200 replies to this topic

[james7ca]

Differential flexure between your guide scope and your EdgeHD is likely to be as big (or bigger) of an issue as any concern about guiding scale (e.g. the focal length of the guide scope).

 

Have you checked your subs for drift during a non-dithered and sequential imaging session that has lasted for more than thirty minutes (better yet, up to one hour)? I'd be surprised if you weren't getting measurable drift to the point where it could be affecting your results.

 

Going to a longer focal length with a heavier guide scope MIGHT actually make that drift worse since flexure could very well be increased with a larger guide scope. Plus, more weight is going to be harder on your mount (for guiding and astrophotography).

 

Frankly, if you want a finer image scale with your current setup I'd think about using a barlow mounted internally to your current guide scope. You can get barlow lenses that have a 1.25" screw thread that can be mounted internal to a standard t-threaded imaging chain using an adapter that is designed to hold 1.25" filters (such as __HERE__ ).

 

Lastly, the link that was referenced by the OP as doing a "good job" of explaining guiding ratios contained some rather speculative statements that were subsequently revised by the original poster. Thus, you should read the followups in that thread for a more complete view of this topic.

Edited by james7ca, 03 March 2022 - 07:39 AM.

[Blackhawk163]

<<John, I'm interested in where you got the focal length requirement from.>>

 

I did mention that earlier, but without details.  I use this website, under "Guidescope suitability," where it allows you to plug in figures:  https://astronomy.to...ope_suitability

 

Using Celestron 9.25" EdgeHD as my scope, Nikon D810a as my imaging camera, and ZWO ASI290MM for guiding camera, if I do not plug anything into guide telescope, but use 400mm for the focal length of the guidescope, it gives me an imaging/guiding ratio of 1/3.49.  As I understand it, 1:3 or better is ideal.  Currently I'm at 1:6.65.

 

Ah okay makes sense. I'm at 1:2.84 just realized that I've been binning my asi 178mm at 2x2 and if I kept it at 1x1 I'd be at 1:1.42. So I'll fix that whenever clear skies present themselves. 

 

As for my experiment, I increased the rigidity on the draw tube on my 80mm by adding 2 new 1/4 x20 set screws to lock it in place, as well as taking out the focuser assembly. One of the screws were stripped, I belive this was the major cause of the apparent flexure. So I drilled and tapped the existing ones. Very solid now. I don't have my equipment at home to test how much flexure there still is, but I'm sure this made a difference.

 I do know, at least in my case, that guiding via OAG just works best in my skies. I’m in the Bortle 4 area, and the only time I would ever lose a star is if clouds rolled in.

[John Verderame]

Differential flexure between your guide scope and your EdgeHD is likely to be as big (or bigger) of an issue as any concern about guiding scale (e.g. the focal length of the guide scope).

 

Have you checked your subs for drift during a non-dithered and sequential imaging session that has lasted for more than thirty minutes (better yet, up to one hour)? I'd be surprised if you weren't getting measurable drift to the point where it could be affecting your results.

 

Going to a longer focal length with a heavier guide scope MIGHT actually make that drift worse since flexure could very well be increased with a larger guide scope. Plus, more weight is going to be harder on your mount (for guiding and astrophotography).

 

Frankly, if you want a finer image scale with your current setup I'd think about using a barlow mounted internally to your current guide scope. You can get barlow lenses that have a 1.25" screw thread that can be mounted internal to a standard t-threaded imaging chain using an adapter that is designed to hold 1.25" filters (such as __HERE__ ).

 

Lastly, the link that was referenced by the OP as doing a "good job" of explaining guiding ratios contained some rather speculative statements that were subsequently revised by the original poster. Thus, you should read the followups in that thread for a more complete view of this topic.

Hi James, and thanks for the response.  No problem with differential flexure with my current setup.  I think you saw the images I provided of what I'm using and it is absolutely rock solid.

