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dan_19991

f ratio for guidescopes

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Hi there, 

Ive recently bought my first proper guide camera (altair GPCAMv2 130 mono) meaning i now have a NEQ6 mount, carrying a 8" SW200p reflecting telescope, a guide camera, and a dslr for the main imaging, but a bit confused about a guidescope, my scope came with a 9x50 straight through finder scope which i could use or i have a 70mm f/12 telescope, which was my first scope.

The thing im confused about is how F ratio of a guidescope affects its guiding capability? would the larger apparture of the 70mm be usable as i have already got guidescope rings for it or is it a better idea to buy an adapter for the camera? 

Thanks 

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I made a scope using an f3.6 lens. Its aperture and focal length (50mm x 183mm) are very close to a 9x50 guidescope so my guess is they have similar f-ratios.

It always seems to find stars without any problems. A longer, slower scope like  the 70mm f/12 might struggle to find bright enough stars in its narrower field of view, however 183mm might be a bit short to guide a 200P well.

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23 minutes ago, Stub Mandrel said:

I made a scope using an f3.6 lens. Its aperture and focal length (50mm x 183mm) are very close to a 9x50 guidescope so my guess is they have similar f-ratios.

It always seems to find stars without any problems. A longer, slower scope like  the 70mm f/12 might struggle to find bright enough stars in its narrower field of view, however 183mm might be a bit short to guide a 200P well.

thanks for the reply, my hope is to upgrade to a st80 or similar in summer but ive just forked out for a NEQ6 and guide camera, so money is a tad tight a the moment, i guess ill try the 70mm f/12 and see how it goes, i just dont understand for f ratios work and what an optimal guide scope would be.

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I think there's no harm in trying a scope with a fl of 840mm. The reason for short focal lengths of guide scopes probably has more to do with fov when using the small chips typically found in guide cams and it's nice to have a decent choice of guide stars. Clearly, some people use oag's which operate with guide cams at the native fl of the imaging scope. Just make sure your f12 scope is well tight with no flex!

Do post how it works for you :)

Louise

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Theory says that F ratio is irrelevant since stars are point sources and, therefore, don't follow the F ratio/exposure rule. It is aperture which determines the number of stars you'll see in a given field of view. In practice, with small basic optics, they probably don't behave strictly as point sources. However, there is no doubt that bigger scopes find more stars. The star count per unit sky is far higher in our TEC140 at F7 than in our Tak 106 at F5. The issue might be field of view but, honestly, I'd be surprised. I'd certainly try the 70.

Ideally any reflector should be be guided by an OAG which can 'see' mirror movement, something which is hard to avoid in such scopes. But would you have the back focus?

Olly

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9 minutes ago, ollypenrice said:

Theory says that F ratio is irrelevant since stars are point sources and, therefore, don't follow the F ratio/exposure rule. It is aperture which determines the number of stars you'll see in a given field of view. In practice, with small basic optics, they probably don't behave strictly as point sources. However, there is no doubt that bigger scopes find more stars. The star count per unit sky is far higher in our TEC140 at F7 than in our Tak 106 at F5. The issue might be field of view but, honestly, I'd be surprised. I'd certainly try the 70.

Ideally any reflector should be be guided by an OAG which can 'see' mirror movement, something which is hard to avoid in such scopes. But would you have the back focus?

Olly

thanks, that makes sense, and im afraid i have little to no backfoucus, im going to barely get a coma corrector in there 

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1 hour ago, Thalestris24 said:

I think there's no harm in trying a scope with a fl of 840mm. The reason for short focal lengths of guide scopes probably has more to do with fov when using the small chips typically found in guide cams and it's nice to have a decent choice of guide stars. Clearly, some people use oag's which operate with guide cams at the native fl of the imaging scope. Just make sure your f12 scope is well tight with no flex!

Do post how it works for you :)

Louise

thanks for the advise, and i certainly will post how i get on when i get the camera and some clear nights, assuming it all works of course.

