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OAG or separate guide scope?


johncneal
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I'm probably going with a EdgeHD 8 on an NEQ6 but can't find much on the relative merits of using an AOG rather than a separate, say, 80mm guide scope. Either way I was thinking of the Lodestar as the guide camera.

Also, anyone any perspectives on whether to let the Lodestar guide the mount directly or to PHD or similar instead?

Input appreciated.

John

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An OAG has a number of benefits:

Lower overall weight

Reduced flexure (no rings, focuser tube to sag)

The big one is that it will adjust for any mirror sag (though the Edge OTA has mirror locks which will help a lot).

The disadvantages are:

More difficult to set up (getting the guide camera and imaging camera both focused and retaining the focal reducer to sensor distance can be tricky)

Reduced number of guide stars.

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An OAG has a number of benefits:

Lower overall weight

Reduced flexure (no rings, focuser tube to sag)

The big one is that it will adjust for any mirror sag (though the Edge OTA has mirror locks which will help a lot).

The disadvantages are:

More difficult to set up (getting the guide camera and imaging camera both focused and retaining the focal reducer to sensor distance can be tricky)

Reduced number of guide stars.

Add to the disadvantages: Make sure that your OAG can reach focus with the guide camera at the same time that the telescope snaps into focus, I found that with my OAG, I think that it is an OVL one, and a filter wheel and reducer I could not reach focus with my imaging set up. The write up on  the FLO site also indicates that if a reducer/ filter wheel is used one is expected to do a lot of fiddling to get the spacer distance correct for the guide scope focus.

A.G

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Add to the disadvantages: Make sure that your OAG can reach focus with the guide camera at the same time that the telescope snaps into focus, I found that with my OAG, I think that it is an OVL one, and a filter wheel and reducer I could not reach focus with my imaging set up. The write up on  the FLO site also indicates that if a reducer/ filter wheel is used one is expected to do a lot of fiddling to get the spacer distance correct for the guide scope focus.

A.G

Getting the whole thing in focus is a royal PITA (or it was for a noob like me, anyway).

On a SCT it should be a bit easier due to the more generous spacing of 105mm. On my televue TRF2008 there is only 56mm to play with, so getting the OAG, filterwheel, and various adapters in was a pain in the bum. Then getting the various threads tight and in the correct orientation so the OAG pickup didn't cast a shadow on to the sensor was another pain. I ended up cutting a few thin plastic spacers from a notebook cover to get it all lined up and spaced bang-on

th_OAG_zps70947229.jpg

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With an SCT, I'd be tempted to go with an OAG  - the pros outweigh the cons on this type of instrument.

The Lodestar is an excellent choice of guide camera.

Setting up the two cameras (imaging and guide) to be parfocal can indeed be a frustrating process but done in the daytime while focusing on a distant flat object like a wall makes it much easier than trying it on the night sky.

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It's certainly well worth the effort to get an OAG configured, particularly for long focal length imaging.  Fortunately a lot of the initial setup of the spacing can be done during the day by pointing the scope at a distant object.

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+1 for OAG, especially if you have a sensitive guide cam like a Lodestar. I used one on my C11 to help rule out a bunch of issues as per above.

Not used it on my Edge 8 in anger yet as I am waiting for the Edge 8 reducer from Celesetron to come out (don't think it is out just yet? These things seem to take years LOL).

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i went for this http://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html and found that for spot on fine-focusing one of these http://www.firstlightoptics.com/adaptors/baader-t-thread-to-125-helical-focuser.html is absolutely brilliant. You can also take it slightly off focus in a controlled way to smooth out the star shape if you want.

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Thanks everyone for their comments - helpful and constructive as always.

Luke: Celestron do appear to sell a focal reducer for the EdgeHD 8 - not cheap but what is !! Having mentioned this - is this an essential component when using an OAG or is it just a question of using it to get a wider field and/or additional back focus?

Nobody commented on whether to let the Lodestar  guide the mount directly or whether PHD or similar is a better option - or have I got the wrong end of the stick here?

John

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Getting the whole thing in focus is a royal PITA (or it was for a noob like me, anyway).

On a SCT it should be a bit easier due to the more generous spacing of 105mm. On my televue TRF2008 there is only 56mm to play with, so getting the OAG, filterwheel, and various adapters in was a pain in the bum. Then getting the various threads tight and in the correct orientation so the OAG pickup didn't cast a shadow on to the sensor was another pain. I ended up cutting a few thin plastic spacers from a notebook cover to get it all lined up and spaced bang-on

th_OAG_zps70947229.jpg

Hi,

that is very interesting, I have exacthe same, CCD, FF/FR and filter wheel but my guide camera is an ASI 120 MM with a backfocus of 12.5mm for the sensor, I 'd have to try to do a routine at datime to see if I can reach focus with this.

Regards,

A.G

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Thanks everyone for their comments - helpful and constructive as always.

Luke: Celestron do appear to sell a focal reducer for the EdgeHD 8 - not cheap but what is !! Having mentioned this - is this an essential component when using an OAG or is it just a question of using it to get a wider field and/or additional back focus?

Nobody commented on whether to let the Lodestar  guide the mount directly or whether PHD or similar is a better option - or have I got the wrong end of the stick here?

John

PHD all the way, you'll need that to guide but its easy as you like and the best price possible (i.e. free) :)

Will

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Coming from the opposite angle, I tried guiding with an ST80, QHY5 and PHD on an 8" SCT and NEQ6 (no mirror lock-up though) and it was not very successful; I think you've got much more chance of success with an OAG at that kind of focal length.  The other benefit of using PHD is that there is so much advice and help around for it, plus PHD 2 is now being worked on apace, which should further improve matters on that front.

