Changing my guide scope for an improvement in guiding

Hey guys,

i am imaging with a esprit 120 and imx571 type camera.
this setup gives me a pixel scale of 0.88.
right now i am guiding with a artesky ultraguide 32mm with 130mm focal length and a 290c camera. The pixel scale of my guiding setup is 4.6.
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

i am sadly don’t have enough backfocus left to us an off axis guider.
would my guiding benefit from getting a bigger guide scope like the evoguide 50 from skywatcher?

I use a 60mm Guide Scope at 280mm FL. with my WO 132FLT. The guide camera is a ASI120mini. This combo delivers with 300sec subs with any of my ZWO cameras. I use it with the 533mcPro, 1600mmPro, 2600mm/mc Pro, 071mcPro cameras only discarding subs due to clouds or wind guests. On a recent run with that refractor GS combo on a CGX-L I captured 72 x 300sec subs with a 2600mmPro with Zero subs rejected inspected by Blink in PI.

Hope that helps.

I think you should use your GS-290 combo and test it under the stars. I believe you should be good. I have a 290 and have used it on occasion for guiding. For whatever reason, I always felt it was over sensitive to the seeing with my various finders - guide scopes. No proof on that, but it was just an observation with a very small sample size… I use PPEC in PHD2, let the mount settle for 20min while the camera cools to acquisition temp. Then let ur rip!

CS - Tim

PS. Make sure your GS is rigidly mounted to eliminate any related issues…

i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

Remember that, regardless of whether you are using a guide scope or off axis guiding, your guide camera does not know or care what your image scale is. So the ratio of guiding scale to image scale is kind of irrelevant to how much guiding improvement can be gained.

What matters is how well the stars are sampled by the guide camera and by the sensor. The general guide line is 1/3 to 1/2 of seeing. So if your seeing is 2", then your guide scale should be 1" for optimal sampling. If your guide scale is coarser than that, a longer focal length guide scope will help. Note that that doesn't necessarily mean you need the improvement, simply that it will show an improvement. See discussion by Jon Rista here:

https://www.cloudynights.com/topic/617386-imaging-vs-guiding-image-scale/

AS you may know, guiding cameras and scopes are just one element here. The weight (or inertial moment) of your setup, quality of your mount and flex in your system will matter as well.

I run a similar imaging scale on my EQ6 with my newtonian at 0.83, but I'm running an OAG. Very much seeing limited but I'm able to pretty consistently achieve 0.2-0.5 RMS with spikes usually no more than 0.6rms.

Prior to OAG I ran a 50mm guide scope and would often times get into 0.3rms territory, but it wasn't as consistent. I remember seeing 0.4-0.6 RMS average, but the spikes were pushing my image scale.

Now with OAG strapped on my 70mm Meade, I'm seeing around 0.4-0.5 average.

I believe you should be able to make a 50mm work assuming it's around 250mm focal length or better. Personally I'd go for something longer tho. Be wary of mounting and preventing any flexure. PPEC tuning is a must IMO.

For what it's worth, I run 20 minute subs on all my narrowband stuff. I rarely reject any since I don't use NB stars anyways. Also, I'm swapping out to a different coma corrector so I don't have to sample at 0.83 anymore. I felt like I was constantly babysitting PHD to keep my guiding in check. 0.8/px is a bit tight for EQ6, but it does work. Maybe a reducer would be an option for you?

Arun H:
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

Remember that, regardless of whether you are using a guide scope or off axis guiding, your guide camera does not know or care what your image scale is. So the ratio of guiding scale to image scale is kind of irrelevant to how much guiding improvement can be gained.

What matters is how well the stars are sampled by the guide camera and by the sensor. The general guide line is 1/3 to 1/2 of seeing. So if your seeing is 2", then your guide scale should be 1" for optimal sampling. If your guide scale is coarser than that, a longer focal length guide scope will help. Note that that doesn't necessarily mean you need the improvement, simply that it will show an improvement. See discussion by Jon Rista here:

https://www.cloudynights.com/topic/617386-imaging-vs-guiding-image-scale/

AS you may know, guiding cameras and scopes are just one element here. The weight (or inertial moment) of your setup, quality of your mount and flex in your system will matter as well.

Well, thats kinda depressing, because it translates into me needing an way bigger guide scope or a way to get an oag in a setup that has no backfocus left.

until now i only ever thought about the guiding setup to imaging setup scale.

thx
andreas

Arun H:
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

Remember that, regardless of whether you are using a guide scope or off axis guiding, your guide camera does not know or care what your image scale is. So the ratio of guiding scale to image scale is kind of irrelevant to how much guiding improvement can be gained.