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3 hours ago, ollypenrice said:

Theory says that F ratio is irrelevant since stars are point sources and, therefore, don't follow the F ratio/exposure rule.

I was so pleased to read this having tried to make this point a while back to someone who came back quoting Dawes Limit and Rayleigh Criterion. In the end I gave up the conversation as I thought I had got it wrong. Thanks Olly.

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With today's guiding algorithms, the software is capable of guiding to within 1/10 of a pixel!

The critical issue is really the size of the guide camera pixel v's the pixel size of the imaging camera.

If both have the same size, then in theory (and usually in practise hence the rise in popularity of the finder-guider) the guide camera focal length only needs to be 1/8 or 1/10 the imaging focal length.

In the bad ol' days when using the Mark I eyeball for guiding we used barlow lenses (x3 in my case) on an OAG to get enough guide control - even then I used to guide to an inside corner of the crosswire (12mm Xwire eyepiece) - hopefully for even better accuracy.

 

 

 

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So from what ive heard the only reason to have a short focal length guidescope is for the extra field of view, in theory the f/12 should work fine just take longer to find a guidestar, at least until i am able to buy an st80 or alternate. thanks all for the help, and once the new guide camera arrives ill be sure to post some pictures. 

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21 minutes ago, dan_19991 said:

So from what ive heard the only reason to have a short focal length guidescope is for the extra field of view, in theory the f/12 should work fine just take longer to find a guidestar, at least until i am able to buy an st80 or alternate. thanks all for the help, and once the new guide camera arrives ill be sure to post some pictures. 

Mainly - but also for ease of handling. A typical guide scope has a fl of 180-200mm and isn't heavy, whereas your f12 would be longer than my main imaging scope! So there could be issues of weight, balance and flexure. Combined with your gpcam you will be guiding at 0.92"/pixel so you might find guide stars are a bit blurry (I'm guessing here!) - depending on where you are and your local sky conditions. But, sure, have a go and report back, please!

Louise

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49 minutes ago, Thalestris24 said:

Mainly - but also for ease of handling. A typical guide scope has a fl of 180-200mm and isn't heavy, whereas your f12 would be longer than my main imaging scope! So there could be issues of weight, balance and flexure. Combined with your gpcam you will be guiding at 0.92"/pixel so you might find guide stars are a bit blurry (I'm guessing here!) - depending on where you are and your local sky conditions. But, sure, have a go and report back, please!

Louise

Thanks for the reply.

Thats a good point which i previously hadnt considered, the weight isnt much of a problem as im stil a fair few kgs under recommended for the NEQ6, and the size, your correct i believe when focused it is geting just longer than my main scope. but the main problem im considering now is flexture especially considering the guiderings are homemade, im currently looking into any ways of reducing flexure. I am a bit confused why you think the stars may be blurry however? and if there would be anything i could do to rectify this (i obviously cant say if your right or wrong for a few days). 

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Hi

Do you have, or can you get, a set of tube rings for the 70mm scope? Then maybe you could bolt directly to the tube rings of your imaging scope. You probably don't want guide rings, as such. I'm not sure how the stars might look - that's something I'd be interested to know!

Louise

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21 hours ago, ollypenrice said:

Theory says that F ratio is irrelevant since stars are point sources and, therefore, don't follow the F ratio/exposure rule.

Agreed as if stars were perfect point sources...

Then pixel size is irrelevant as each star would only hit one pixel.

The signal received by each star's pixel would be proportional to the square of aperture alone.

 

Clearly even  with the most advanced scopes brighter stars look BIGGER as well as brighter.

Why is this as even modest stars that clearly aren't bright enough to fill a pixel well can appear much larger than their supposed airy disk? Twinkling?

 

 

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2 hours ago, Thalestris24 said:

Hi

Do you have, or can you get, a set of tube rings for the 70mm scope? Then maybe you could bolt directly to the tube rings of your imaging scope. You probably don't want guide rings, as such. I'm not sure how the stars might look - that's something I'd be interested to know!

