200 replies to this topic

[John Verderame]

I've been doing a good bit of research on this, and am a little frustrated, so any help will be much appreciated.

 

Wanting to simplify life a bit, I'm pretty much down to just one scope for imaging now, my 9.25" EdgeHD.  I want to concentrate on DSOs, particularly some of the more difficult ones, to get out of the envelope of just shooting the "usual" stuff.  Currently I'm using a 50mm Stellarvue F050G for guiding on my CGX, and on good nights of seeing I can get as low as .5 - .6 arcseconds RMS using PHD2.  Normally seeing here (Green Valley, Arizona), despite a good dark sky, is usually pretty average, so more like .8 - .9 arcseconds on a normal night.  I'd like to do better, but I definitely do not wish to go the OAG route.  Been there, done that, did not like it at all, and I'm getting nice star images even with 3-4 minute exposures.  Longer than that starts to get iffy.

 

I get excellent star fields and pinpoint stars with the SV50, but it only has a focal length of 210mm.  Using a Nikon D810 for imaging, and doing the calculations, my imaging/guiding ratio is worse than 1:6.  So, I assume if I can bring that down closer to 1:3 I can get even better guiding and imaging results.  I have tried an Astromania 60mm (240mm f/l), but was not real happy with the star images and field.  I tried an Astro-Tech 72ED, and it needed an extension tube to come to focus so I never used it.  My current setup is rock solid - I've done everything possible to eliminate flexure and am sure it has worked, so I have no desire to add an extension tube.

 

Again, no desire for OAG either.  I'm looking for suggestions for a good, solid guidescope with around 400mm focal length, for a few hundred dollars at most.  If it turns out that my SV50 is the best I'll be able to do, so be it.  It's a great scope.  Of course, another issue at the present time is availability, but please don't let that be a consideration.  Any and all suggestions welcome (EXCEPT OAG! ).

 

Thanks for your time!

[Notdarkenough]

I, too, am not an OAG fan. Mine is back in the box. Regarding a guidescope, I assume you are looking at a small fraction?  Do you have any weight restrictions in addition to the budget, or budget constraints only? Lastly, what Guidecam are you using? 

[Tapio]

I'm not sure if you can get better guiding with longer fl guide scope (but I'm interested if you can make comparison ).

1/6 ralation works fine nowadays.

[ETtheExtraterrestrial]

I’m curious why the not a fan of OAG?

[John Verderame]

My apologies on the guidecam: ZWO ASI290MM mono.  I have it threaded directly onto the SV guidescope.  Love that feature, and it's a great cam.

 

The only weight restriction I have is my increasing inability to lift heavy stuff cuz I'm gettin' old   But I've still got enough juice to bring everything outside and set up and break down each time I go out.  However, the 50mm SV being nice and small, I'm kinda spoiled with it and don't want to get too large a guidescope.

 

I tried a nice quality OAG.  It was a pain in the butt, at least for me.  Even tho' I have dark sky, it was difficult to find a good guide star and I did not like having to rotate it for that little prism to pick something up, while simultaneously having to watch the computer screen to see if I found anything.  With the guidescope, I've immediately got an excellent star field right on my screen, with lots of choices for guide stars.  With virtually zero flexure, works fine for my needs.

 

Another thing is, one less item hanging off the back of the scope.  The way I have it, the guidescope, along with its clamps and rings, acts like a counterweight for my T-adapter and imaging camera, as I have it moved forward on a mounting rail.

[KTAZ]

Keep the SV50 or go OAG. The OAG is a solid and well machined assembly with plenty of adjustability. I love mine and would never consider a independent guide scope on a large SCT.

 

As for those that say they just lock their mirror, that doesn't make sense either. With the OAG, I can set up a multi hour session, setup my autofocus routine (can't do that with a locked mirror, unless you spend bookoo bucks on a separate focuser), and run right through the meridian flip without having a little mirror flop screw up my session.

[John Verderame]

Diff'rent strokes!  Not at all interested in another OAG.

