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OAGs and suitable guide cameras


halli

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Hi All

just bought a GSO RC8 and decided to go down the OAG route for guiding.

I was hoping that my ASI 120MC camera which I had used successfully with a ST80 for guiding my 130pds would work ok with an OAG.  I therefore took the plunge and bought an OAG from Astromarket which seemed good value at 122 Euros including a nose piece.  

I had read that to set one of these up was quite fiddly, the main issue being making the guide camera parfocal with the main camera which was a 1100d .

I set to work in my garden focusing on a distant object and found that the ASI camera needed around 21mm or so extra focus distance with my set up to achieve the correct focus to make it equal to the 1100d. It was useful that the RC8 focusser had a mm scale on it which could be easily used to determine the difference between the two cameras.  This was outside the range of what  the OAG offered as an adjustment !  I therefore obtained a 20mm T2 to T2 extender to put between the OAG and guide camera and lo and behold I could achieve parfocus.  

I then turned to the moon to fine tune the guide camera focus whilst having the main camera in focus  already.  I appeared to at least have achieved the first aim in the process.  Now onto a  guide star - almost there I thought  !  But not a chance I cant pick anything up even by rotating  the focuser through 360 deg !

I have just about given up on it and have read similar threads on using the ASI 120MC with an OAG  which doesn't appear to be sensitive enough.

So what do I do? - go back to using the ST80 which would be a shame or spend more money on a better guide camera !   The latter is a possibility but which one ?

It appears that the Lodestar has a great reputation but costs over £400 with the x2 coming in at £450!   I am very reluctant to pay this much for a guide camera only.  

Has any one out there got any views, therefore, on a cheaper sensitive camera that would do the trick with an OAG and a 1.6m focal length scope ?

I guess it would need to be mono with a CCD ?   I would appreciate any thoughts on the way forward.

Thanks in anticipation

Ian

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I use a Lodestar with the OAG and with the 8" RC never failed to find a guidestar. If that's a little too expensive though, have a search on the new QHY5 (which I have read is as sensitive as a Lodestar) or there's a new Altair GPCam that I had read will be very sensitive as well http://www.iankingimaging.com/show_products.php?category=115

Hope that helps. Not an answer as such as I've not used anything but the Lodestar, but some ideas for further investigation. :)

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Over the last 5 years, I have guided with a SynGuider, Atik Titan, Atik GP, ZWO ASI120MM-S and recently a Lodestar X2.  I agree with Sara that Lodestars are great, and my X2 is certainly the best of all of those, rivalled very closely by the Titan (which despite being very good was a bit big for mounting on an OAG).  

I've been forced to abandon OAGing for now due to the size of sensor I'm using, but the X2 is so so sensitive I have no doubt it would be as good on an OAG as it is for me on a finderguider.

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Thanks Sara - appreciate your input

I may have a word with one of the suppliers about the camera your link refers to get their views. Certainly the price seems great. I will also look into the new QHY5 as you suggest. The lodestar certainly appears to be the benchmark the problem is just quantifying the difference in performance with the other cameras to get some certainty about their ability with an OAG before buying one !

Cheers

Ian

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Thanks as well for your response Fatwoul our posts must have crossed!

The lodestar certainly appears to be the market leader but alas is also quite expensive.

I note you have had success with the mono version of the camera I have

Cheers

Ian

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I suggest the Altair Astro GPCAM mono, it uses the same MT9M034M sensor as the newer QHY5L-IIM and will be very similar in sensitivity to the original Lodestar. I have all of those cameras - the Lodestar, QHY and GPCAM (also the older QHY5L-II mono with 1/2" sensor and the colour QHY5L-IIC, both of which are far less sensitive). The advantage of the GPCAM is that right now the introductary price offers a saving of at least £80 compared to the QHY but as the say - once they're gone they're gone! Available now from Altair Astro.

ChrisH

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Thanks Chris -  it may be worth taking a punt at £135  - I guess  you will never know if something will definitely work with your set up unless you try it !

I have noticed that the GPCAM mono uses the same sensor as the mono version of my ASI camera (ASI120MM).  

I guess I should try and evaluate the difference on paper between the mono  MT9M034 sensor which the GPCAM and ASI both use and its colour partner AR0130C which I am using currently and is not picking up guide stars.

