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using a barlow for guiding ?


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Hi all,

I'm wondering if I can use a barlow between my guidescope and guide cam for improved guiding accuracy ?

My guidescope is a Celestron T70 travelscope (70mm ap, 400mm fl, f/5.7 refractor) which sits piggybacked on top of my main scope (150mm ap 750fl f/5 reflector) and I'm currently using a Meade LPI as a guide cam.  It's not the most sensitive camera, so I've not tried barlowing it yet.

However, I've now got a brand new shiny QHY5l on order (yay !) so I'm wondering if with the increased sensitivity of that camera, I could put a barlow in the quiding image train, and assuming I can get focus, will that give me improved guiding accuracy ?  My spare barlow is a bog-standard Celestron 1.25" 2x barlow.

Anyone tried this ?

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I shouldn't think it would make much difference.

Guiding software can measure the position of a star to better than a tenth of a pixel, which with your setup is less than the best seeing would allow (this will improve with the QHY5 over the LPI anyway as it has smaller pixels).

In my opinion it would be more advantageous to have a larger fov for better star choice, and less light loss.

Where I work we use 700mm fl guidescopes for 4064mm fl telescopes, and have never had a problem keeping stars round and small. Past a certain point you're at the mercy of the atmosphere.

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You could but with more focal length you reduce the available choices for good bright guide stars. A guide scope with a focal length of around 100mm -200mm is what you want.  Orion or Zhumell make a decent one. Stellarvue makes a great one.

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Hi all,

I'm wondering if I can use a barlow between my guidescope and guide cam for improved guiding accuracy ?

My guidescope is a Celestron T70 travelscope (70mm ap, 400mm fl, f/5.7 refractor) which sits piggybacked on top of my main scope (150mm ap 750fl f/5 reflector) and I'm currently using a Meade LPI as a guide cam.  It's not the most sensitive camera, so I've not tried barlowing it yet.

However, I've now got a brand new shiny QHY5l on order (yay !) so I'm wondering if with the increased sensitivity of that camera, I could put a barlow in the quiding image train, and assuming I can get focus, will that give me improved guiding accuracy ?  My spare barlow is a bog-standard Celestron 1.25" 2x barlow.

Anyone tried this ?

I use an SW 80mm Equinox for my SCT and 70mm TV Pronto for the refractor, both short f/l scopes. Never felt the need to barlow either and guiding is good enough. More important is to ensure there is no flexure between the two OTAs.

ChrisH

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Guiding software can measure the position of a star to better than a tenth of a pixel, which with your setup is less than the best seeing would allow (this will improve with the QHY5 over the LPI anyway as it has smaller pixels).

In my opinion it would be more advantageous to have a larger fov for better star choice, and less light loss.

Actually yes, good point there.  Doing the sums, my T70 with meade LPI without barlow has 4.1" per pixel (using your excellent fov calculator), and the limiting resolution of that scope (Dawes limit) is 1.65" so I only need PHD to be guiding to 0.4 pixel accuracy to be doing the best job it can do, which it easily does.  As you say, will be even better with the QHY.

Interesting, I never thought that the T70 was almost too long for my guiding - I guess key now is to use longish exposures for the guiding so that I'm not chasing seeing too much, and instead let it average out.  Saved myself some pain trying to use the barlow there.

Less said about flexure at the moment the better tbh, I'm still struggling to get a robust solution.  I have a couple of guidescope rings, but am struggling to mount them to my main OTA tube rings, wrong screw diameters, screw pitch etc etc  - argh

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I wouldn't bother. Your guiding errors (and no guidng is perfect) will not arise from the focal length of your guidescope. They will come from backlash, most likely, and then imperfect communication between mount and software (ie calibration) and finally flexure. Obvioulsy nobody can put these in a definitive order for a rig they haven't seen but I'm giving you my best guess. For me backlash has been the root of all evil which is why I like zero backlash systems like Mesu and Avalon.

Olly

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i use a microsoft lifecam 300 hd on the 50mm finder to guide my skywatcher 200pds mounted on the az eq6 gt and it works beautifully, but when it was all on the celestron cg5 gt the weight was to much and it suffered guiding errors and backlash i would lose around 60 percent of the subs i would take. since upgrading to az eq6 gt  i have'nt lost more than 5 percent due to guiding, only from human error.

so to me i needed a better mount.

with your setup i would just try a 50 mm finder as a guidscope. it will cut down the weight and give you a wider field of view.

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