[drd715]

While an oag is probably going to track better than a guide scope at such a long focal length you can use a separate guide scope.  Im using a 90mm refractor At 700mm focal length on a 6 inch 1200mm refractor.   This works well for me.  Tracking really becomes challenging at 2800mm.  Most imagers tend to use oag guiding on focal lengths longer than 1500/2000mm.  

[Blackhawk163]

I recently bought a celestron 80mm guidescope for $100 for my edgehd 925. I haven't tried it though. I don't like the rings on it and it needed an extension tube as well. So now I'll have to worry about flexing I guess. I really didn't like my oag either...

...Well, that is until everything started to click with the oag. For some reason now, I'm getting great guiding here with it. Usually around .3-4 depending on seeing, average nights I'm at .5-6. All this with a single star at times.

That being written, I still plan on using the 80 just to see if it's actually worth it.
I use the svbony 60mm (240mm) when I'm using the hyperstar, oag at 1645mm f/l and I'll be trying the 80mm (600mm) soon.

[KTAZ]

While an oag is probably going to track better than a guide scope at such a long focal length you can use a separate guide scope.  Im using a 90mm refractor At 700mm focal length on a 6 inch 1200mm refractor.   This works well for me.  Tracking really becomes challenging at 2800mm.  Most imagers tend to use oag guiding on focal lengths longer than 1500/2000mm.  

He is at 2350mm focal length.

[John Verderame]

<<you can use a separate guide scope.>>

 

I AM using a separate guide scope.  The question is, is there a better guidescope option out there for my setup than what I'm using.  My search for one has been frustrating.  Not at all interested in using an OAG, and based on my research here and elsewhere over the years, there are plenty of folks out there who do not use OAGs, even for longer focal lengths.  Separate guide scopes have served me well with every scope I've used them on.  The trick is getting a good star field, and eliminating flexure.

 

Appreciate the responses, but can we please not discuss OAGs any more?   

[John Verderame]

I recall reading some unfavorable stuff about the Celestron 80mm (sorry!), so that one's not a consideration.  Perhaps one of them was that it needed an extension tube, I don't recall.  But yes, that would definitely knock that one off the list!

 

Oh, and as KTAZ noted above, the 925 EdgeHD is at 2350 f/l.

[drd715]

He is at 2350mm focal length.

At 2350mm a guide scope in the 700/800mm size should work.  I don't think I would go shorter than 600mm.

[John Verderame]

I'm not a big math guy, so I use the guidescope suitability calculator found here:  https://astronomy.to...ope_suitability

 

According to that, a guidescope with around a 400mm focal length should work fine, giving me about a 1:3.5 imaging / guiding ratio.  So, I'm thinking something in the 400-500mm f/l range should work great.  There doesn't seem to be anything out there in that range that's 1) not expensive; 2) doesn't need an extension tube; 3) would get a quality star field.  The closest I've come is an Astromania 70mm (400mm f/l), but I do not trust the optical quality of it (both from personal experience and that of others).  Some Astromania users have even found that the objective lenses were in backward, and they're basically the generic type that are rebranded under different marketing names.

[ram812]

I'm not a big math guy, so I use the guidescope suitability calculator found here: https://astronomy.to...ope_suitability

According to that, a guidescope with around a 400mm focal length should work fine, giving me about a 1:3.5 imaging / guiding ratio. So, I'm thinking something in the 400-500mm f/l range should work great. There doesn't seem to be anything out there in that range that's 1) not expensive; 2) doesn't need an extension tube; 3) would get a quality star field. The closest I've come is an Astromania 70mm (400mm f/l), but I do not trust the optical quality of it (both from personal experience and that of others). Some Astromania users have even found that the objective lenses were in backward, and they're basically the generic type that are rebranded under different marketing names.