On Altair's website, they give the specs for both colour and mono sensors for the GPCAM - the chief difference being the dynamic range as below

Dynamic Range:
GPCAMAR0130C: >85.3dB
GPCAMMT9M034M: >115dB

I'm not sure how the 30db benefit in dynamic range  of the mono version translates into a practical increase in sensitivity but 30dB appears to be significant.

Is any one able to interpret this dB benefit in real terms wrt how well it will see guide stars ?

Thanks

Ian

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If you can get something close to the original Lodestar for £135 don't faff about, grab it! The original Lodestar is the perfect guide cam, bar those grindingly stupid little cables which cause me a lot of grief. (Please don't bombard me with the fixes. I've tried them all and they work for a while and then you take them off and that works for a while, etc etc!)

I don't know the camera in question but Chris seems to know it. 

Olly

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It may be rather old school these days but I've successfully been using a Meade DSI with an OAG on my RC8 for a good few years. They often come up on the second hand market at reasonable prices. Clearly they'll be nowhere near as sensitive as the Lodestar but I've always been able to find a guide star.

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I suggest the Altair Astro GPCAM mono, it uses the same MT9M034M sensor as the newer QHY5L-IIM and will be very similar in sensitivity to the original Lodestar. I have all of those cameras - the Lodestar, QHY and GPCAM (also the older QHY5L-II mono with 1/2" sensor and the colour QHY5L-IIC, both of which are far less sensitive). The advantage of the GPCAM is that right now the introductary price offers a saving of at least £80 compared to the QHY but as the say - once they're gone they're gone! Available now from Altair Astro.

ChrisH

Thanks Chris -  it may be worth taking a punt at £135  - I guess  you will never know if something will definitely work with your set up unless you try it !

I have noticed that the GPCAM mono uses the same sensor as the mono version of my ASI camera (ASI120MM).  

I guess I should try and evaluate the difference on paper between the mono  MT9M034 sensor which the GPCAM and ASI both use and its colour partner AR0130C which I am using currently and is not picking up guide stars.

On Altair's website, they give the specs for both colour and mono sensors for the GPCAM - the chief difference being the dynamic range as below

Dynamic Range:
GPCAMAR0130C: >85.3dB
GPCAMMT9M034M: >115dB

I'm not sure how the 30db benefit in dynamic range  of the mono version translates into a practical increase in sensitivity but 30dB appears to be significant.

Is any one able to interpret this dB benefit in real terms wrt how well it will see guide stars ?

Thanks

Ian

There's a huge difference between the colour and mono cameras in terms of their ability to pick up faint stars, but I;ve never used both in the same role for guiding. I _have_ tried both the colour version of the QHY5L-II (which is the same as your colour cam) and the M034M mono in my AllSky camera, for that I used 3 seconds exposure for the mono, but had to use 10 seconds for the colour and still couldn't capture the faint stars that show with the mono camera. The mono is also sharper, more contrasty, and the noise is noticeably lower. I'm as sure as I can be without actually using one in an OAG that the GPCAM-mono would work perfectly - not as sensitive as the X2 perhaps (which also has larger pixels) but should be a match for the orginal Lodestar. There is a comparative review of the those two cameras (Lodestar and MT9M034M QHY5) on CloudyNights somewhere, the conclusion was that they were equivalent for sensitivity. I'm sorry I don't have that link to hand just now...

ChrisH

OK, found it:  http://www.cloudynights.com/topic/425482-detailed-lodestar-vs-qhy5l-ii-study/?hl=%20qhy5l#8208ii%20%20lodestar

Quite a lot to take in but you would have to read it all to understand what he's on about :-)  Might be more info than you really wanted!

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Hi Chris

I currently had strapped my ST80 to the back of my RC8 to try it out guiding at he next clearing.  However even if it worked, I had come so far with the OAG solution that I felt committed to solve the problem.  I guess thats one of the reasons that some of us folk have taken up astrophotography -  for the technical challenge -  as well as exploring the universe !  The end result then feels far more satisfying if you have solved some  problems on the way !

I feel that using an OAG seems a much more elegant solution and can perhaps lead to better guiding performance which seems to be the holy grail for long focal length scopes like RCs. 

Anyhow thanks for your last post which has helped me to push the button on the GPCAM mono !