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Nobody commented on whether to let the Lodestar  guide the mount directly or whether PHD or similar is a better option - or have I got the wrong end of the stick here?

 

John

It's not either/or, John; the Lodestar can't do it on its own. You need both a camera, e.g. Lodestar, to image a guide star, and a software auto-guiding program such as PHD (other brands are available) to process the guide-star images and issue drive corrections to the mount.

Adrian

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I'd also recommend an OAG for this set-up.  In the end I think you'll get much better results at the relatively long focal length of the EdgeHD 8.


 


A focal reducer will be very useful too.  It makes guiding considerably easier and will lower the f-ratio to something more usable.  The Optec Lepus reducer is a less expensive alternative to the Celestron one and offers slightly faster optics.  The downside is it has a smaller flat field imaging circle, but it's still big enough for most sensors.


 


Yes setting up an OAG is a PITA, but as has been said, you can do this in daylight (at least to a first approximation) and you only have to do it once.  Another advantage for me is that I can take the whole imaging gear (OAG, guide cam, filter wheel, imaging camera) off of one telescope and drop it straight into another rig with no need for re-adjustment.


 


How easy it is to find a guide star depends on what you are imaging.  It's easy enough for anything in or around the milky way but more challenging for some galaxies in sparser star fields... 


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I chose a camera with built in OAG to keep weight and bulk down, also its all in one box which is a big plus i feel, It was easy to focus and worked a treat first time. I doe use a lodestar which is lightweight and sensitive.

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I use Atik 314, Atik EFW2, Atik OAG and Loadstar on 10" SCT  wasn't a problem to set up, maybe because it is all the same make.

Previously used piggyback guide scope and although I couldn't detect any flexure it's 100% better with OAG.

Dave

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It's not either/or, John; the Lodestar can't do it on its own. You need both a camera, e.g. Lodestar, to image a guide star, and a software auto-guiding program such as PHD (other brands are available) to process the guide-star images and issue drive corrections to the mount.Adrian

Adrian: ok, so that's interesting - how does this work then?

The Lodestar has an ST-4 output on it as well as a USB connector - I had (perhaps, naively) assumed that meant it could send signals directly to the mount rather than the computer directing the mount- is this not the case then? And if the computer has to send the ST-4 signal to the mount what is the purpose of the ST-4 port on the Lodestar?

New to this so happy to be advised.

John

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Adrian: ok, so that's interesting - how does this work then?

The Lodestar has an ST-4 output on it as well as a USB connector - I had (perhaps, naively) assumed that meant it could send signals directly to the mount rather than the computer directing the mount- is this not the case then? And if the computer has to send the ST-4 signal to the mount what is the purpose of the ST-4 port on the Lodestar?

New to this so happy to be advised.

John

If you use EQMOD and enable pulse-guiding then you don't need the ST4 cable.

As I understand it (and I am probably waaay out here) if you select the "On Camera" option in PHD (the guide program) then PHD issues the guide commands to the mount via the ST-4 port on the camera. If you select pulse-guiding in EQMOD, then EQMOD sends the guide commands direct to the mount. Pulse guiding has advantages over ST-4, such as closer integration with PEC.

Edited by Zakalwe
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The SX website (and most dealers) describe the Lodestar as an 'Autoguider', but the user manual (http://www.sxccd.com/handbooks/Handbook%20for%20the%20Lodestar.pdf) calls a 'Guider'.  Now there is a world of difference between the two things.  The accepted definition of 'Autoguider' is a camera with on-board processing plus a connection to the mount (typically ST4) which allows guiding with no computer or other external input.  For example the Celestron NexGuide (http://www.celestron.com/astronomy/celestron-nexguide-autoguider.html) is an example of an Autoguider.

A camera with an ST4 port but that requires external software to use is not (to my mind, and probably to most imager) an autoguider.  It is a guider or just a camera if you prefer.  I think it is obvious this is likely to mislead potential buyers (hence this conversation), whether it is intentional mis-description or not.

The ST4 port might be useful if you are controlling your mount via the handset, but if you are using a mount and computer driver that supports ASCOM pulse guiding, there is no huge advantage to using ST4.  In the case of EQMOD it may be better to use ASCOM pulse guiding, since the software will integrate pulse guiding commands and PEC (if you are using it) to avoid instructions sent for PEC conflicting with guide commands over ST4.

Edited by IanL
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I'll join the chorus in favour of an OAG with your scope. For smaller refractors give me a guidescope every time, but we guide the big ODK with an OAG. You need the focal length and the use of the same light cone to combat mirror movement.

The Lodestar is a great camera but its cable connections are dismal. They are so bad that Telescope Service make a widget to help matters. However, I don't think you need it. I would bring the cables round the Lodestar in a loop and then tape them to the body of the camera. I find it utterly exasperating that SX haven't backtracked on their original mistake in choosing the micro ST4 cable which has absolutely no place out of doors, in the dark, in the damp, on something which moves, as autoguided telescopes have a tendency to do. On top of that, the cables are so close together that when you have to replace the mini USB you will probably find that the one you buy has too much plastic round the bottom of the cable and you'll need to shave some off. For goodness sake, SX, sort it out. This could and should be the last guide camera anyone would ever want or need but I've had three cable failures and a couple of lost nights. Yes, I do this for a living and my kit works hard, but there is no need for this issue to exist.

Olly

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... For smaller refractors give me a guidescope every time...

Olly

Why's that then Olly?  I don't doubt you're right but I am curious as to why!  I swap my OAG off of my SCT and happily on to my FSQ85; it guides just fine and with the shorter focal length I can always find a suitable guide star.  No worries about differential flexure so what's the big advantage of a separate guide scope?

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