Someone should make an OAG/rotator

[codemonkey (Lee)]

I love my Celestron OAG, best OAG I've found so far, but it has one flaw. You can rotate the guide camera and main camera simultaneously by undoing three screws on the "front" side of the OAG, and rotating the whole train.

To rotate the guide cam independently of the imaging cam, you have to undo three screws on the front, and three on the back, which means you have to hold your imaging cam in place by hand while rotating the guide cam. It's hard to keep your composition "just so" while doing that. Especially annoying when you do a meridian flip and lose your guide star.

It'd be great if someone made a rotator with a built-in OAG that could rotate your main cam, while independently rotating the guide cam, all motorised so you could do it remotely...

[Slawomir (Suavi)]

Great idea Lee; however, it would require the pick off prism to be at a relatively large distance from the optical axis, so it would not obstruct main camera's CCD at some angles. Few manufacturers of excellent Astro cameras permanently secure the pick off prism as close as possible to the optical axis and the main camera's CCD for good reasons.
I think an alternative solution (probably also cheaper and simpler than motorised solution) would be getting a very sensitive guide camera so you would always have an adequate guide star available.

[codemonkey (Lee)]

Hmm... not sure why the prism would need to be any further away than usual? Either way, it's pie in the sky, I'm not going to make one so unless someone else does...

Interesting point about the guide cam. I don't think there's much out there that beats the QHY5L-IIM. Peak QE is about bang on compared to the Lodestar, the latter of which seems to get most of the benefit from having bigger pixels, but since you can bin the QHY, I'm not sure how much better it truly is. I did find a thread by the guy that makes Metaguide on Cloudy Nights where he normalised some values in comparing the two (image analysis, not just specs) and found the QHY to be, sensitivity-wise, better.

Having said that, I'm using a NIR filter at the moment and the Lodestar does have better sensitivity there. Either way I'm not sure if I'm going to get enough extra sensitivity to turn a complete lack of apparent stars into ones that I can guide on. I think I'd really just have to get a much bigger chip, which when combined with that kind of sensitivity... I'm probably looking at big bucks, and probably something more traditionally used for imaging than guiding.

[RobF (Rob)]

Where'd you get your Celestron OAG out of interest Lee? I've heard they're a great piece of kit but hard to source in Australia.

[codemonkey (Lee)]

I'm not sure if there's anywhere in Australia that stocks them. I bought mine from the US, from B&H Photo Video. Didn't even know they sold astro gear until then.

[Slawomir (Suavi)]

Originally Posted by codemonkey

Hmm... not sure why the prism would need to be any further away than usual? Either way, it's pie in the sky, I'm not going to make one so unless someone else does...

Interesting point about the guide cam. I don't think there's much out there that beats the QHY5L-IIM. Peak QE is about bang on compared to the Lodestar, the latter of which seems to get most of the benefit from having bigger pixels, but since you can bin the QHY, I'm not sure how much better it truly is. I did find a thread by the guy that makes Metaguide on Cloudy Nights where he normalised some values in comparing the two (image analysis, not just specs) and found the QHY to be, sensitivity-wise, better.

Having said that, I'm using a NIR filter at the moment and the Lodestar does have better sensitivity there. Either way I'm not sure if I'm going to get enough extra sensitivity to turn a complete lack of apparent stars into ones that I can guide on. I think I'd really just have to get a much bigger chip, which when combined with that kind of sensitivity... I'm probably looking at big bucks, and probably something more traditionally used for imaging than guiding.

Hi Lee,

I was thinking that the prism needs to be far enough from optical axis so it does not block light falling on the CCD, in particular at 45degrees relative to the main CCD (cutting a corner); I feel that for larger CCDs that could be an issue. It is great that your OAG gives enough clearance at all angles - so obviously this can be overcome.