Cheers

Ian

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I tend to feel like a good guide camera is actually more important than the imaging camera.

A lower-grade imaging camera can be compensated for, either by giving it more light or in post process. But a cheap guide camera will utterly paralyse a good imaging sensor; without the ability to get the photons on target, what use is all that expensive CCD real estate?

I would invest in a Lodestar, or failing that a new-model cheap mono like the Altair GPUSB; I use a Lodestar and it never fails to find a guide star, even with an OAG. I felt very apprehensive moving to OAG after so many horror stories about invisible guider stars, but the first time I got it focused, everything was totally fine. 

And yeah, as others have said; a colour CCD is *entirely* unsuited to guiding, if it works it is more by luck than anything else :). Colour chips are naturally much less sensitive, but that isn't the only issue. Consider that a CCD can achieve subpixel guiding by comparing the signal from adjacent pixels; if a star moves slightly, the signal bias between the pixels will change predictably.

With a colour CCD each 'pixel' is actually a four-colour array of bins; RGBG. No camera has equal sensitivity to colour and no guide star issues light equally in all three bands. This makes subpixel guiding less accurate.

Get a cheap mono, or bite the bullet and get a standard Lodestar :)

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Thanks Stratis - I also think the same about the importance of guiding.  It is the most difficult system to set  up but most satisfying when it works !  The need to improve the PHD graph and push for longer exposures is one of my main ambitions.  However,  I still feel that £450 for a lodestar is a lot of money for a guide camera so will go for the GPCAM mono and hope for the best.

cheers 

Ian

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I absolutely agree with the theory that a mono guide cam should be more sensitive than an OSC guide cam. Trouble is, I used both (Atik 16ic cameras) together for several years and found no perceptible difference! This may be because I had far more light in my F5 guidescopes than I needed so the difference in sensitivity was never explored.

Olly

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I have both the colour and mono GPCAM cameras (for development purposes with SharpCap).... I tested them against each other at one point and found that I needed 4-5 times longer exposures with the colour camera to get the same level of histogram as the mono camera (on the same subject, which was an indoor scene).

That being said, I've guided quite successfully with the colour camera, PHD2 and a 50mm guidescope.

cheers,

Robin

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I have both the colour and mono GPCAM cameras (for development purposes with SharpCap).... I tested them against each other at one point and found that I needed 4-5 times longer exposures with the colour camera to get the same level of histogram as the mono camera (on the same subject, which was an indoor scene).

That being said, I've guided quite successfully with the colour camera, PHD2 and a 50mm guidescope.

cheers,

Robin

Yes, the more I think about it the more I think I just had a surfeit of light using the colour cam in an ST80. When I first saw the Lodestar's faint stellar images from an OAG on PHD's screen I thought, 'I'll never be able to guide on that,' but if I could see it I could indeed guide on it.

Another OAG parameter to throw into the mix: I've only used an OAG on the very accurate Mesu. On this mount it's possible - and advantageous - to guide on 4 or 5 second subs. This makes the camera's job easier. On mounts needing correction at sub second intervals a poor camera will clearly struggle.

Olly

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I use a neq6 mount and it seems to be happy with 2 to 3 secs exposures using a ST 80 and a colour ZWO 120MC.

However an oag with the 120MC didn't appear to be any good wrt getting guide stars unless of course it was down to my ineptness !

I have ordered the mono GPCAM now so hoping it will pick up sufficient guide stars with the oag. The proof of the pudding ....................

It seems that your 4 to 5 times difference in exposure between mono and colour Robin, roughly lines up with ChrisH's experience above. Well fingers crossed. My only worry is the small size of the sensor in the GPCAM cf a Lodestar and the effect that this will have on peering through an oag on a 1.6m focal length RC8 scope !!

Cheers

Ian

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I use a neq6 mount and it seems to be happy with 2 to 3 secs exposures using a ST 80 and a colour ZWO 120MC.

However an oag with the 120MC didn't appear to be any good wrt getting guide stars unless of course it was down to my ineptness !

I have ordered the mono GPCAM now so hoping it will pick up sufficient guide stars with the oag. The proof of the pudding ....................

