much smaller there will be less stars to choose from

[kalasinman]

I appreciate your response, but your terms aren't clear.

I am not interested in comparing between the OAG and anything else. I know what it does and does not do quite well.

I am only interested in this 50mm scope I have with or without a Barlow, and the net effect on guiding.

I have plenty of stars to guide with without the Barlow.

My guide cam is quite sensitive.

I fail to see, other than from a reduction of FOV, why an increase in FL would give me less stars.

If you read the sticky here on SGL about guiding, it is stated that without the Barlow , I am right at the limit of undersampling. That is what I seek to correct.

[ollypenrice]

I think there are two potential drawbacks. 1) The reduced FOV will present fewer guidestars, as Ole says, but you might still fine sufficient. 2) The F ratio will be enormously slowed. Now you might argue that F ratio has no effect on point sources and this would be true if, in a tiny amaterur scope, stars really were point sources. By the time their beam has passed through a fairly rustic lens I doubt that they are and I think the slow F ratio will impact badly on the number of workable stars. I'm only surmising here. I've no first hand experience other than guiding with a 10 inch F10 SCT (carrying an apo piggyback.) It was a lousy guide scope and found very few stars!

My other thoughts are these. Depending on what the problem was with the small guider you might simply multiply it up by the addition of a Barlow. If it were some kind of flexure you could easily make it much worse, I'd have thought. As for sampling, my understanding is that a guider first calculates a centroid for the guide star and then uses that as its reference for the scope's current position. Craig Stark actually recommends softish focus to help in this calculation so our usual desire for resolution doesn't apply when guiding.

I know it isn't what you're asking but I suspect another look at the small guider and its settings might be in order.

Olly

[kalasinman]

Fair points .  I would say that flexure is the most likely candidate as it's mounted in a finder style stalk rather than rings.

The greatest strength of the Barlow idea is low capital at risk. I'll most likely go back to the OAG for now, and keep the 50mm for its' intended use as a guider for a 200mm camera lens.

Even though the graph looked bad, 15 out of 22 5 minute subs looked good with round stars. Keeping with the flexure idea, I'm not correcting for PE at the moment, although the mount will do non-permanent PEC. Seems like when the PE hits, it's going to be magnified by any flexure.

[Tim]

I wouldnt worry about PE, you wont notice it if you are guiding properly, unless your mount happens to be a real stinker.

Stick with the OAG. With a sensitive enough guidecam (not sure what the qhys are like, I use a lodestar) you can guide pretty much anywhere in the sky, even at long slow focal lengths.

Forget the barlowed guider, its a non starter.

[kalasinman]

I do appreciate the opinions. I do find it interesting that the idea of using a Barlow in this way was suggested by a guide scope manufacturer. Has anyone actually tried this?

[ollypenrice]

I do appreciate the opinions. I do find it interesting that the idea of using a Barlow in this way was suggested by a guide scope manufacturer. Has anyone actually tried this?

I'd give more credence to what is said by astrophotographers. If we were to listen to what is said by American SCT manufacturers about DS imaging we'd all be up a gum tree.

Olly

[kalasinman]

Well, this was a EU. manufacturer, but nevermind. I find it interesting that not a single commentator has actually tried this. What does being American have to do with anything?

[Thalestris24]

Hiya

I had a go a while ago with my 60mm guide scope (fl=240mm) and cheap SW Barlow but the outcome wasn't satisfactory. I don't know if it was because of my lp and noisy sky but I couldn't focus properly on guide stars and the s/n was very poor. I decided it wasn't worth spending the time fiddling with. I now have a Celestron Travelscope 70 which only cost me £39 at the beginning of June and for the whole kit including backpack and eyepieces. It was a special offer on Amazon. Almost wish I'd got 2! Anyway, it makes a good guide scope, 70mm/400mm, and is very light Some people have picked them up even cheaper previously but they seem to be back up to £69.99 now.

Louise

Edit: They are only $64.95 on Amazon.com at the moment

[Psychobilly]

 What does being American have to do with anything?

The two main SCT manufacturers used to be Meade and Celestron... Which  were American owned....  Olly has an SCT thing   Cant blame him really...  I fell into the SCT trap and spent quite a bit of time turning it into something "useful" a mount for a frac...

Peter....

[Zakalwe]

I find it interesting that not a single commentator has actually tried this.

Possibly because most people know there is little to be gained? I've never tried commuting to work by pogo-stick either but I know that it is probably possible. I also know that it'd take a long time, I'd look very silly, it'd hurt and going by car allows me to listen to the radio.

Many, many people are using converted 50mm finderscopes (finderguiders) successfully. I have used one in the past and they are easily capable of guiding at 800mm. They do need to be solidly mounted (as does any external guiding solution) to prevent flexure. Finderguiders have a real ace up their sleeve in that they do not (usually0 have focusers on them- the cameras are normally screwed into the barrel with an adapter. Most cheap scopes have wobbly focusers that are a source of flexure.

Using a finderscope bracket is a recipe for disaster, especially as most use a spring-loaded plunger at the rear and a rubber O-Ring at the front. Putting a Barlow onto that will make things worse, not better. I think that you would be far better to use the $$$ to get a proper clamping arrangement. Clamps that sound and lighting guys use for hanging lights off rigging kit are cheap and are rock solid (don't bother with fancy guide rings that have screw adjustments- the wide FoV means that you will never need to move the finderguider). Whatever you choose you are looking for a SOLID mounting system that will couple the finderguider to the main OTA.

[ollypenrice]

Well, this was a EU. manufacturer, but nevermind. I find it interesting that not a single commentator has actually tried this. What does being American have to do with anything?

Heheh, the connection was rather loose, I'll grant you.  I just don't think scope manufacturers are necessarily the best source of information on AP. If you read Meade and Celestron SCT ads you might conclude that the best astrograph in the world was a fork mounted F10 catadioptric... 

For the rest, my experience concurs precisely with Zakalwe's. My ST80s are set up as guidescopes - forever! The de-lensed Barlow bodies (for backfocus) are Araldited into the drawtubes and the drawtubes are screwed down hard. They're not Araldited into position but I never refocus them. I find no need to have the guidescope adjustable, never havng failed to find a guide star in thousands of hours. The one thing that does matter is no flexure.

(Caveat: dual rigs are funny critters and, for these, I do centre the guidescope in the FOV of the two parallel scopes. I've never got to the bottom of this though.)

Olly