 

No, I've not checked my subs for drift, in part because, on nights of good seeing, I'm getting satisfactory results even with 4-minute subs.  But, as I'm sure you know, there are so many factors involved - seeing, flexure, focus shift, polar alignment, field rotation, tracking anomalies (I do have occasional, non-repetitive "blips" in tracking whose cause I've not been able to pinpoint), camera shift, optical train instability (I use a Starizona t-adapter, to which my camera is attached directly, and it too is rock solid), and on and on.  It's often hard to pinpoint just one cause of star anomalies (speaking from experience here, believe me!).

 

I mentioned that I tried a Barlow, but that made the guiding image train once again susceptible to flexure.  I have an in-line (Vernonscope) 1.5x Barlow, but to use that I'd have to use a nosepiece (as with my 2x Barlow that I tried), once again leaving my image train susceptible to flexure.  As it is, my ASI290MM is threaded directly onto the focus tube of the SV50, and there is no possibility of flexure.  I hope I'm being clear here.

 

As I mentioned too, my current setup allows for a nice balance between the guide scope assembly moved forward on the dovetail rail, and the T-adapter/camera (the D810a is heavy!) setup on the rear of the OTA.

 

Anyhow, to be honest, this is getting way more complicated than I had hoped.  Just simply looking for a 400+mm f/l scope to try as a guide scope.  If it does not work, so be it.  I just like to try stuff.

[james7ca]

Well, you're always going to have some flexure, it just depends upon now much. If you really have zero flexure you should see no drift in the registration of the subs (only small and random variations in the registration caused by seeing and your guider's lag in any corrections needed to account for tracking errors). One easy way to check for this is to just stack your images without doing any registration. That will likely result in some loss in resolution or larger stars but there should be no obvious elongation in the stars, or at least nothing that large unless you have a constant drift in one direction (i.e. likely flexure). Or, you can just run a blink utility on the unregistered subs looking for any obvious direction in the drift (but make certain that the sort order in the blink is time correct, for example PixInsight usually doesn't sort the loaded files in the correct order).

 

IMO, differential flexure is the main reason why some users have so much trouble with walking noise in their stacked images. And, as you probably know, differential flexure doesn't just come from the guide scope. It can happen from temperature shifts and bending of the optical tubes, sag in your main focuser, and movement in your mounting rings or whatever mechanism you use to attach your guide scope to the main imaging scope.

 

As for the barlow, you should be able to configure a barlow so that there is no added flexure (other than what might come from the added weight and the small amount of extension needed to accommodate the barlow's working distance). If the barlow lens is mounted internally to either the camera or the extension tubes in an all threaded connection it should be pretty "rock solid."

 

I once mounted a 1.25", threaded barlow lens to a 1.25" eyepiece extension tube and then screwed that into one of ZWO's thin, t-threaded filter holders that is designed to fit within the nosepiece of their cameras. Since the 1.25"barlow and eyepiece tube will fit within a standard t-extension tube all of the connections were by threaded components and the barlow was completely enclosed by the t-threaded extension tubes that ran between the camera's t-threads and the threaded connection on the guide scope.

 

In fact, I use a similar technique for lunar and planetary photography with my 2" Astro-Physics Advanced Convertible Barlow since that barlow has male M48 threads that can be screwed into a 2"filter holder that is designed for Tele Vue's 2.4" threaded extension tubes. So the A-P barlow is mounted internally to the large Tele Vue extension tubes that are screwed directly into the camera and the imaging scope.

[John Verderame]

James, your responses are always really helpful.  Thanks.  Yes, of course there's going to be "some" flexure, but I think it can be mostly negated in a really solid setup, and I do believe the longer f/l would further mitigate it.  Really really just simply want to at least try it.  Then I can come back here, head hung low, and say I've capitulated to the OAG culture 

 

I have never stacked images without registering first.  I'll try that.  Always open to learning something new.

 

I purchased the Vernonscope 1.5x (1.25") Barlow specifically for lunar and planetary, and it made a huge difference, as it brings my EdgeHD to f/15, which seems to be the sweet spot for getting nice detail.  I don't even use guiding for L&P of course, as it's not necessary.

Edited by John Verderame, 03 March 2022 - 03:08 PM.

[james7ca]

If you stack your UNREGISTERED images make certain that you disable any pixel rejection since it's possible that small amounts of pixel shift will be "masked" or removed by the rejection process that is typically enabled when you integrate a series of subs.