What matters is how well the stars are sampled by the guide camera and by the sensor. The general guide line is 1/3 to 1/2 of seeing. So if your seeing is 2", then your guide scale should be 1" for optimal sampling. If your guide scale is coarser than that, a longer focal length guide scope will help. Note that that doesn't necessarily mean you need the improvement, simply that it will show an improvement. See discussion by Jon Rista here:

https://www.cloudynights.com/topic/617386-imaging-vs-guiding-image-scale/

AS you may know, guiding cameras and scopes are just one element here. The weight (or inertial moment) of your setup, quality of your mount and flex in your system will matter as well.

Well, thats kinda depressing, because it translates into me needing an way bigger guide scope or a way to get an oag in a setup that has no backfocus left.

until now i only ever thought about the guiding setup to imaging setup scale.

thx
andreas

What are you running that has taken up all your backfocus? Curious

Arun H:
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

Remember that, regardless of whether you are using a guide scope or off axis guiding, your guide camera does not know or care what your image scale is. So the ratio of guiding scale to image scale is kind of irrelevant to how much guiding improvement can be gained.

What matters is how well the stars are sampled by the guide camera and by the sensor. The general guide line is 1/3 to 1/2 of seeing. So if your seeing is 2", then your guide scale should be 1" for optimal sampling. If your guide scale is coarser than that, a longer focal length guide scope will help. Note that that doesn't necessarily mean you need the improvement, simply that it will show an improvement. See discussion by Jon Rista here:

https://www.cloudynights.com/topic/617386-imaging-vs-guiding-image-scale/

AS you may know, guiding cameras and scopes are just one element here. The weight (or inertial moment) of your setup, quality of your mount and flex in your system will matter as well.

Well, thats kinda depressing, because it translates into me needing an way bigger guide scope or a way to get an oag in a setup that has no backfocus left.

until now i only ever thought about the guiding setup to imaging setup scale.

thx
andreas

What are you running that has taken up all your backfocus? Curious

- camera
- filter drawer
- falcon rotator

i am at 55.5 mm backfocus with that. 55mm are just not that much space.

Arun H:
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

Remember that, regardless of whether you are using a guide scope or off axis guiding, your guide camera does not know or care what your image scale is. So the ratio of guiding scale to image scale is kind of irrelevant to how much guiding improvement can be gained.

What matters is how well the stars are sampled by the guide camera and by the sensor. The general guide line is 1/3 to 1/2 of seeing. So if your seeing is 2", then your guide scale should be 1" for optimal sampling. If your guide scale is coarser than that, a longer focal length guide scope will help. Note that that doesn't necessarily mean you need the improvement, simply that it will show an improvement. See discussion by Jon Rista here:

https://www.cloudynights.com/topic/617386-imaging-vs-guiding-image-scale/

AS you may know, guiding cameras and scopes are just one element here. The weight (or inertial moment) of your setup, quality of your mount and flex in your system will matter as well.

Well, thats kinda depressing, because it translates into me needing an way bigger guide scope or a way to get an oag in a setup that has no backfocus left.

until now i only ever thought about the guiding setup to imaging setup scale.

thx
andreas

What are you running that has taken up all your backfocus? Curious

- camera
- filter drawer
- falcon rotator

i am at 55.5 mm backfocus with that. 55mm are just not that much space.

Ah yea that rotator occupies alot of space. Id love one but I'm not giving up OAG

Arun H:
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

Remember that, regardless of whether you are using a guide scope or off axis guiding, your guide camera does not know or care what your image scale is. So the ratio of guiding scale to image scale is kind of irrelevant to how much guiding improvement can be gained.

What matters is how well the stars are sampled by the guide camera and by the sensor. The general guide line is 1/3 to 1/2 of seeing. So if your seeing is 2", then your guide scale should be 1" for optimal sampling. If your guide scale is coarser than that, a longer focal length guide scope will help. Note that that doesn't necessarily mean you need the improvement, simply that it will show an improvement. See discussion by Jon Rista here:

https://www.cloudynights.com/topic/617386-imaging-vs-guiding-image-scale/

AS you may know, guiding cameras and scopes are just one element here. The weight (or inertial moment) of your setup, quality of your mount and flex in your system will matter as well.

Well, thats kinda depressing, because it translates into me needing an way bigger guide scope or a way to get an oag in a setup that has no backfocus left.

until now i only ever thought about the guiding setup to imaging setup scale.

thx
andreas

What are you running that has taken up all your backfocus? Curious

- camera
- filter drawer
- falcon rotator

i am at 55.5 mm backfocus with that. 55mm are just not that much space.

