Guide scope question

[WbRaDy]

Im looking to swap over my Samyang 135 setup to my Stellalyra F12 and have a question on guideing, the samyang set up has a ZWO mini guide scope on it and was wondering if the same guide can be used on the F12? it will be using an older ZWO290 as the guide camera so i dont suppose it would be a problem, i was using an evoguide and the same ZWO290 with the F12 last year but looking to lighten things up a bit..

Cheers

[Skipper Billy]

I would think the ZWO mini guide scope would struggle at that focal length - it would work but probably not be very effective. Its useful range is probably up to about a metre focal length.

 

[vlaiv]

Depend on your mount and your wanted guiding precision. It has nothing to do with scope you put on the mount (except for the weight and moment arm - but again, that is mechanical side of things and has nothing to do with telescope focal length).

Focal length of imaging scope is rather irrelevant for this whole process.

There seems to be large jump from 135mm to what scope is that again? You say Stellalyra F/12, but which one? In any case, what mount are you using, and what camera will you be imaging with? Even 6" version has focal length of 1800mm. Do you have camera with at least 12µm pixel size, or do you know how to bin your data?

[WbRaDy]

Afternoon Guys, its the StellaLyra 8" f/12 M-LRS Classical, focal length is 2436 i believe on a skywatcher AZ_EQ6 GT, camera is a ZWO 1600MM, i tried it on a galaxy at the beginning of the year but only for 10 minutes on the Pinwheel before i swapped over and would like to get the best out of it for some more..the image below although not very good was with the evoguide.

 

https://flic.kr/p/2mQ8oCG

 

Edited December 12, 2021 by WbRaDy

[vlaiv]

You can guide with 120mm FL guide scope.

Here are some calculations to help you out figure it if is worth while.

Your guide resolution is ~5"/px (120mm FL and 2.9µm pixel size). This guide resolution will provide you with around 0.25" - 0.3" measurement precision, which is in turn good for guiding in 0.75"-0.9" RMS - which is what you'll probably get with AZ_EQ6 GT mount.

In good seeing of about 1.5" FWHM with 8" scope and guide RMS of 0.8" you'll be able to achieve ~2.5" FWHM stars which require 1.5"/px sampling rate.

With 2436mm of FL and 3.8µm pixel size you will be sampling at 0.32"/px, so you'll need to bin at least x4 if not x5 to get closer to 1.5"/px (x4 will give you 1.29"/px and x5 will give you 1.61"/px - it is more likely that you'll need second one at 1.61"/px).

In the end - yes, you can guide with such combination 120mm FL and ASI290 with 2.9µm pixel size, but your scope and camera combination will be waaay over sampled as is and you'll need to bin x5 in software to get decent results at 100% zoom level and optimum SNR.

You will also fill up FOV:

and I think that a bit larger FOV would suit this target better.

I'd personally consider doing 2x2 mosaic on that target with said scope and camera and definitively going for x5 or even x6 bin.  With such large bin factor - I'd also go for rather long exposure lengths - like at least 5-6 minutes, due to read noise.

[WbRaDy]

Thanks for that vlaiv, much appreciated, some head scratching to do i think, do you think it would be easier/better using an OAG instead of the guide scope? or would it be just too much with the 290?, i may just stick to

luna and planets after all thats what it was purchased for, and thanks again

[vlaiv]

2 minutes ago, WbRaDy said:

Thanks for that vlaiv, much appreciated, some head scratching to do i think, do you think it would be easier/better using an OAG instead of the guide scope? or would it be just too much with the 290?, i may just stick to

luna and planets after all thats what it was purchased for, and thanks again

I think that OAG will certainly help. It will at least remove one thing from the equation and that is centroid accuracy. It will also help remove any differential flexure if there is potential for any.

Do be careful - you'll also need to bin guide camera as well as 2400mm and 2.9µm gives huge over sampling - 0.25"/px.

You don't need that much resolution (by again factor of x4 or x5) and that much oversampling will make finding guide stars much harder.

I have 8" RC and guide it with ASI185mc and OAG. It has 3.75µm pixel, so my guide scale is 0.48"/px. I bin my guide camera x2 to get to about 1"/px. You can set this in PHD2 - there is option to bin image prior to determining star position

Under noise reduction - you can select x2 bin (among other options). If you use ASCOM driver - I think that you can also select x2 bin there as well. This will give you total bin of x4 - which will be ok for OAG.

Maybe consider getting a reducer for your scope for DSO imaging. That CC has pretty flat field, right? Maybe CCDT67 would be good combination with it. It would make scope F/8 and that should not be a problem with small sensor like ASI1600 that has ~22mm diagonal. Since CCDT67 has x0.67 reduction, actual imaging circle will be 22mm / 0.67 = 33mm. Scope should still have good correction at that imaging circle size.

 

[WbRaDy]

I didnt know they did reducers for these scopes, shows how much i know, i think that is the route to take to be honest vlaiv, saves a lot of trouble i think, a question to FLO to see if there are there any other compatibles for this at all, im having trouble find it overe here..Thanks for your time though vlaiv i think this problem is solved 👍

[ollypenrice]

I'd always prefer to guide a reflector with an OAG because of the potential for mirror movement.

Essentially one problem, these days, is that very small pixels and very long focal lengths are not good bedfellows. You could have the same real resolution and a much wider FOV if using a shorter focal length.

The required guiding precision depends on the sampling rate of the imaging rig and is affected both by FL and pixel size. In the old days pixel size didn't vary all that much so focal length was a good shorthand but now cameras vary in pixel size by a factor of more than three times so FL alone is no longer useful.

Olly

[newbie alert]

I'm really enjoying playing with the sct at 1280mmfl.. not too sure if say the same at double the fl.. but it's screaming oag

Guiding is no more difficult than my other setup at 400mm fl

 

[Tim]

OAG for sure, but then I use them for everything, who needs two scopes to muck about with

 

Tim

[WbRaDy]

Thanks guys, much appreciated, plenty to think about 👍

Are any of you using the zwo oag with a skywatcher 80ed at all? I remember trying the V1 when it came out but the threads on the 0.85 reducer used to hit the prism and bend it back slightly so returned, hence why I never bothered again,  just wondering if anything changed on the V2 , would be a bonus to use it for both scopes.

Edited December 13, 2021 by WbRaDy