Colour guide camera? Good/bad idea?

[Ouroboros]

What are the real disadvantages of using  a colour camera for guiding? I've read different views on this ranging from sensitivity issues to others saying they've had no problems using one. 

Do colour cameras effectively have less resolution? Or does PHD2 simply take each and every pixel irrespective of the colour? 

The reason I ask is that FLO has a reduced colour Starlight Xpress Lodestar X2 Autoguider  on offer.  I'd quite like to do some colour imaging with it too. 

[ollypenrice]

You get less light on the chip because of the colour filters over every pixel but, when I used an Atik 16ic colour on one rig and a 16ic mono on the other for guiding, I found no perceptible difference.

This is such a sensitive camera that I really can't see it being an issue.

You don't need resolution in a guide camera. According to Craig Stark, who ought to know, a slightly soft focus helps the calculation of a stellar centroid, the notional sub-pixel point on which the guide command is based.

Olly

[JohnSadlerAstro]

Hi,

I'm not by any means a "pro", but I've always used a colour asi120 and it seems to work fine. To be honest, I'm not sure that the difference between colour an mono is really enough to be an issue, anyway. Half a second extra exposure and you'd probably have countered it. PHD already makes all the images monochrome as far as I can see, never spotted any colour in the stars or noise.

John

[Ouroboros]

6 hours ago, ollypenrice said:

You get less light on the chip because of the colour filters over every pixel but, when I used an Atik 16ic colour on one rig and a 16ic mono on the other for guiding, I found no perceptible difference.

This is such a sensitive camera that I really can't see it being an issue.

You don't need resolution in a guide camera. According to Craig Stark, who ought to know, a slightly soft focus helps the calculation of a stellar centroid, the notional sub-pixel point on which the guide command is based.

Olly

Yeah, I'm aware of that view.  I just wonder whether I'm pushing the resolution limit using the Starlight Xpress Lodestar X2 I have now with its 8.5 um pixels in a 9x50 Finder. Using the guidescope utility on this site I get imaging/guiding ratios of 1:4.6 and 1:7.6 with my ED80 and SW 200p respectively. The SX Superstar would improve both those ratios by a factor of two. 

Now this is not a definitive experiment but I tried guiding using my lodestar in the main scope ED80 just to see how good the guiding could be. After changing all the parameters in PHD2 the guiding was a factor of two-ish better. Now OK the improvement could be down to no flexing between guide scope and main scope ..... or it could be down to better imaging/guiding ratio. 

[JohnSadlerAstro]

13 minutes ago, Ouroboros said:

Imaging/guiding ratios of 1:4.6 and 1:7.6 with my ED80 and SW 200p respectively. 

Ouch. I assume that means 1":4.6"/px for the main and guide cameras? If so that sounds quite nasty.

I'm wanting to try a 2x Barlow in my 9x50, Ive got it focused etc but I don't know whether it will make any real improvement. It will be 1.8":2"/px with the Barlow.

John

[ollypenrice]

40 minutes ago, Ouroboros said:

Yeah, I'm aware of that view.  I just wonder whether I'm pushing the resolution limit using the Starlight Xpress Lodestar X2 I have now with its 8.5 um pixels in a 9x50 Finder. Using the guidescope utility on this site I get imaging/guiding ratios of 1:4.6 and 1:7.6 with my ED80 and SW 200p respectively. The SX Superstar would improve both those ratios by a factor of two. 

Now this is not a definitive experiment but I tried guiding using my lodestar in the main scope ED80 just to see how good the guiding could be. After changing all the parameters in PHD2 the guiding was a factor of two-ish better. Now OK the improvement could be down to no flexing between guide scope and main scope ..... or it could be down to better imaging/guiding ratio. 

I suppose you might be pushing it. I use ST80s for guiding, so 400mm FL with the Mk 1 Lodestars. As ever, I think you did the right thing to experiment. I've never used a finder guider.