Louise

i dont currently have a set but im sure i could, but im concerned that the small field of view of the f/12 wont be able to consistently give me a guide star. ill be sure to post pictures when i can.

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Don't forget that if lack of stars is a problem, you can increase the exposure length to compensate (within reason). I find with my NEQ6 that a four second exposure is sufficient to maintain good guiding, and no doubt you could go for longer if needed. Obviously you'll eventually reach a point where the guiding inputs are too infrequent to keep up with the errors, but 1 to 2 second exposures can lead to fairly erratic guiding on this mount.

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28 minutes ago, Stub Mandrel said:

Agreed as if stars were perfect point sources...

Then pixel size is irrelevant as each star would only hit one pixel.

The signal received by each star's pixel would be proportional to the square of aperture alone.

 

Clearly even  with the most advanced scopes brighter stars look BIGGER as well as brighter.

Why is this as even modest stars that clearly aren't bright enough to fill a pixel well can appear much larger than their supposed airy disk? Twinkling?

 

 

I've no idea. Twinkling, dispersion, an assortment of aberrations... It's not surprising that scopes don't perform perfectly. But what is clear both in theory and practice is that bright stars are 'less larger' in equivalent quality large scopes than small ones. So far as the OP is concerned, his slow F ratio but larger aperture 70mm scope will probably find more stars per unit sky but see less sky. How will this play out? Only testing will find out.

1 hour ago, IanL said:

Don't forget that if lack of stars is a problem, you can increase the exposure length to compensate (within reason). I find with my NEQ6 that a four second exposure is sufficient to maintain good guiding, and no doubt you could go for longer if needed. Obviously you'll eventually reach a point where the guiding inputs are too infrequent to keep up with the errors, but 1 to 2 second exposures can lead to fairly erratic guiding on this mount.

This doesn't agree with my own EQ6 mount experiences (I don't think! But it's hard to know.) I find 4 seconds ideal with our Mesus but the EQ sixes seem to thrive on more frequent corrections. Trouble is, shorten the guide interval and you'll inevitably improve the guide trace but that's a trace based on the apparent, rather than the real, position of the guide star. The dreaded 'chasing the seeing.' How do we measure the optimal guide sub length? FWHM on the captured stars based on an assortment of guide sub durations/intervals? Is there another way?

Olly

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On 13/01/2018 at 18:58, ollypenrice said:

Trouble is, shorten the guide interval and you'll inevitably improve the guide trace but that's a trace based on the apparent, rather than the real, position of the guide star.

Is that right? I've always found that longer guide exposures give me a slightly calmer trace; because of that dreaded seeing being more "averaged out". I still haven't figured out the ideal interval, though. I find myself frequently changing it throughout a session. Probably to no real effect!

As for guiding with the 840mm guide scope, it's certainly possible but you will find fewer stars - and therefore fewer bright stars - in your narrow FOV. If that's an issue on a certain target, you can tweak the pointing of your guidescope relative to the imaging scope if you have to, but don't go silly far away or you'll end up with field rotation on longer exposures.

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11 minutes ago, Shibby said:

Is that right? I've always found that longer guide exposures give me a slightly calmer trace; because of that dreaded seeing being more "averaged out". I still haven't figured out the ideal interval, though. I find myself frequently changing it throughout a session. Probably to no real effect!

As for guiding with the 840mm guide scope, it's certainly possible but you will find fewer stars - and therefore fewer bright stars - in your narrow FOV. If that's an issue on a certain target, you can tweak the pointing of your guidescope relative to the imaging scope if you have to, but don't go silly far away or you'll end up with field rotation on longer exposures.

I find the difference between 2 seconds and 4 seconds is not that obvious or consistent in terms of RMS error or actual image FWHM on my NEQ6 (with ST80 and QHY5 guiding). What I do find is that the 4s intervals don't suffer from mini-runaway situations - with 2s exposures sometimes the trace can end up going away from the centre line for two or three exposures before it is pulled back in. Occasionally it can then get away from PHD2 as a result for a longer period. With 4s exposures, guiding tends to be more stable and corrections pull back to the centre on the next exposure - the deviation overall tends to be the same (hence similar RMS errors), but I don't get the occasional glitches that I do with 2s.  Ultimately every mount has its own mechanical issues, especially at this end of the market, and you'll have to experiment.

