imaging / guiding ratio from a Guide scope to an OAG
Hi, sorry for the confusing title there.
I am thinking about moving away from a Guide scope to an OAG
So i have a Esprit 150 with a ASI2600mc main imaging camera
I have a ASI290mm mini guide camera and an Orion 60mm Guide scope.
I was looking at swapping out the Orion guide scope with an ZWO OAG.
Now the question apart from if the two are compatible Esprit 150 and ZWO OAG,
Is the imaging / guiding ratio
So with my setup with the Orion Guide scope, my imaging / guiding ratio is 1:3.37
So when i go to an OAG and please correct me if i am off track here, the ASI290mm mini is now imaging through the Esprit 150, so the imaging / guiding ratio now changes to 1:0.77
I don't think this would be ideal, so would i be correct in saying that i would now have to bin the guide camera to say 4x4 to get the imaging / guiding ratio up to 1:3.09
Sorry for the confusing question, Been trying to do a bit of research in this and not too much luck yet.
Any advice would be appreciated.
Thanks.
Peter.
Last edited by bluesilver; 09-12-2021 at 01:56 PM.
Reason: Typo
Do you mean a ratio of image scale (Arcseconds per pixel) between the imaging and guiding cameras? If so I would not worry too much. I use an ASI2600 on both a 10"F4 newtonian (About 1100mm focal length with the Paracorr in place, similar to the Esprit) and my Stellarvue SVX80T (480mm focal length) using a 290MM mini on both and both guide just fine. I tried the one on the newt at 2x2 binning but it did not make any measurable difference to 1x1.
Within reason any decent OAG will be fine so long as the clear aperture is big enough to avoid vignetting, there are adapters available to fit it in the image train and you have enough backfocus to play with.
Hi, Thanks for the reply and sorry for the confusing post here
I was actually referring to the imaging/ guiding ratio that is found on the astronomy.tools page
I was just trying to find out if this figure of 1:0.77 is poor.
I think i read somewhere that a scale of 1:3 would be ideal
I currently have an Orion 60mm guide scope attached and the scale is 1:3.37
So was just basically trying to find out of moving to an OAG with this 1:0.77 is going to work or not.
Hope that makes sense?
Any advice is appreciated.
Thanks.
Peter.
Peter,
Any reason why you want to move to OAG ? Are you having trouble locking onto a good guide Star ?
Interested in your experience or problems
As your aware , OAG is generally to reduce weight, flexure problems with long OTA’s and for focal lengths greater than 1200mm
Both my rigs ( 900mm and 1000mm focal lengths ) have Orion 60mm guide scopes with an ZWOASI120MM-S USB3 guide camera using PHD2 Multistar guiding and it works really well , there’s always a good selection of guide stars for PHD2 to use ( unless your imaging some tiny remote galaxy somewhere with a limited star field )
Martin
Hi Martin,
Thanks for the reply,
Just looking into a few options and possible solutions at the moment,
The last few times i have been out imaging i have been loosing the guide star for some reason.
Also using PHD2 multi star guiding
A bit hard to pin point what is going on yet, the only thing that has changed is my scope and tuned up the worm worm gears to reduce backlash on the AZ/EQ6
No binding on the mount, so that part is good.
I will have to take a shot of the PHD2 graph next time i get a clear night, can't remember what the numbers were now off hand.
The only thing i haven't tried in PHD2 and only just now realized it, is to run the guiding assistant to see what it comes back with.
But was just thinking an OAG might just improve things just that little bit more.
Hi Martin,
Thanks for the reply,
Just looking into a few options and possible solutions at the moment,
The last few times i have been out imaging i have been loosing the guide star for some reason.
Also using PHD2 multi star guiding
A bit hard to pin point what is going on yet, the only thing that has changed is my scope and tuned up the worm worm gears to reduce backlash on the AZ/EQ6
No binding on the mount, so that part is good.
I will have to take a shot of the PHD2 graph next time i get a clear night, can't remember what the numbers were now off hand.
The only thing i haven't tried in PHD2 and only just now realized it, is to run the guiding assistant to see what it comes back with.
But was just thinking an OAG might just improve things just that little bit more.
Hmm.....