It seems that your 4 to 5 times difference in exposure between mono and colour Robin, roughly lines up with ChrisH's experience above. Well fingers crossed. My only worry is the small size of the sensor in the GPCAM cf a Lodestar and the effect that this will have on peering through an oag on a 1.6m focal length RC8 scope !!

Cheers

Ian

The smaller 1/3" sensor will actually work in your favour - it's difficult to properly illuminate with the small pick-off prism/mirror in the OAG at the best of times. Apart from the points mentioned above, the star shape is often not as good with an OAG so with limited choice your're not only forced to use a dimmer star than you would with a guidescope, but also one that is smeared and not round. PHD doesn't care much about the shape because it will calculate the centroid anyway, but the smearing will reduce the peak light intensity and thus the s/n ratio is also reduced. None of this is new information and experienced OAG users take such problems in their stride, but it is extra work. Being resolutely lazy I hate extra work so always preferred to use a guidescope :-)

ChrisH

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I use a neq6 mount and it seems to be happy with 2 to 3 secs exposures using a ST 80 and a colour ZWO 120MC.

Cheers

Ian

Interesting. Our pair of EQ sixes like shorter guide intervals than that, as do our Avalon and Tak EM200, though those can tolerate longer subs. I'm sure this is just the luck of the draw but anyone setting up a system from scratch should bear in mind this variability, I guess.

Olly

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Well I guess I will take the OAG experiment to its end point with the GPCAM and then decide whether  to return back to the ST80 if it gets to be all too fiddly.  Nothing worse than getting frustrated about missing good imaging time when the conditions are right.  I managed to get into a grove with the 130pds with vey short set up time before I could image   I will see how it goes........  If I do regress back to my ST80 I guess the GPCAM mono should be a better guidecam than the ZWO colour so I at least I should get some improvement.

Wrt the exposure setting and mount type I will certainly experiment a bit more with shorter intervals to see if I can get a better PHD performance in the light of your experience Olly.  

I guess a more sensitive guidecam should help me to do that !

I am now feeling better about spending the money on the GPCAM if the OAG route fails  !

Thanks for your input ChrisH and Olly.

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Just my 2 cents. Go for the lodestar guide camera, it is the only option for guiding via an oag with long focal lengths in my opinion. When i started imaging with an oag at my 8'' rc i first started with a qhy5 and i struggled to find a guide star even at dark skies. After a few months of hair pulling sessions i went for the lodestar camera and never went back. At each target i point my scope there will always be a guide star and i have been able to find a guide star even at heavily light polluted skies. Craig Stark at his phd site has a paper that compares various guide cameras, the lodestar is at first place followed by the the atik 16 camera at last place is the qhy camera (the article is old but it is very helpful)

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Halli,

With the camera(s) you have available, have you had ANY success in finding and using a guide star?

I've used many guide cameras in OAG without problems.

Just check what guide images you can achieve - there may be other issues involved.

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Thanks for your input Kookoo and Merlin.   Well I now have a GPCAM mono and am awaiting for a clear sky for its first use - hopefully by this weekend. In the meantime i had resorted back to the trusty 130pds and ST80 combination as I was wasting too much clear sky imaging time on the RC8 and OAG !  

I made a decision to go this way prompted by the ridiculous price of the Lodestar and comments re the GPCAM sensor having an equivalent sensitivity for around £140.   i will soon find out how good it is.  Not sure if I persevered enough with my colour ZWO but one of the problems was the difficulty in adjusting the focus of the OAG I had which had a turret mounted prism controlled by a tiny grubscrew.  I thought I did capture a funny shaped guide star at one point but the guiding wasn't very successful.  I had also downloaded the latest version of PHD2 (v2.5.0) which I subsequently found was giving funny results even on my ST80 which was cured by reverting to the previous version.

The focus of the GPCAM should be easier to adjust as it has a 1.25 eyepiece fitting  - I also thought that if I couldn't get the oag to work effectively at least I would have a better gudecamera for my ST80.  

I have tried the RC8 with the ST80 and the guiding seemed reasonable although it was a temporary lashup and perhaps I was a bit optimistic by starting with 1200s subs !  The stars were slightly eggy under high mag however although hopefully I can improve things with a permanently mounted ST80 arrangement.  However I will be disappointed if I cannot continue with my OAG and the new GPCAM ! 

Cheers

Ian

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