Will the Orion ST80 (400mm) work with that camera directly threaded to it? I use the SSAG with mine and also have to use an extension, but does come up with plenty of stars to choose from. I used to use it on my XT10 (1200mm F4.7) but downsized to the Orion 60mm Helical Guide scope for weight. I have also successfully used it on my AT6RC (1350mm F9) and have since come to the conclusion the best mounting for a top-mounted guider on an OTA is with fixed scope rings. I just don't trust those plastic tipped adjustable ring sets. I also, since having adjustable guide scope rings have yet to adjust them at all to find a suitable guide star to lock on to😁. So next time out I'll test the theory of fixed rings, (which I have both) vs. adjustable- easier to balance with a lower profile GS on top vs. a vixen rail w/ ADM ring mount brackets higher above the main OTA (AT6RC, XT10) (Lower profile/ center of gravity). If properly torqued, my extension tube (Which have been modified by my drilling and tapping a 3rd. torque locking screw equa-distant on it from the other 2). Lots to think of when mounting a larger guide scope on top of our kit!

Good luck in your hunt for the "Perfect" guide scope!😁

CS, Ralph

[Tapio]

Unless you are going to image with guide scope, you don't need 'quality star field'.

[Notdarkenough]

Sounds like no one has a better guidescope suggestion. If you find something, please share! I'll keep looking as well and PM if I find anything.

Mike

[Sacred Heart]

I've been doing a good bit of research on this, and am a little frustrated, so any help will be much appreciated.

 

Wanting to simplify life a bit, I'm pretty much down to just one scope for imaging now, my 9.25" EdgeHD.  I want to concentrate on DSOs, particularly some of the more difficult ones, to get out of the envelope of just shooting the "usual" stuff.  Currently I'm using a 50mm Stellarvue F050G for guiding on my CGX, and on good nights of seeing I can get as low as .5 - .6 arcseconds RMS using PHD2.  Normally seeing here (Green Valley, Arizona), despite a good dark sky, is usually pretty average, so more like .8 - .9 arcseconds on a normal night.  I'd like to do better, but I definitely do not wish to go the OAG route.  Been there, done that, did not like it at all, and I'm getting nice star images even with 3-4 minute exposures.  Longer than that starts to get iffy.

 

I get excellent star fields and pinpoint stars with the SV50, but it only has a focal length of 210mm.  Using a Nikon D810 for imaging, and doing the calculations, my imaging/guiding ratio is worse than 1:6.  So, I assume if I can bring that down closer to 1:3 I can get even better guiding and imaging results.  I have tried an Astromania 60mm (240mm f/l), but was not real happy with the star images and field.  I tried an Astro-Tech 72ED, and it needed an extension tube to come to focus so I never used it.  My current setup is rock solid - I've done everything possible to eliminate flexure and am sure it has worked, so I have no desire to add an extension tube.

 

Again, no desire for OAG either.  I'm looking for suggestions for a good, solid guidescope with around 400mm focal length, for a few hundred dollars at most.  If it turns out that my SV50 is the best I'll be able to do, so be it.  It's a great scope.  Of course, another issue at the present time is availability, but please don't let that be a consideration.  Any and all suggestions welcome (EXCEPT OAG! ).

 

Thanks for your time!

My suggestion,  use what you have.  If you have a 72MM F6 or F7 and all you need is an extender for focus,  get the extender and try it.     I have a 76MM, a 92MM refractor also a 7" mak 2600MM.  I have not done anything with the mak yet.  Using the 76 to guide the 92 I get .4 to .8 arcsecond / pixel tracking.   Back in the day when I only had my mak  guiding was done with a dual chip camera, SBIG 9E,  similar to OAG.  Never paid attention as to how good the tracking was, my picture taking skills were 0.  Stars were round though.  If the main imager was focused so was the guide camera.

 

Anyway, if you have the 72 use it, don't go and buy another guide scope. Get the extension tube for focusing.    Joe

[DuncanM]

I've been doing a good bit of research on this, and am a little frustrated, so any help will be much appreciated.