Ah yea that rotator occupies alot of space. Id love one but I'm not giving up OAG

The rotator is probably the most useful thing in my imaging train.
i can image 2 or more targets in one night without doing anything apart from the planing in nina.
no fiddling around with the manual rotator of the esprit in the middle of the night.

i have seen solutions where the falcon rotator was put before the field corrector to safe backfocus but i am not sold on that solution.

PHD2 is capable of detecting (on average) displacements of about 1/10 the FWHM of the guiding scope (which could be the same you're imaging with). A small scope is essentially limited by just the image scale so having a very sharp focus helps in guiding (with an off-axis scope), as seeing fluctuations are reflected in scintillation rather than random positional shifts. You're best bet is to improve upon the image scale but not to the point of chasing the seeing, which would require considerably longer integrations.

andrea tasselli:
PHD2 is capable of detecting (on average) displacements of about 1/10 the FWHM of the guiding scope (which could be the same you're imaging with). A small scope is essentially limited by just the image scale so having a very sharp focus helps in guiding (with an off-axis scope), as seeing fluctuations are reflected in scintillation rather than random positional shifts. You're best bet is to improve upon the image scale but not to the point of chasing the seeing, which would require considerably longer integrations.

Thank you for your answer.
it seems to me that it isn’t that easy to make an 100% correct answer on that topic.

if i understand you correctly, a guide scope with a bit more focal length or even the one i use now should to the job just fine.
on the other hand i have the comment from arun that suggests that i should guide at 0.5x my seeing (which could be around 2 on good nights). That would require a substantially longer focal length guide scope or better an oag.

on the other hand i have the comment from arun that suggests that i should guide at 0.5x my seeing (which could be around 2 on good nights). That would require a substantially longer focal length guide scope or better an oag.

I would definitely recommend reading that thread in its entirety which answers the question you asked. See James's comment on the second page in particular, which simplifies things and recommends a 1:3 or 1:4 ratio of guide to image scale. The point I did not cover in my initial post was that, for a small aperture guide camera, the size of the star is less controlled by seeing and more by diffraction from the guide scope itself - so you might have great seeing, but the size of the star on the guide cameras sensor will be larger than seeing would suggest due to the small aperture of the guide scope. This will help you, since the larger size of the star will make the location of the centroid more precise to calculate. Incidentally, these considerations are why people go to OAG, since it makes these kind of ratios very, very easy to achieve and also largely takes care of flex.

Thank you for your answer.
it seems to me that it isn’t that easy to make an 100% correct answer on that topic.

if i understand you correctly, a guide scope with a bit more focal length or even the one i use now should to the job just fine.
on the other hand i have the comment from arun that suggests that i should guide at 0.5x my seeing (which could be around 2 on good nights). That would require a substantially longer focal length guide scope or better an oag.

My rule of thumb is to use a guiding scope with at least 1/3 of the focal length of the scope I'm imaging with, assuming same pixel width (or thereabout). For my 600mm FL scope that means around 200mm (I'm using a 240mm). For my 300mm I'm using a 128mm and for my 1200mm 450mm. When I was using a 2000mm FL scope I'd have to use a 600mm guidescope but I had to bin the guide camera most of the times because good guide stars could be quite rare otherwise.

Most definitely you want to avoid guiding on the seeing so I see no point in what Arun said. Besides, if that were true we wouldn't be able to guide with off-axis guiders, which is patently untrue.

andrea tasselli:
Most definitely you want to avoid guiding on the seeing so I see no point in what Arun said.

I didn't recommend guiding on the seeing. Where are you getting that from my post? The question is one of sampling. PHD2 settings can be adjusted to avoid guiding on the seeing regardless of the image scale of the guiding setup.

Andrea brings up what is for me, a most important point with scintillation and integration times. One thing I see many others do is try to guide at 1/2 or 1 sec. subs. I always guide with the camera set at 4 second subs. Occasionally will go up to 6 seconds if needed to get the guide star bright enough. This eliminates the effects of bad seeing for the most part and also gives me dozens of stars to guide on. Also eliminates the need to rotate the camera looking for guide stars.

A good piece of advice here is to calibrate close to the intersection of the equator and the medirian, then run PHD2's guiding assistant at the start. That will take care of finding the optimal parameters and integration times and avoid guiding on the seeing regardless of your guide scale.

An easy way to do this is to click on the drift align tool in PHD2 as shown here:

https://openphdguiding.org/PHD2_Drift_Alignment.pdf

Note that you are not doing a drift alignment, the purpose is to bring up the window on page 2, then click on slew, which will slew your scope to the intersection of the equator and meridian. Do the calibration there, slew to your target, spend two minutes doing a Guiding assistant run. This is time well spent and can be done just before astronomical dusk.