Perhaps it is true that QHY5L-IIM is more sensitive than Lodestar, but with 3.75micron pixels vs 8.2-8.4micorn pixels I honestly doubt it, in particular that to my knowledge binning 2x2 does not necessarily increase sensitivity by a factor of 4. Also, not sure whether 12bit with QHY would not induce some limitations as for guiding on brighter stars (saturated stars). I have been using the old version of Lodestar at 560mm fl (f5.6) and always have at least few good stars to choose from, and I image about 2km from Brisbane's CBD.

Another reason I can think of is that the size of a pick off prism can play its part too- a small prism would not fully illuminate the guide chip making some stars dimmer - as would placing the pick off prism far away from the optical axis - vignetting would limit the amount of light hitting the prism plus the stars could be spread over several pixels at larger distances from the optical axis.

[codemonkey (Lee)]

Cheers Suavi. I see what you mean now, hadn't really thought about that. I've only got a small Sony sensor and that's never been a factor for me, but that's probably much more of an issue for those with APS-C sized sensors and larger formats.

Here's a link to the thread I was talking about:

http://www.cloudynights.com/topic/42.../#entry5489591

I actually don't know whether the QHY does hardware binning or software, so there's that as well.

So while the QHY might technically be more sensitive, practically it might work out in favour of the Lodestar, especially in NIR where the Lodestar is going to have a significant advantage.

Thing is though, I'm not convinced that it'll make the difference between detecting no stars and detecting them well enough to guide on.

Just to clarify, are you suggesting that you don't have to rotate your OAG at all to find a guidestar? I always find one, I just have to rotate to do so, and sometimes when I do a meridian flip, I lose it.

[Slawomir (Suavi)]

Thanks for the link to the thread Lee - will check it this evening.

Yes, my OAG is fixed relative to the main camera, and to date I never had to rotate it due to the lack of suitable guide stars. After meridian flip I would select a different star because the camera is "upside-down" relative to its orientation prior to the flip, thus the OAG points at a different area near the main target.

I have a motorised rotator though - it is very convenient (and fun to use!) for planning the photograph and precisely orientating the camera

[rally]

The existing solution works ! - you have a rotator that is positioned in front of the OAG/guide camera and the main camera and with ASCOM and the appropriate image acquisition software, after a meridian flip the software automatically rotates the whole system 180 degrees for you.
This way you keep using the same guidestar and there is no change in position.

SBIG cameras have a fixed position internal guide chip and its rare not to be able to get a good guide star, so the same should apply to any setup if the mount is stable and accurate then you simply keep exposing for longer to improve the SNR of the guide image
Usually >20 SNR is what you want, but on a Paramount for example you can still use a guide star around 10 SNR.

Having a chilled guide camera with a shutter so you can take darks and have them applied automatically during the guiding process is also something that really helps improve the SNR.

Rally

[codemonkey (Lee)]

Originally Posted by Slawomir

Thanks for the link to the thread Lee - will check it this evening.

Yes, my OAG is fixed relative to the main camera, and to date I never had to rotate it due to the lack of suitable guide stars. After meridian flip I would select a different star because the camera is "upside-down" relative to its orientation prior to the flip, thus the OAG points at a different area near the main target.

I have a motorised rotator though - it is very convenient (and fun to use!) for planning the photograph and precisely orientating the camera

Wow, that'd be nice. I've actually only just started doing flips recently, I can count the number I've done on one hand. But yeah, at 840mm with a QHY5L-II, I usually have to rotate to find a guidestar, and I don't currently have a motorised rotator.

Originally Posted by rally

The existing solution works ! - you have a rotator that is positioned in front of the OAG/guide camera and the main camera and with ASCOM and the appropriate image acquisition software, after a meridian flip the software automatically rotates the whole system 180 degrees for you.
This way you keep using the same guidestar and there is no change in position.

SBIG cameras have a fixed position internal guide chip and its rare not to be able to get a good guide star, so the same should apply to any setup if the mount is stable and accurate then you simply keep exposing for longer to improve the SNR of the guide image
Usually >20 SNR is what you want, but on a Paramount for example you can still use a guide star around 10 SNR.