Losing a guide Star could mean many things like PHD2 settings , PHD2 calibration or Guide scope focus, or really poor conditions like high cloud etc.... An OAG won’t fix these issues
Attached are my settings using the Orion 60mm guide scope ( focal length 240mm ) and ZWOASI120MM-S guide camera ( pixel size 3.75uM ) for PHD2 Multistar guiding
Also my procedures and notes on using PHD2
They may help , they may not
But at least you can check and compare
Hmm.....
Losing a guide Star could mean many things like PHD2 settings , PHD2 calibration or Guide scope focus, or really poor conditions like high cloud etc.... An OAG won’t fix these issues
Attached are my settings using the Orion 60mm guide scope ( focal length 240mm ) and ZWOASI120MM-S guide camera ( pixel size 3.75uM ) for PHD2 Multistar guiding
Also my procedures and notes on using PHD2
They may help , they may not
But at least you can check and compare
Cheers
Martin
Also an excellent video from Bruce Waddington ( PHD2 developer)
Very much appreciated.
I did see a few things there that are different and the main one was the focal length.
For some reason my settings got changed, ( most likely me trying things out in the dark and miss clicking on things )
But my focal length was off for a start, it showed up at 260mm.
So that is one thing i will fix,
Calibration steps is slightly different, but this can be from different mounts, mine is sitting at 812 while yours is at 900
So some good things there to try out.
Will have to watch that video also.
Within reason, if you have the wrong focal length entered in PHD2 all that will happen is that your guide stats will be wrong when expressed in arc-seconds. It might calculate an oddball calibration steps figure but if it calibrates properly it will probably guide fine. That ratio it is calculating is your guiding image scale compared to your main image scale, both expressed in arc-seconds per pixel. If you use an OAG you will have that ratio regardless of what you put them on. My ratio is identical when the guide cam is binned 1x1 and both my scopes guide very nicely using that.
It would be worth downloading PHD2 log viewer and seeing what your logs say in terms of the guide star mass and SNR, you might find that as you loose the star you see mass and SNR falling away which could be passing cloud, dewing up of the guide scope etc. This is an example of mine when cloud got in the way. The yellow and white traces are the guide star stats.
Very much appreciated.
I did see a few things there that are different and the main one was the focal length.
For some reason my settings got changed, ( most likely me trying things out in the dark and miss clicking on things )
But my focal length was off for a start, it showed up at 260mm.
So that is one thing i will fix,
Calibration steps is slightly different, but this can be from different mounts, mine is sitting at 812 while yours is at 900
So some good things there to try out.
Will have to watch that video also.
Appreciated.
When you change your guide scope focal length, you / PHD2 will have to calculate the calibration step again and therefore you will have to do a new calibration
Calibration is best done at Dec =0 or near the junction of the celestial equator and just before the north meridian ( in that area )
Your probably aware of this but I thought to mention it again as it catches a lot of folk out
Hope you resolve your guide Star issues
Also I use a 2 sec exposure which I’ve used for years under all conditions, some folk say it should be 1.5 to 1.0 under good conditions but I just leave it at 2 sec and it’s worked well for both my rigs
Thanks heaps, lots of very good information there.
I am going to now go back and recreate a new profile using the wizard so that everything is back to standard settings and start over, i think i am just chasing my tail at the moment so to say.
Started watching that video and i have to say it is pretty much the best thing i have found in regards to PHD2, sort of thing that needs to be made a sticky i guess.
But lots of information there and a few things that i need to do to set things up again.
Thanks again for all the advice and links,
Very much appreciated.
Peter.
Thanks heaps, lots of very good information there.
I am going to now go back and recreate a new profile using the wizard so that everything is back to standard settings and start over, i think i am just chasing my tail at the moment so to say.
Started watching that video and i have to say it is pretty much the best thing i have found in regards to PHD2, sort of thing that needs to be made a sticky i guess.
But lots of information there and a few things that i need to do to set things up again.
Thanks again for all the advice and links,
Very much appreciated.
Peter.
Glad to help where I can
I’ve asked for help too , many many times on this forum and folk are always obliging
Here’s a photo of my 8” rig and guide scope set up ( solid as a rock ) everything is bolted down with stainless steel hex head metric bolts