 

Wanting to simplify life a bit, I'm pretty much down to just one scope for imaging now, my 9.25" EdgeHD.  I want to concentrate on DSOs, particularly some of the more difficult ones, to get out of the envelope of just shooting the "usual" stuff.  Currently I'm using a 50mm Stellarvue F050G for guiding on my CGX, and on good nights of seeing I can get as low as .5 - .6 arcseconds RMS using PHD2.  Normally seeing here (Green Valley, Arizona), despite a good dark sky, is usually pretty average, so more like .8 - .9 arcseconds on a normal night.  I'd like to do better, but I definitely do not wish to go the OAG route.  Been there, done that, did not like it at all, and I'm getting nice star images even with 3-4 minute exposures.  Longer than that starts to get iffy.

 

I get excellent star fields and pinpoint stars with the SV50, but it only has a focal length of 210mm.  Using a Nikon D810 for imaging, and doing the calculations, my imaging/guiding ratio is worse than 1:6.  So, I assume if I can bring that down closer to 1:3 I can get even better guiding and imaging results.  I have tried an Astromania 60mm (240mm f/l), but was not real happy with the star images and field.  I tried an Astro-Tech 72ED, and it needed an extension tube to come to focus so I never used it.  My current setup is rock solid - I've done everything possible to eliminate flexure and am sure it has worked, so I have no desire to add an extension tube.

 

Again, no desire for OAG either.  I'm looking for suggestions for a good, solid guidescope with around 400mm focal length, for a few hundred dollars at most.  If it turns out that my SV50 is the best I'll be able to do, so be it.  It's a great scope.  Of course, another issue at the present time is availability, but please don't let that be a consideration.  Any and all suggestions welcome (EXCEPT OAG! ).

 

Thanks for your time!

All that matters is that the guidescope/camera can guide to the limit of atmospheric conditions and that it not suffer from differential flexure; once this is accomplished guidescope FL is almost irrelevant, as autoguider software can find the guidestar centroids with great precision. But if you want to test that hypothesis, just add a barlow in front of the guide camera.

 

I have a 10in SCT working at F5 (ZWO OAG/ASI120mm mini)  and another 10in SCT at F7 (ZWO OAG/ASI 178mm) . I find guidestars in any random field about 90% of the time with the ASI120mm mini and 95% of the time with the 178mm. The ASI120mm mini gives me a 9.7 x 13 arc min field and the 178mm a 9.6 x 14.3 arc min field, respectively. If you used a ZWO OAG-L and an ASI174mm mini and a C-9.25 at F10, you would have a guidecamera field of 10.4 x 16.6 arc/min and you'd find a guidestar in almost any random field. The OAG will guide better because it completely eliminates guider errors due to mirror movement and flexure.

Edited by DuncanM, 28 February 2022 - 08:53 AM.

[John Verderame]

Thanks very much for all the good information.  With regard to star fields, yes, I know the important part is the centroid, but first you need a few stars to choose from to find one.  I use the SNR function in PHD to see if I have a good guide star.  Generally if the SNR is at least 30 or thereabouts (averaging, that is, as seeing comes and goes) I find my tracking is better.  With the Stellarvue scope, I have many more stars to choose from, and now that PHD also has that "multiple star" function, where it chooses other stars to track besides the guide stars for more accuracy, it's important to have a good star field.

 

As for <<guidescope FL is almost irrelevant>>  that I believe is the first time I ever heard that (of course "almost" is the qualifier there!).  Just about everything I've read so far says that for a long f/l scope like my edge you need a longer f/l guidescope so the image scales of the guide cam and the imaging cam are better matched to avoid star bloat or elongation in your images, and that an imaging/guiding ratio of 1:3 or better is ideal.  I think post #11 in this thread does a good job explaining that:  https://www.cloudyni...ng-image-scale/

 

So, it's looking more and more like I'll be sticking with my trusted 50mm SV.  Really do not want to get into extensions (and btw, I do not have a 72mm guide scope - sold that long ago when it didn't work due to the extension need), and I also did try using a 2x Barlow, and of course that's the equivalent of an extension of sorts, so the image train was not as secure and flexure became an issue.