Olly

[Ouroboros]

3 hours ago, JohnSadlerAstro said:

Ouch. I assume that means 1":4.6"/px for the main and guide cameras? If so that sounds quite nasty.

I'm wanting to try a 2x Barlow in my 9x50, Ive got it focused etc but I don't know whether it will make any real improvement. It will be 1.8":2"/px with the Barlow.

John

I tried the 2X Barlow that comes with the SW 200p in the finder scope but found I couldn't reach focus.  

[JohnSadlerAstro]

1 hour ago, Ouroboros said:

Found I couldn't reach focus.  

Hi,

I have a pair of adapters for the 50mm finder inside ring, one to female and one to male T-Ring. Using the thinner one, (female I think) I screwed the top of the focuser barrel from my old 130M into it. Unfortunately, I could reach focus, so I tried screwing the focuser end onto the inside and it worked fine. I had to replace the tightening screws with one tiny one, as the originals were to large to fit inside the finder barrel.

This is the result:

John

[Ouroboros]

@JohnSadlerAstro I can't quite work out what you've done there but it works that's the main thing. I can't recall now whether the Barlow made the focus in too far or out too far. I'll look up the optics formula for Barlows. 

[JohnSadlerAstro]

Hi,

I'll get a photo of how it goes together and post them up later today--you may well have the bits required for it to work. I found the Barlow was focusing too far in, i.e. I couldn't push it inwards far enough to focus. The reason for that was the length of the barrel and the adapter I was using--obviously the focus point is actually further out when using a Barlow.

John

[JohnSadlerAstro]

Hi,

The photos.

Sorry they are in dreadful quality, I assure you that I used a British baked potato to take the photos. ?? Also the map was from an old (1939) issue of the Daily Express, I'm not a regular reader. ?

^ The overall guide assembly. Behold the venerable rubber bands to hold the red-dot on!

^ With the cam unscrewed. You can see the pink 3d printed adapter with female T-thread on the inside. Not my choice of colour.  

Now the key to the entire assembly:

I've screwed the end of a 1.25" focuser barrel into it. It's the end part of an all plastic one that comes with the SW 130M, there's a male t-Thread on the top of it which screws into the pink adapter. 

Because the eyepiece tightening screws have large heads and are too long to fit inside the barrel of the finder, I took them out and replaced them with a single, short screw. The Barlow slots into that from the other side and is tightened up as usual. 

Obviously it cant be adjusted once I screw the assembly into the finder, but I've done it up nice and tight so the Barlow wont work loose. But for a simple system which gives 360mm focal length, I'm quite pleased. Haven't tested it yet, though.

John

[Ouroboros]

@JohnSadlerAstro yes, I can sort of see what you've done there.  I'll have another bodge about and see what other bits and bobs I've got. I'm using one of those Finderguiders from Astronomy365, or whatever it's called, which has a 1.25" eyepeice adapter that screws into the end of the finder tube. Essentially what you're saying I think is that the Barlow needs to sit in place of that 1.25" eyepeice holder so that it's far enough forward to focus on the camera. 

 

[JohnSadlerAstro]

7 minutes ago, Ouroboros said:

Essentially what you're saying I think is that the Barlow needs to sit in place of that 1.25" eyepeice holder so that it's far enough forward to focus on the camera.

Hi,

Yes, that's correct. Basically the eyepiece holder is inside the finder, not outside it.

Nice setup you've got there. ? Got any good photos with it?  

John

[Ouroboros]

Thanks.  The most recent was this one of the gamma Cygni nebula near Sadr.  I was quite pleasantly surprised how much I picked up considering my DSLR is unmodded. 

 

[michaelmorris]

On 27/08/2018 at 09:54, ollypenrice said:

This is such a sensitive camera that I really can't see it being an issue.

Olly

What he said. 

I switched from a mono Lodestar (a very sensitive camera) to a mono Lodestar x 2.  I reckon on getting somewhere between 20% and 40% more available guide stars as a result.