Having a chilled guide camera with a shutter so you can take darks and have them applied automatically during the guiding process is also something that really helps improve the SNR.

Rally

Yep, fair point Rally. The reason I was thinking one such that I described would be useful is for those of us who have to rotate to find a guide star. How would you do that remotely? With existing solutions either you don't rotate and you need to have a sensitive enough cam (or big enough FOV) to get stars wherever you land, or your composition is entirely dependent on guide star availability (because you rotate the whole train to find a star).

Increasing exposure length isn't always an option; fine if you have a premium mount that only needs to correct low-frequency, smooth errors, but for the majority running on workhorses like the EQ6...

So while the existing solution works, it's not without its issues.

[PRejto (Peter)]

Just to play devil's advocate.....why not an ONAG? You get to guide in IR, huge FOV, no need for rotator. If you figure the cost to fit a rotator the ONAG might be less expensive. Then you also get the benefit of auto focusing through FocudsLock. No, I don't work for IF. But, maybe I should!!

Peter

[codemonkey (Lee)]

Funny you should say that, Peter. That was what started all of this. I was thinking about getting an ONAG, but it occurred to me that I didn't know how it provided a bigger FOV as I'd read before, since the FOV should only get bigger if the sensor size changes, or the focal length changes, or if the sensor size remains fixed but it gets more illumination.

The ONAG, according to my understanding simply gives you access to the entire FOV available, as opposed to just the edges that an OAG gives you, but it doesn't make it all available at once, so it seems unlikely to make any significant difference to me... if I rarely found a guidestar, it would make a difference, but I always find one, I just have to rotate... and with the ONAG it means rather than rotating, you move the "stage" around if I understand correctly.

[Eden (Brett)]

Originally Posted by codemonkey

The ONAG, according to my understanding simply gives you access to the entire FOV available, as opposed to just the edges that an OAG gives you, but it doesn't make it all available at once, so it seems unlikely to make any significant difference to me... if I rarely found a guidestar, it would make a difference, but I always find one, I just have to rotate... and with the ONAG it means rather than rotating, you move the "stage" around if I understand correctly.

This is spot on -- it gives you access to pretty much the entire FoV and when you can't find a guide star straight away, you just adjust the X/Y stage. How much adjustment is required depends on where you are pointing, e.g. the availability of red dwarf class stars and the size of your guiding sensor. Peter uses a Starlight Xpress Ultrastar on his rig, which has the more generous sensor size. Some folks use their "old" imaging camera for guiding, eliminating the need to adjust the stage at all!

Give me a holler if you want to try the ONAG out at some stage, I'd be more than glad to send it up to you.

[rally]

Lee,

If you are remote imaging then you will be using the tools that allow you to plan and manage your remote imaging session
So you are selecting your guide star in the planning stages rather than trying to find one by trial and error on the night.

For example - if you are using TheSky with an OAG/guide camera - you either download (if its a standard known arrangement like SBIG cameras) or create from scratch your own camera Field of View Indicator (FOVI) use the create tool
This is a little scaled diagram of the geometry of your main camera and guide camera that has the correct relative positions, sizes and correct image scale for your system - OTA, Flattener, camera etc
Once enabled in TheSky, the good thing about this is that providing you are using ASCOM compliant hardware, the rotator is calibrated to astronomic North, It not only shows you what you are pointing at and what stars the guide camera can see by the overlay on TheSky's planetarium view, it also allows rotation information to be sent from TheSky to your planning software and in real time from the rotator back to the TheSky.
You use the GSC catalog to select a suitable star based on your experience with system - eg magnitude and exposure time and now your imaging session is able to be planned on the desktop beforehand.

There is plenty of info on this availble on the SB site that can guide you through the process.

