Guidescope

[Stub Mandrel]

I have a 300mm x 50mm achro lens (I bought a 150 x 50mm achro to make a finderscope out of the scope I took it out of) and have just bought a cheap R&P focuser to make it up into a scope. Assuming the Gods are kind and I get a guide camera, is a 300mm f6 scope likely to be able to find me enough guide stars or would I be better reversing the swap and having a 150mm f3 guidescope?

[Stub Mandrel]

>bump<

[SonnyE]

I have no idea Stub, but I'll take a crack at it....

I use a 50mm guide scope on my equipment. It has a Focal Length of 162mm. I have an Orion SSAG camera as my guide camera. This combination is serving me very well.

I'm of the opinion that a guide scope and guide camera only has to lock onto a single guide star to do it's job. And if it can hold that tracking for hours on end, then it is doing it.

Recently some friends and I were talking about the guide scope and its relationship size-wise to the telescope it needs to guide for. Seems that about 1/3 is adequate.

I'm far more simplistic in my thinking. I figure I use a triple APO telescope that is 80mm, so a 50mm guide scope should be sufficient. I didn't want my guide scope to be bigger than my imaging telescope. In practical application it has been more than adequate.

Clear as mud...

[Stub Mandrel]

4 hours ago, SonnyE said:

Clear as mud...

Not at all, that's really useful, just knowing something similar works, and having a mere EQ3 mount I want to keep it as light as possible.

I think 50/162 is close enough to 50/150 that I can have confidence that if the longer lens doesn't work out, I can use smaller one instead.

[iapa]

There was a previous thread,  https://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/?page=1, that suggested a ratio of guide camera imaging scale (arcsec/pixel) to imaging camera image scale of 4 or lower is what you are looking for.

camera pixel scale = pixel size/focal len x 206.3 (400m f5 refractor and 1920x960 guide cam)

my imaging pixel scale is 0.86 (a bit low so some chasing of seeing there), (900mm f5 reflector)

gives a ratio of 2.25 - gets me 30sec exposures on an AVX.

 

 

[Stub Mandrel]

I think I follow, although i haven't chosen a guide cam yet (probably mono ZWO for planets and small DSOs as well)

I'm hoping for more than 30 seconds, I can do 2 minutes unguided :-)

[SonnyE]

With Guiding, if not right away, as you fine tune things you should be able to really stretch the exposure time frame out. I will typically do 300 second increments. (5 minutes)

Nebulousity is the only stacking program I've every manged to get a modicum of success with, but they want $95 USD for it. I'm not sure enough yet to open my purse for it. So instead, I've taken longer, and longer, One Shot Color images (OSC).

One friend told me, "Sonny, you just stack with time."

Typically for me, I will do lengthier and lengthier shots to find one I like. (Often overshooting, then choosing something lower.) But the guiding is the reason I can do such long OSC imaging.

The 50 / 150 should do fine for you Neil. And like you said, you can always try the other way if you'd like. Even just to satisfy a curiosity.

[iapa]

@Stub Mandrel

you didn't mention the OTA you are using?

With f5 8" (200mm)  reflector, and f5 80mm specified I have run up to 2min as well unguided, although it varies depended on the target altitude. 300sec easily guided.

Disclaimer - this is as I understand things to be

With an f5 400mm OTA you will get longer unguided exposures without trailing than with an f5 800mm OTA because of the wider field of view of the former - assuming you use the same guide cam in each case.

You do become dependent on the pixel size of the guide camera as the software is looking at pixel illumination to do its calculations.

I've not actually tried imaging with either of my 80mm aperture refractors (f5 400mm fl and f6.3 500mm fl) Think it might be worth the time to give a shot - there's a 142mm fl 'scope somewhere around and a QHY6..... one for long clear nights

@SonnyE with all those long exposures what are you doing about noise? Lots of darks bias and flats? And or dithering?

i find DSS most of the time gives good first estimate of results

anyway - deviation, but no repitition or hesitation tho LOL.

You could work out the ratios for different guideacope based on your preferred guide cam

Or, given you are looking at the best of two guidescope options - simple arithmetic to work out the pixel size needed for guide cam for each guidescope option when used with your main OTA. Let's you see if that camera is affordable ?

 

[Stub Mandrel]

The main OTA is the 130P-DS which is about 580mm with coma corrector.

I doubt I will be doing too much guiding here - LP puts the histogram at 25% on 2 minutes, but I will use it at dark sites. I also fancy getting a Ha filter and trying Ha + OSC, although I know Ha with a DSLR isn't ideal.

[SonnyE]

15 minutes ago, iapa said:

@Stub Mandrel

you didn't mention the OTA you are using?

With f5 8" (200mm)  reflector, and f5 80mm specified I have run up to 2min as well unguided, although it varies depended on the target altitude. 300sec easily guided.

Disclaimer - this is as I understand things to be

With an f5 400mm OTA you will get longer unguided exposures without trailing than with an f5 800mm OTA because of the wider field of view of the former - assuming you use the same guide cam in each case.

You do become dependent on the pixel size of the guide camera as the software is looking at pixel illumination to do its calculations.

I've not actually tried imaging with either of my 80mm aperture refractors (f5 400mm fl and f6.3 500mm fl) Think it might be worth the time to give a shot - there's a 142mm fl 'scope somewhere around and a QHY6..... one for long clear nights

@SonnyE with all those long exposures what are you doing about noise? Lots of darks bias and flats? And or dithering?

i find DSS most of the time gives good first estimate of results

anyway - deviation, but no repitition or hesitation tho LOL.

You could work out the ratios for different guideacope based on your preferred guide cam

Or, given you are looking at the best of two guidescope options - simple arithmetic to work out the pixel size needed for guide cam for each guidescope option when used with your main OTA. Let's you see if that camera is affordable ?

 

Oh, I've always been noisy. My Mum use to say she could just stop and listen and she'd know right where I was by my singing or humming.

Since DSS simply will not complete a task for me, and Registax won't either, I've resigned myself to what does work for me, time. Time and Guiding.

Whatcha think? 1800 second piece of the Veil Nebula.

My camera tends to do the rainbow sprinkles with the longer and longer time windows. I know there is room for improvement, just haven't found the magic key yet.

[iapa]

I've  the 200pds - got that as I use a modded 600D Canon and the PDS brings focal plane out of the focusser  a bit more. OTA itself is knackered - damaged mirror ? good for practice with the various software packages tho' until I can replace it.

Anyway, easy enough to estimate guide camera pixel size for 300mm f6 and 150mm f3 with the 130pds and your DSLR.

the f3 could mean you can maybe get away with  a less sensitive guide cam. Need to consider that you prob alt do not want guide cam exposures less than a couple of secs - or you end chasing seeing and loosing guiding.

Man - I do go on ?

Disclaimer - all based on my understanding of this stuff (2yr noob so no expert).