 

In PixInsight you can also look at the rejection maps (if you integrate the subs with rejection ENABLED) since those might show rejected pixels around the unregistered stars. These rejection maps can also reveal focus drift and changes in the transparency of the sky as rejection-caused halos around the brighter stars.

Edited by james7ca, 03 March 2022 - 11:41 PM.

[John Verderame]

Ok, thanks.  Don't have PI, but I do use DeepSkyStacker.

[james7ca]

So, I revisited my barlow setup for a guide camera and you can see two stages in the assembly in the below image. This just requires fairly standard parts, a GSO shorty 2X barlow (or any barlow that can be unscrewed from its eyepiece holder to present a standard male, 1.25" filter thread), a short 1.25" eyepiece extension tube (this one happens to have a length of 25mm), and a ZWO 1.25" filter holder (allows you to mount a 1.25" threaded component -- usually a filter -- into the 2" nosepiece of one of their small cameras). Add to this a standard t-threaded extension tube (mine happens to be about 13mm long) that will mate to the female, t-threads on the ZWO camera.

 

What you end up having is a combination that can be threaded into any scope that has a male t-thread at its end. In my case, I've attached this to a Laida 50mm, f/4 finder scope (more on this in a separate and later thread) which in this case produces a 1.6X barlow factor and a resultant focal length of 325mm at f/6.5 (as 1.6 x 200mm ≈ 325mm). You can get a greater amount of magnification with a further extension of the barlow and in fact I've already done that by inserting a 1.25" filter between the barlow and the 1.25" extension tube (more on that in a separate and later thread).

 

Anyway, this is a 100% threaded solution and the barlow itself sits totally inside of the larger t-threaded extension tube (thus, when you thread this into the back of a scope you can't even see the barlow, just the outside of the t-threaded extension tube). Of course, whatever you thread this into might need to have an opening that is large enough to pass a 1.25" diameter tube which should be the case for anything that uses a t-thread (M42x0.75) for attachment and that will accept 1.25" eyepieces.

 

The Laida finder is relatively inexpensive and has some unique features, like a standard M48 threaded optical tube which means you can make fine adjustments to the backfocus by adding/removing M48 extension tubes between the front objective and the Crayford focuser on the output end.

 

You can learn more about the Laida finder scope in the following thread:

 

https://www.cloudyni...2#entry11084417

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Edited by james7ca, 04 March 2022 - 01:32 PM.

[John Verderame]

Wow, James, that's a really innovative idea!  I love tinkering like that, so I'm going to go into my astro junk drawer and see if I have some way I could duplicate what you did using a GSO 2x Barlow and retaining my SV50mm guide scope that I like so much.  Actually, I assume any Barlow could be used that has a threaded-on lens cell.  I don't think the GSOs are available for now.

Edited by John Verderame, 04 March 2022 - 01:21 PM.

[james7ca]

You'll probably have to use a longer t-threaded extension tube to get focus with your SV50. With my Laida finder scope I could add 20mm to the optical tube (between the objective and focuser) with a standard M48 extension which produced a focus point (with the barlow) that was just a millimeter or two from having the focuser completely racked inward, so basically no overhang or focuser extension on the rear of the scope (best to prevent any flexure in the focuser).

[John Verderame]

I'm playing around with ideas now.  I think I've got some useable stuff in my astro junk.  Might just use the ZWO insertion barrel after all and see how that works.  Threaded-on would be best though.  Nice, solid connection.

[james7ca]

Here is how the final setup looked, this image is a little bit smaller than life size. From the objective (not the dew shield) to the rear of the camera is about 10 inches, but with the barlow the effective focal length is about 13 inches (≈ 325mm). As shown it is focused at infinity.

 

It was almost pure chance that the spacing worked out this well, although I think I might try for a bit more magnification from the barlow which will require a different set of extension tubes. I did, however, add a 1.25" filter to the spacing between the barlow and the camera which should increase the barlow factor somewhat, but I won't know by how much until I get a chance to take some images, maybe tonight and if the rain clouds clear I can try a shot of the early crescent moon. Could be a tight fit on the moon, given that I'm using a relatively small format ASI462MC camera for this initial testing, but my normal guide camera is an ASI290MM (however, I have used the ASI462MC for guiding with a 850nm IR-pass filter).