Edited by IanL

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23 minutes ago, Shibby said:

Is that right? I've always found that longer guide exposures give me a slightly calmer trace; because of that dreaded seeing being more "averaged out". I still haven't figured out the ideal interval, though. I find myself frequently changing it throughout a session. Probably to no real effect!

As for guiding with the 840mm guide scope, it's certainly possible but you will find fewer stars - and therefore fewer bright stars - in your narrow FOV. If that's an issue on a certain target, you can tweak the pointing of your guidescope relative to the imaging scope if you have to, but don't go silly far away or you'll end up with field rotation on longer exposures.

I've always found I could get a cute looking trace by using short guide subs but that this didn't equate to the best real guiding as indicated by the FWHM of the image. However, this may just be something that I notice on a couple of mounts and may be partly down to chance. I mostly now use our pair of Mesus and these just don't misbehave so I never experiment with them. Maybe my theory is simply wrong...

Olly

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just thought id update this thread, after having a clear night, i found that in any given position even with the F/12 scope, i would get at least 4 stars that were suitable for guiding, i did however have a little trouble getting it properly focused at first, and also i think it is the camera, occasionally the the noise of the camera would flash up so id get one exposure that was incredibly noisy and PHD would loose the guide star, this did slow and eventually stop towards the end of night but other than the camera being faulty i cant figure an explanation for this.

I i will definitely be looking at getting an st80 or similar to replace the scope but for now its certainly usable, i also found that now my guiding was pretty decent and the stars are looking relatively round, i need a coma corrector as it is now very evident. this is the image i got of M1 the crab nebula at 20 x 500 second subs. 

Crab nebula 18-1-18.jpg

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Late last year there were simultaneous discussions on this very subject on the Open PHD Guiding Forum and on the Stark Labs forums, which had input from the PHD2 coders.

They settled on :

0.2 x guide pixel size x 1000 / imaging pixel size

You haven't specified which DSLR you have, but for 3um the guide focal length would be 250mm, for 4um it's 187mm, for 5um it's 150mm, for 6um it's 125mm.

So the 183mm finderscope, if that is it's FL, is well worth a try.

You will not need adjustable rings to find a guidestar, so bolt it down solidly front and back.

As Louise has said, avoid the weight of an ST80 or the 70mm F12 as you will only get flex - we're talking microns here.

Michael

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Hi

I bought a Celestron travel scope 70 for a mere £39 from Amazon about 2 1/2 years ago - a bargain! Anyway, I substituted it for the TS 50mm guide scope I was using on the 115mm TS APO. That was so I could use the 50mm guide scope on the recently purchased eq3 pro and to guide a dslr and lens. The travel scope is 70mm/400mm and it will be interesting to see how it performs as a guide scope. It's obviously similar to an ST-80 - but lighter :). Unfortunately the mostly cloudy, snowy weather has prevented proper testing. It's terrible how the snow makes the skyglow so much worse! I've been able to have a couple of quick goes and both systems seem to work ok. Calibration was fine and the few single subs I did on both systems were fine. Looks like the snow will be gone by morning but still no sign of clear skies. Oh well.

Dan - yes, focus is quite critical with guide cameras. The 'noise' you saw may have been clouds as it was accompanied by loss of guide star. Guide cams themselves are intrinsically quite noisy which is why it's a good idea to make a set of darks/bad pixel map. They can make the guide cam image a lot clearer.

Louise

Edited by Thalestris24

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16 hours ago, dan_19991 said:

occasionally the the noise of the camera would flash up so id get one exposure that was incredibly noisy and PHD would loose the guide star,

This is what happens if the stars all disappear behind cloud, PHD stretches the starless image (and the noise appears).

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