[John Verderame]

Thought I'd share some pics of my current setup.  It's pretty low-profile, with the rings secured to the clamps, and the Delrin-tipped setscrews are very snug.  Then I have the guide cam cord secured by that little wood block that fits snugly in the finderscope base, so there's no chance of any movement from that cord being yanked or flopping in the wind.

 

 

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[bearpig]

z73??? Available used for very reasonable prices.

[John Verderame]

<<z73??? Available used for very reasonable prices.>>

 

From what I'm seeing, something like $600.00 or more for a used one?  I'd like to keep it to about half of that at most, but it is a good f/l.  Any idea if that needs an extension for autoguiding?  Have you used yours for that purpose?

[DuncanM]

Thanks very much for all the good information.  With regard to star fields, yes, I know the important part is the centroid, but first you need a few stars to choose from to find one.  I use the SNR function in PHD to see if I have a good guide star.  Generally if the SNR is at least 30 or thereabouts (averaging, that is, as seeing comes and goes) I find my tracking is better.  With the Stellarvue scope, I have many more stars to choose from, and now that PHD also has that "multiple star" function, where it chooses other stars to track besides the guide stars for more accuracy, it's important to have a good star field.

 

As for <<guidescope FL is almost irrelevant>>  that I believe is the first time I ever heard that (of course "almost" is the qualifier there!).  Just about everything I've read so far says that for a long f/l scope like my edge you need a longer f/l guidescope so the image scales of the guide cam and the imaging cam are better matched to avoid star bloat or elongation in your images, and that an imaging/guiding ratio of 1:3 or better is ideal.  I think post #11 in this thread does a good job explaining that:  https://www.cloudyni...ng-image-scale/

 

So, it's looking more and more like I'll be sticking with my trusted 50mm SV.  Really do not want to get into extensions (and btw, I do not have a 72mm guide scope - sold that long ago when it didn't work due to the extension need), and I also did try using a 2x Barlow, and of course that's the equivalent of an extension of sorts, so the image train was not as secure and flexure became an issue.

I have a 50mm/F4 guidescope on my 10in F7 OTA and a 50mm/F3.3 on my 10in F5 OTA  and both have ASI120mm mini (or equivalent). I use these as backup guiders if I can't find a suitable guidestar or for guiding on comets. There are literally dozens of guide stars in any random field with either guidescope (even the 50mm F4 has a ~1.5 degree diagonal FoVwith the ASI120mm mini) and both guide very accurately for shorter subs (3-4mins) but for longer subs mirror movement and flexure starts to degrade the tracking.

 

With my OAGs I can use sub exposures of almost any length (15-30 mins for example) and still get very accurate tracking. 

[John Verderame]

Thanks, Duncan.  I can lock the primary on the EdgeHD, so mirror movement's not an issue, and I'm not even aware of movement effects when I don't lock it.  I generally don't do more than about 2 hours of exposures at a time, and never do meridian flips, so mirror movement's not a big issue here.  Where you say, "if I can't find a suitable guide star..." well, there's one of my quibbles with OAGs (and you're in a Bortle 1 location!?).  Don't have that problem in the least with the guidescope.  Even in a sparse star field there are plenty to choose from.  Let's put it this way:  I spent a lot of my early life in astronomy wasting time looking for the objects I was trying to see (visually).  Now with computerized mounts, problem solved (hey, I'm not one of those "old-fashioned" guys who sings the praises of the good old days of star hopping!  You can have 'em!).  So I don't want to have to go back now to spending time searching for GUIDE stars!  I turn on PHD and voila', a nice full field of stars to choose from (and I can often see my target too), no adjustment needed.  I've actually done 10-minute subs where the stars were nice and round, but I keep most of my exposure times to 5 minutes or less.  It's just that I have to have great seeing for a long exposure like that (10 minutes), and the seeing at my location (Bortle 3-4) is often not good due to desert thermal differentials and mountainous terrain.