Autoguiding a CPC 800

[skogpingvin (Bill)]

Hi folks, Bill here

I've just started a part-time job at Optics Central in Mitcham, being a technical support person for astrophotographers as well as doing some birdwatching stuff as well.

I've got a customer with a Celestron CPC 800. He wants to use it for astrophotography, and says that he's not fussed about the field rotation that will come about from the alt-az, but he is worried about poor tracking. It seems that he's getting spaghetti stars. So he wants to autoguide it for five minute exposures.

We've got guide scopes and stuff, but I'm not sure how it would all fit together. I don't know what the appropriate setup is, all I can say is that his setup is going to be different to mine (I have a pretty standard NEQ6/EQMOD and an OAG/PHD2 with SGP).

The CPC mount has an autoguide port - I think. Does anyone use it? Is this the best way to go?

[garymck (Gary)]

Your customer is never going to be able to do 5 minute exposures in altaz configuration. The limit will be somewhere between 5 seconds and 30 seconds (can't recall how to calculate it exactly) depending upon how high the object is in the sky - longer exposures will always show field rotation even if autoguided. There is no way around this unless you add another piece of equipment (field derotator)...The stars will always be elongated, less if autoguided - perhaps macaroni rather than fettuccini length :-)

Gary

[skogpingvin (Bill)]

Originally Posted by garymck

Your customer is never going to be able to do 5 minute exposures in altaz configuration. The limit will be somewhere between 5 seconds and 30 seconds (can't recall how to calculate it exactly) depending upon how high the object is in the sky - longer exposures will always show field rotation even if autoguided. There is no way around this unless you add another piece of equipment (field derotator)...The stars will always be elongated, less if autoguided - perhaps macaroni rather than fettuccini length :-)

Gary

Thanks Gary. I was kinda afraid this was going to be the outcome. Because I'm on an alt-az I'm totally unfamiliar with field rotation. I think there's a wedge for the CPC 800. I'm no fan of wedges, as it makes the mount unstable, but this might be his only option.


Bill

[The_bluester (Paul)]

That really will be your customers only workable option. I never tried guiding on my old CPC mount but they should work like any other guider port (ST4 I think) but field rotation will show up after about 15 seconds.

The only workable option to be able to guide it and get a useful result would be to put it on a wedge. I have no idea how well the CPC mounts behave for AP. Even guided, 5 minutes might be pretty challenging. It would also probably need to be done using an off axis guider as a guidescope will show a lot of differential flexure type issues, particularly with the movable main mirror in the C8 tube.

[skogpingvin (Bill)]

Originally Posted by The_bluester

That really will be your customers only workable option. I never tried guiding on my old CPC mount but they should work like any other guider port (ST4 I think) but field rotation will show up after about 15 seconds.

The only workable option to be able to guide it and get a useful result would be to put it on a wedge. I have no idea how well the CPC mounts behave for AP. Even guided, 5 minutes might be pretty challenging. It would also probably need to be done using an off axis guider as a guidescope will show a lot of differential flexure type issues, particularly with the movable main mirror in the C8 tube.

Thanks Paul, I'll discuss it with the client. I was in a similar situation a few years back, and succeeded in (kinda sorta) autoguiding a Meade alt-az mount. It was such a pain that I eventually pulled the pin and upgraded to an NEQ6, tearing the OTA off the mount and bodging it onto a Vixen dovetail. Never looked back.

[The_bluester (Paul)]

More or less what I did. I took my CPC925 off it’s mount and put it on an Orion Atlas Az/Eq mount.

[skogpingvin (Bill)]

Originally Posted by The_bluester

More or less what I did. I took my CPC925 off it’s mount and put it on an Orion Atlas Az/Eq mount.

Oh, cool. How do you find it? Apart from this client, I've got a mate who is considering a wedge for another alt-az. I told him it might get wobbly.

[The_bluester (Paul)]

I am far happier with the Orion mount. I ended up really disliking having to wait for the CPC to slew and never being able to manually point it. It did have advantages though, the main one being it was very stable once set up, you could touch the OTA or eyepiece without setting the thing shaking.

I mainly use my current mount in Alt-Az mode but even then having the dual encoders is great as if you want to quickly change targets (After aligning it properly of course) you can just release the clutches and aim it yourself and the mount keeps on tracking once you do the clutches up again.

To be clear, the Orion mount I have is dual mode, GEM or alt-az natively, it is not on a wedge. Any wedge for a decent sized mount would need to be a pretty solid affair.

[skogpingvin (Bill)]

Originally Posted by The_bluester

To be clear, the Orion mount I have is duel mode, GEM or alt-az natively, it is not on a wedge. Any wedge for a decent sized mount would need to be a pretty solid affair.

Hmm, I'm a little confused. Did you buy it in Australia? I think it's sold here as the Skywatcher AZEQ6 GT, like this one: www.opticscentral.com.au/skywatcher-azeq6-gt-goto-mount-w-tripod.html.


I'll pass on your experiences with the wedge. Many thanks. It was what I suspected, but I don't have any actual experience with them.


Bill

[The_bluester (Paul)]

The Orion Atlas pro is the same mount as an AZEQ6. I bought mine from Bintel in Melbourne a few years ago (Before they closed the Melbourne store obviously)


At the time the Orion was several hundred dollars cheaper than the AZEQ6 with the difference being it was painted black!