The difference is you will know before you start - exactly which star you intend to guide on before you do the image session and you will have already calculated the centre of field position and the camera angle of rotation for both composure and for the selection of a suitable guide star - now everything is known and planned

This assumes of course that your mount is polar aligned and calibrated and doesnt suffer from any significant backlash.
A friend of mine was doing this on his EQ6 - so its quite possible.
But if you are remote imaging I am not sure how you would be doing it on an EQ6 - you ideally want a mount that is capable of remote imaging - has a home position and accurate homing process or absolute encoders.

Rally

Originally Posted by codemonkey

. . . .

Yep, fair point Rally. The reason I was thinking one such that I described would be useful is for those of us who have to rotate to find a guide star. How would you do that remotely?

. . . .

[codemonkey (Lee)]

Originally Posted by Eden

This is spot on -- it gives you access to pretty much the entire FoV and when you can't find a guide star straight away, you just adjust the X/Y stage. How much adjustment is required depends on where you are pointing, e.g. the availability of red dwarf class stars and the size of your guiding sensor. Peter uses a Starlight Xpress Ultrastar on his rig, which has the more generous sensor size. Some folks use their "old" imaging camera for guiding, eliminating the need to adjust the stage at all!

Give me a holler if you want to try the ONAG out at some stage, I'd be more than glad to send it up to you.

Thanks Brett, appreciate that mate I've decided at this point to take a pass on the ONAG as I don't think it'll make a enough of a difference to me to justify the expense, especially with the exchange rate the way it is at the moment.

Originally Posted by rally

Lee,

If you are remote imaging then you will be using the tools that allow you to plan and manage your remote imaging session
So you are selecting your guide star in the planning stages rather than trying to find one by trial and error on the night.

For example - if you are using TheSky with an OAG/guide camera - you either download (if its a standard known arrangement like SBIG cameras) or create from scratch your own camera Field of View Indicator (FOVI) use the create tool
This is a little scaled diagram of the geometry of your main camera and guide camera that has the correct relative positions, sizes and correct image scale for your system - OTA, Flattener, camera etc
Once enabled in TheSky, the good thing about this is that providing you are using ASCOM compliant hardware, the rotator is calibrated to astronomic North, It not only shows you what you are pointing at and what stars the guide camera can see by the overlay on TheSky's planetarium view, it also allows rotation information to be sent from TheSky to your planning software and in real time from the rotator back to the TheSky.
You use the GSC catalog to select a suitable star based on your experience with system - eg magnitude and exposure time and now your imaging session is able to be planned on the desktop beforehand.

There is plenty of info on this availble on the SB site that can guide you through the process.

The difference is you will know before you start - exactly which star you intend to guide on before you do the image session and you will have already calculated the centre of field position and the camera angle of rotation for both composure and for the selection of a suitable guide star - now everything is known and planned

This assumes of course that your mount is polar aligned and calibrated and doesnt suffer from any significant backlash.
A friend of mine was doing this on his EQ6 - so its quite possible.
But if you are remote imaging I am not sure how you would be doing it on an EQ6 - you ideally want a mount that is capable of remote imaging - has a home position and accurate homing process or absolute encoders.

Rally

Thanks for the detailed post :-)

My main point was: If you could remotely rotate the OAG and imaging camera independently of each other, you'd be a lot more likely to be able to keep your composition exactly as you'd like, and find a good guide star. I wasn't talking about finding one through trial and error necessarily, and this is true regardless of prior planning or trial and error.

I consider myself to operate in a "semi-remote" fashion because I'm as lazy as anybody you'll ever meet. My gear sits up in the paddock about 300m away from the house, and I'm trying to reduce the amount of time I spend walking back and forth. I seriously spend most of the nights that I image walking back and forth... recently I motorised a focuser which resolved most of that, but there's still some things that could be improved, like being able to keep guiding after doing a flip.

Ideally I'll get it to a point where I can go up once at the start to open things up and turn everything on, and once at the end to pack everything up.