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Edited by james7ca, 04 March 2022 - 02:35 PM.

[John Verderame]

Ok, I'm going to go ahead and get that ZWO filter holder and a 2x Barlow with a threaded-on lens cell and take it from there.  That's a super idea and I never would have thought of it.  Might take some time to get all this together and try it out, but I'll keep you posted.  This is what I'd really like to do instead of going the OAG route, at least for now - give it a try.  Thanks again!

[Rlakjdlsj]

Can't believe there are 3 pages to this thread for such a simple question...   anyway, ain't read them all, so if I missed something, sorry,

 

 But I've recently going through the same process.    I don't have nearly the good seeing you do, but recently found I had some flexure going on while imaging at low Dec's,  with using a piggybacked C80ED w/ FR scope I use for both guiding and wider field imaging or solar observing.  I also wanted to 'keep' the 80mm aperture to make sureI'd get plenty of guide stars.

 

The C80Ed weight with rings and all was about 10 lbs.  My main F.L. is just over 1200mm.   That all did fine on objects from the equator up but guiding went to heck somewhere below the equator.  The system was just too heavy with GEM & Newt in near horizontal position.  

 

So, after digging around like you for something light, 400mm and 80mm or so,   I decided on an Orion guidescope version of the ST-80.     Pretty cheap $,  lightweight at 2 1/4 lbs and supposedly has a newer design better focuser for guide camera.  The scope is in the mail now, so can't give you a report yet.   But might be worth a look.   If its crap I'll send it back but its got to cause a lot less probelm just from the lack of mass.  

 

I also have a ZWO 60mm guidescope so I tested my too much 'weight-mass' theory on and it worked fine at imaging an object at -13 DEC the other night for nearly an hour.   Nice round stars in all sub frames.   I was also able to remove a 3rd 11# counterweight from the main mount counterbalance which substantially reduced the mass-inertia required to make tiny PHD corrections and that load on gears/clutch.   If the CT 80 doesn't work out with larger aperture, I'll just use the 60mm and lengthen exposures for more SNR if I need to or can.   

 

https://www.telescop...ts?keyword=CT80

[John Verderame]

<<Can't believe there are 3 pages to this thread for such a simple question...>>

 

  Welcome to a typical CN discussion!

 

I'll look forward to your results with that Orion scope, but it doesn't have the best reviews, so hope it works out ok for you.  As you can see, perhaps the discussion has gotten so long because there's just no simple solution to it.  Seems like there might even be a niche opportunity here for a good quality 400mm+ f/l guide scope here.

 

I liked your point aboutthe mass-inertia.  That was something I hadn't seriously considered till watching a YouTube video yesterday.  Another good reason to keep my lightweight, low-profile SV50mm.

[james7ca]

Ok, I'm going to go ahead and get that ZWO filter holder and a 2x Barlow with a threaded-on lens cell and take it from there.  That's a super idea and I never would have thought of it.  Might take some time to get all this together and try it out, but I'll keep you posted.  This is what I'd really like to do instead of going the OAG route, at least for now - give it a try.  Thanks again!

That ZWO filter holder is nice to have. However, unless you have some way to add spacing between that and the GSO barlow you aren't going to get much magnification since the barlow will be so close to the sensor. That's why I used that 25mm long 1.25" barrel extender and yet that only produced about a 1.6X boost.

 

I just checked on OPT and Agena Astro looking for some 1.25" spacers/extenders and I couldn't find any. They both have a 2" barrel extender but I couldn't find anything in the 1.25" size. I guess it was lucky that I purchased my 25mm extender several years ago from OPT (I actually ended up with two, but I think 50mm of extension would be too much). Next best would be to take an old 1.25" filter and remove the glass to produce an additional 5mm or so of extension (for my setup). That would give me a total of about 30mm (25 + 5) which might be somewhat closer to 2X. Plus, as I noted earlier I've mounted a 1.25" filter already so that might get me up to 34mm (25+5+4) and the 1.6X factor was without any filter, just the 25mm extender.