Arun H:
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

Remember that, regardless of whether you are using a guide scope or off axis guiding, your guide camera does not know or care what your image scale is. So the ratio of guiding scale to image scale is kind of irrelevant to how much guiding improvement can be gained.

What matters is how well the stars are sampled by the guide camera and by the sensor. The general guide line is 1/3 to 1/2 of seeing. So if your seeing is 2", then your guide scale should be 1" for optimal sampling. If your guide scale is coarser than that, a longer focal length guide scope will help. Note that that doesn't necessarily mean you need the improvement, simply that it will show an improvement. See discussion by Jon Rista here:

https://www.cloudynights.com/topic/617386-imaging-vs-guiding-image-scale/

AS you may know, guiding cameras and scopes are just one element here. The weight (or inertial moment) of your setup, quality of your mount and flex in your system will matter as well.

*Thanks for this @Arun H - very useful!!

Just a note on your backfocus value of 55mm. The ESPRIT 120ED manual on page 8 of my old manual (don’t know about any later manuals) has a backfocus of 75mm.

Greg McCall:
Just a note on your backfocus value of 55mm. The ESPRIT 120ED manual on page 8 of my old manual (don’t know about any later manuals) has a backfocus of 75mm.

True. Still has. But that is measured from the field flattener.
right after that comes a m62? To m48 adapter that eats up a whole lot of those 75mm ;)
but it would be an idea to get an adapter done that does not eat that much backfocus.

Hey guys,

i am imaging with a esprit 120 and imx571 type camera.
this setup gives me a pixel scale of 0.88.
right now i am guiding with a artesky ultraguide 32mm with 130mm focal length and a 290c camera. The pixel scale of my guiding setup is 4.6.
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

i am sadly don’t have enough backfocus left to us an off axis guider.
would my guiding benefit from getting a bigger guide scope like the evoguide 50 from skywatcher?

Just my two cents here: I usually guide with an Asi120mm-s and a 60/228 guide scope, using an Asiair pro.
With this combo, I guide an Asi294mm riding on a C11@2210mm. Cem70 standard and 0.5" rms on average nights.
So I think that you won't have any problem with you guiding configuration.
Ciao.
Lorenzo

Greg McCall:
Just a note on your backfocus value of 55mm. The ESPRIT 120ED manual on page 8 of my old manual (don’t know about any later manuals) has a backfocus of 75mm.

True. Still has. But that is measured from the field flattener.
right after that comes a m62? To m48 adapter that eats up a whole lot of those 75mm ;)
but it would be an idea to get an adapter done that does not eat that much backfocus.

And that is what backfocus is, from flattener to sensor. I think Sky-Watcher provides an adapter to simplify things. I’m is Australia and we have a guy in Western Australia who is great at adapters.
I’ve a new OTA now so going by memory but I would suggest an adapter that’s a little shorter. You can always extend a length but not shorten it later.

Lorenzo Siciliano:
Hey guys,

i am imaging with a esprit 120 and imx571 type camera.
this setup gives me a pixel scale of 0.88.
right now i am guiding with a artesky ultraguide 32mm with 130mm focal length and a 290c camera. The pixel scale of my guiding setup is 4.6.
i initially bought the guide scope for a smaller setup.

my imaging / guiding ratio is 1:5.2.

i am sadly don’t have enough backfocus left to us an off axis guider.
would my guiding benefit from getting a bigger guide scope like the evoguide 50 from skywatcher?

Just my two cents here: I usually guide with an Asi120mm-s and a 60/228 guide scope, using an Asiair pro.
With this combo, I guide an Asi294mm riding on a C11@2210mm. Cem70 standard and 0.5" rms on average nights.
So I think that you won't have any problem with you guiding configuration.
Ciao.
Lorenzo

That’s encouraging. Your pictures look great 👍

Greg McCall:
Greg McCall:
Just a note on your backfocus value of 55mm. The ESPRIT 120ED manual on page 8 of my old manual (don’t know about any later manuals) has a backfocus of 75mm.

True. Still has. But that is measured from the field flattener.
right after that comes a m62? To m48 adapter that eats up a whole lot of those 75mm ;)
but it would be an idea to get an adapter done that does not eat that much backfocus.

And that is what backfocus is, from flattener to sensor. I think Sky-Watcher provides an adapter to simplify things. I’m is Australia and we have a guy in Western Australia who is great at adapters.
I’ve a new OTA now so going by memory but I would suggest an adapter that’s a little shorter. You can always extend a length but not shorten it later.

I think lacerta in austria (near me) could maybe do such adapters. Guess i will just ask them.

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