Which Guide scope for a Celestron CPC 9.25

[Ramsees]

Hi Folks,

I just bought the new Celestron CPC 9.25 HD-EDGE and have attached to it an F6.2 Lepus focal reducer . I am guiding with a 60mm APM Deluxe guide-scope which is 284 Focal length. after a few discussion with the American company OPTCORP they said that to guide properly you need a minimum guide-scope that is 4 times less than the original focal length of the main scope. with the focal reducur I will have a FL of 1457mm divided it by 4 = 364 FL required for the guide-scope. would the APM still work in your opinion.

Thanks

[Amaranthus]

Have you considered an off-axis guider instead? That is what I use on my C8.

[John K]

Unless you have a super duty mount permanently set up, at that focal length, you might find OAG is the way to go. Consider spending the cash on a sensitive guider and OAG set up.

[Ramsees]

Thanks guys , I bought a new QHY guider with 3 micron pixel size and a high resolution . I tried the OAG but it is proving too difficult becuase finding a guide star is a challenge, then the stars are never round and then once I find the star it wobbles immensely and PHD goes crazy . Because the Celestron is a big instrument along with the dew shield attached any slight wind shakes it making it difficult to guide on an OAG. Any ideas are helpful but I do have the permanent pier in concrete and a High resolution small pixel guider so hopefully that will suffice .

[Amaranthus]

Which QHY guider? The 5Lii should be more than sensitive enough for an OAG. Don't worry if the stars in the OAG are not round, that is irrelevant for guiding.

Look, I virtually guarantee you that you'll get better guiding on a C9.25 with an OAG than you will with any guidescope. Post up a screenshot of your PHD session and let's have a look at what's wrong.

[Merlin66]

I agree with Barry....
If the star is wobbling in the OAG FOV then I'd expect the stars to be wobbling in any image you take....
Could be seeing conditions, or some mechanical factor.
A good guide camera or a good OAG will give you results second to none.
I use PHD2 (and AstroArtV5) for guiding with an on-axis guider (reflective slit spectroscope) on a C11 at f10 using the Lodestar and can easily get ten minute subs on a 20 micron gap.......

[Ramsees]

Guys,

thanks for the feedback, I bought the QHY guider 5Lii and this apparently is a good camera and then it is attached to an APM Deluxe guide-scope. I can't get an OAG because I bought all the gear thru OPTEC which is the Lepus Focal reducer designed for the CPC Celestron HD-EDGE and that comes with a special tube length. I just need some assurance that with the APM and guiding camera I hopefully should be fine. I use PHD and therefore I am testing all the new gear sometime this week once I get my guide camera and Bahtinov Mask for focusing

[gb44]

The settings in PHD are critical - what settings do you use for the setup?
I can send a recommended set that I gleaned elsewhere and have used with success for my finderscope/refractor combo.

GlennB

[Ramsees]

Glen,

I am now using PHD 2, I just put in the camera pixel size and then the focal length and that is pretty much it apparantly. Can you please send me your recommended settings which will work with my setup and equipment. That will be greatly appreciated. Thanks heaps

[Peter.M]

Please keep in mind, many people move up to an OAG, watch the graphs they are used to seeing in PHD which are flat, and complain that the OAG star is bouncing around. This is fine and valid if you are using phd2 and have it in arc seconds, and you have your program setup correctly. The guider he is using has 3 micron pixels, a binned lodestar has 17 micron pixels. A movement of 4 pixels in your QHY guider is still less than 2 arc seconds.

I myself would go with the OAG too, if you can find a guidestar with the guider, and I would definitely think about binning it!

[Ramsees]

Thanks Peter, I appreciate your email here but am not much familiar with the terminologies of Arc seconds and pixel sizes. Everyone's saying that PHD 2 is better because it works on arc seconds and that the Camera I have has small pixels therefore it should pickup the movement better and guide hopefully better. I can't goto an OAG because I specifically ordered the Lepus F6.2 Focal reducer for the CPC HD-Edge and everything has been made up to get the right distancing to the Camera. if I go with an OAG then all of that will have to be scratched and then I would have to sell my new APM guide scope and buy new spacers which is a bit of a loss and long journey.

My relevant question is will my curent setup work if I take say 8 minutes exposures as I can't go too long due to light pollution issues.

any thoughts ?

[torsion]

Hi Ronny,

I think you will be fine, just take care to make a solid connection for mounting your APM onto the Edge HD to 'remove' and differential flexure.

As for the 'mirror flop' you can mitigate that with the mirror locks on the back after you have finished focussing. You may have to undo and refocus if there are large temperature changes.

As for the PHD2 settings start off with the defaults, although you may need to adjust the calibration steps, but there is a handy calculation option to get yoy going. Theae settings are mount specific.

I use Metaguide and it works great, even used the finder scope and an analogue video camera to guide. Now I use the QHY5lii and it runs fantastic.

As mentioned before, as long as your error is around +/-2 arc seconds you will/should have round stars.

See to post a screen shot of your guide plots, with any modified settings.

Cheers,
Bram

[Ramsees]

Thanks Bram, appreciate your input here. how do I check for the error in arc seconds, any idea where that is displayed in PHD, if Metaguide is better where can I download it from please.

[Peter.M]

These are not available in phd1, In phd2, there are settings that define your pixel size and focal length. Then the graph has a button that changes the scale between arcseconds per pixel and pixels.

This is what I was getting at, if you use phd1 and have been using a guidescope the graph that you are used to has been hiding errors that you will begin to see if you use a longer focal length guider (namely OAG), or move to PHD2 which shows arc seconds per pixel. For this reason I think that people believe OAGs are inferior (guide star jumps around a lot), when in actual fact the graph is just scaled differently.

[Rod771]

Hi Ronny

This may be a silly question but..... Do you have your CPC mounted on a EQ wedge? To do long exposure photography effectively with ALT AZ mount it needs to be mounted on a wedge so it can be aligned with the south celestial pole.

You may know this already, just checking. :)

[torsion]

Hi Ronny,

I would stick with PHD2 for the moment, if you get your head around Peter's comment you could/should investigate alternatives (Metaguide is http://http://www.astrogeeks.com/Bliss/MetaGuide/ ).

I am not a PHD2 expert, but I made a screen shot of the 'History'. To view this window select 'Display Graph' in the main 'View' drop down menu (unfortunately the naming and labeling is not consistent).

Then you can set the 'y-axis units' in the 'Settings' drop down on the left. The options are 'Arc-seconds' and 'Pixels'.

When in 'Arc-seconds', and you have provided the guide scope focal length (brain - Global) and guider ccd pixel size in micrometers (brain - Camera), it will represent physical pointing error of the guide scope/camera. Assuming the rigid connection of the guide scope to the main scope then this will represent the pointing error of the main scope as well.

If in 'Pixels' it will show the guide star movement on the guide ccd. It can be handy for trouble shooting (say for calibration see if the star moves with the cal steps), but it has no real representation and you can't compare guide performance because it depends on the focal length and pixel size on how much the scope is moving. This actually comes back to Peter's comment.

Oh, yes I assume you have a wedge :)

Hope this helps.

cheers,
Bram