Edited by james7ca, 04 March 2022 - 03:06 PM.

[John Verderame]

I have a couple of those 25mm spacers in my junk drawer, and also one or two empty filter rings.  Just came up with an idea for another problem I was trying to solve, so I'm on a roll here (hopefully, anyhow).

[james7ca]

I found something on Amazon that should work to add spacing and it doesn't need the optional ZWO filter holder. I think most of the small (planetary) cameras from ZWO come with a short, c-mount lens that can be used with the camera for all-sky capture. With that they include a c-mount adapter that threads into the nosepiece of the camera (kind of like the 1.25" filter adapter but with a smaller c-mount thread -- about 1" in diameter). Well, SVBONY sells a c-mount to 1.25" filter adapter that looks to have a fairly long tube (how long, I don't know, but it seems to be about 20mm in length). So, if you took that SVBONY part you could use the ZWO c-mount adapter (which you may already have) and this SVBONY adapter to create a threaded connection between the camera and any male-threaded, 1.25" accessory.

 

In any case, here is the link to the SVBONY part ($16 from Amazon and that includes a 1.25" UV/IR cut filter):  

 

  https://www.amazon.c...,aps,182&sr=8-5

 

[UPDATE]

Well, Agena Astro does have the 1.25" barrel extenders, I had to do a search using Google because the Agena search turned up nothing (which I thought was strange because Agena usually has more adapters than anyone else). Anyway, here is the link in case someone needs one of these:

 

  https://agenaastro.c...JRoCYcoQAvD_BwE

 

They also have 0.25", 0.5", and 0.75" long versions of the extender which could be used to tune the magnification, but they are out of stock on those.

[/UPDATE]

Edited by james7ca, 04 March 2022 - 03:55 PM.

[Rlakjdlsj]

<<Can't believe there are 3 pages to this thread for such a simple question...>>

 

  Welcome to a typical CN discussion!

 

I'll look forward to your results with that Orion scope, but it doesn't have the best reviews, so hope it works out ok for you.  As you can see, perhaps the discussion has gotten so long because there's just no simple solution to it.  Seems like there might even be a niche opportunity here for a good quality 400mm+ f/l guide scope here.

 

I liked your point aboutthe mass-inertia.  That was something I hadn't seriously considered till watching a YouTube video yesterday.  Another good reason to keep my lightweight, low-profile SV50mm.

thanks...I read the reviews, and also compared reviews to the ST-80 (far better reviews) and I think most of the poorer reviews were made with the previous cheaper focuser on that scope.   But yeah,  we'll see, after I get it.      Maybe there is a simple solution.  I find far too many often 'overthink' a simple problem here on CNs.  Thats just the old ATMr in me talking...   K.I.S.S.  

Edited by Ron359, 04 March 2022 - 03:35 PM.

[John Verderame]

Thanks, James.  Already ordered the ZWO filter holder and don't really need the SVBONY thingy, nor that filter.

 

Ron, I like to keep things simple too, which is the very reason why I'd like to stick with the guide scope if possible and avoid an OAG.

 

Speaking of old ATM stuff, the first decent scope I had was as a teenager - an 8" reflector that a friend of our family's father had made.  It had a mount made of iron pipes, and the counterweight was a large juice can filled with concrete.  Don't want to get too far off topic here, but just had to mention that.  It was a great scope to cut my astro teeth on!

[Oort Cloud]

That ZWO filter holder is nice to have. However, unless you have some way to add spacing between that and the GSO barlow you aren't going to get much magnification since the barlow will be so close to the sensor. That's why I used that 25mm long 1.25" barrel extender and yet that only produced about a 1.6X boost.

I just checked on OPT and Agena Astro looking for some 1.25" spacers/extenders and I couldn't find any. They both have a 2" barrel extender but I couldn't find anything in the 1.25" size. I guess it was lucky that I purchased my 25mm extender several years ago from OPT (I actually ended up with two, but I think 50mm of extension would be too much). Next best would be to take an old 1.25" filter and remove the glass to produce an additional 5mm or so of extension (for my setup). That would give me a total of about 30mm (25 + 5) which might be somewhat closer to 2X. Plus, as I noted earlier I've mounted a 1.25" filter already so that might get me up to 34mm (25+5+4) and the 1.6X factor was without any filter, just the 25mm extender.

Agena is the only place I've been able to find 1.25" extensions (blue fireball brand), as I did the same thing you did, except with a reducer instead of a barlow - to do EAA with my C6 when I was first starting out. I also found some at BH, but don't get those. They look proper, but have the wrong male threads so they're useless. These are the ones I got, but they only have this 1" version in stock, the 0.5" and 0.75" are out of stock:

https://agenaastro.c...-extension.html

Edit: looks like I jumped the gun on this one...should have read through to the end of the thread

Edited by Oort Cloud, 04 March 2022 - 04:05 PM.

[Rlakjdlsj]

Ok, I'm going to go ahead and get that ZWO filter holder and a 2x Barlow with a threaded-on lens cell and take it from there.  That's a super idea and I never would have thought of it.  Might take some time to get all this together and try it out, but I'll keep you posted.  This is what I'd really like to do instead of going the OAG route, at least for now - give it a try.  Thanks again!

Just reading your discussion of adding a barlow +.    I tried that a with screw on barlow  a couple years ago on a (different than ZWO) small 60mm scope and found it didn't work very well.   A barlow ups your f/ratio so cuts out some number of potential dim guide stars while also reducing the FOV to chose them from.  Your already small 50mm aperture is effectively cut to some lesser number depending on the barlow spacing etc.

 

  I found the ZWO 60mm 240mm f.l. was 'adequate' for accurate guiding at my 1200mm main scope, but sometimes it was tough getting bright enough stars.   A good friend successfully guides his C11HD @ f/7 with FR,  with a 60mm 240 also.   

[John Verderame]

Just reading your discussion of adding a barlow +.    I tried that a with screw on barlow  a couple years ago on a (different than ZWO) small 60mm scope and found it didn't work very well.   A barlow ups your f/ratio so cuts out some number of potential dim guide stars while also reducing the FOV to chose them from.  Your already small 50mm aperture is effectively cut to some lesser number depending on the barlow spacing etc.

 

  I found the ZWO 60mm 240mm f.l. was 'adequate' for accurate guiding at my 1200mm main scope, but sometimes it was tough getting bright enough stars.   A good friend successfully guides his C11HD @ f/7 with FR,  with a 60mm 240 also.   

I don't think I'll have a problem getting stars.  Good dark sky and the star fields I'm getting sans Barlow are really nice - almost overwhelming at times.  My SV50 guide scope has only a 210mm f/l, nowhere near adequate for a 2300mm f/l main scope.  I'm not using a focal reducer (to me, yet another complication, more weight on the back, and more glass) though if one comes along again at a good price, I might finally try it out once I get the guiding going just a bit better.  Working on one thing at a time...

[james7ca]

It looks like Agena sells a 70mm, f/5.7 finder/guider that has a focal length of 400mm. It uses a sliding drawtube and rear-mounted helical for the focusing and even though the scope has locking screws for both it could still be subject to sag or movement (IMO). My smaller (60mm, f/4.1) Astro-Physics (Baader) Vario-Finder also uses a sliding drawtube for the course focus but it comes with a third ring to support the drawtube and camera and the fine focus is at the objective end (lens cell is threaded with a locking ring).

 

Price for the Agena finder is $165, which seems pretty good since it also includes a dual set of rings and a 3.5" long Vixen/Synta-style dovetail mounting bar.

 

But, it's currently "OUT OF STOCK."

Edited by james7ca, 05 March 2022 - 04:07 AM.

[John Verderame]

<<But, it's currently "OUT OF STOCK.">>

 

Yes, I had seen that one and might have considered it if not for the above.  But I'm thinking we've got it licked with the other idea for the 50mm.

 

Got a friend who recently started using an OAG and he's going to call so we can discuss it this morning.