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Sammyb

Celestron CPC series and field rotation

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Hello,

I understand these scopes require a wedge for long exposure astrophotography due to field rotation from the AltAz mount.... but I have been wondering how long is "long"?

At what exposure length does the field rotation become evident? Is it possible to take 30s or 1 minute exposures?

Sam

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Depends on the altitude of the subject and the focal length of your scope. The lower in the sky the object is the longer the exposure you can make; the higher the object is the shorter the exposure you can make until near the zenith it's almost impossible without trailing. The shorter the focal length the longer the exposure you can make. I have managed to photograph the Orion Nebula with an f6.3 focal reducer with an exposure of about 25 seconds. That object is both reasonably bright and low in the sky. This is with a CPC 925 GPS. Really, you need an EQ mount to get successful long exposures (or a wedge).

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The best answer is, "have a go", as it will vary depending on what part of the sky you are looking at. Targets in the Northern sky for instance appear to move less in a given period than those in the southern sky.

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and the focal length of your scope

As far as I am aware, field rotation is not affected by focal length.

NigelM

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Wedge is the way to go for long exposure AP. after about 30s you start to get blurring due to Field rotation. I am selling my CPC 1100 because a wedge sounds a bit too much of a faff to me.

Sent from my iPhone using Tapatalk

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Field rotation rate is zero to the East and West and a maximum at the meridian. It increases as the altitude increase and is infinite at the zenith.

What I've tended to use as a rule of thumb is that 20 seconds will be OK over most of the sky for altitudes of less than 60 degrees. You may be able to manage 30 seconds looking more or less East or West.

Most scopes can't be guided and this will become an issue at much more than that.

Chris

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Field rotation rate is zero to the East and West and a maximum at the meridian. It increases as the altitude increase and is infinite at the zenith.

What I've tended to use as a rule of thumb is that 20 seconds will be OK over most of the sky for altitudes of less than 60 degrees. You may be able to manage 30 seconds looking more or less East or West.

Most scopes can't be guided and this will become an issue at much more than that.

Chris

Quite. Also remember that because of the focal length 20-30s is no time at all on DSO on these scopes since they are slow in photographic terms. They are great for planetary AP and visual, GREAT for DSO visual but not so good for DSO AP, unless of course you mount it on a wedge. If you are in a position to leave on the said wedge - especially if you have an observatory - then one of these would make a great telescope!!! But for folks who have to break down every time I think a wedge is a little too much messing about and defeats the whole object of the CPC concept. However, that's because I don't have a wedge. Maybe its straightforward...... ???

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Opinion on the proprietory wedges and the performance of the Amercian forks on such wedges varies. I'm squarely located in the 'not impressed' camp. Ken, on the other hand, has made them work.

I find them a pain to polar align and my LX200 never got anywhere near being able to image without trailing. The nearest I got was piggybacking a short FL refractor on it and that did work well enough under guiding.

Long FL scopes need good mounts and I don't think I'd call either Meade or Celestron mounts 'good' in photographic terms. I wish I had never ever gone anywhere near a wedge and I was glad to be rid of it. I loved the LX200 in alt az for visual use and should have left well alone.

Olly

Edited by ollypenrice

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I have a wedge with my CPC11'' ; along with auto-guider it a single best investment. This turns your scope into fully-fledged astro-imaging system without the mess with meridian flip and balancing.

I have two systems sitting side by side and Alt-Az on a wedge is a lot easy to work with. You start it and leave for whole night from East to West.

If you have Hyperstar you can try 20-30 seconds sub exposure. With full F/10 it sounds like no go - may M42 or M27 only.

Mark

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Opinion on the proprietory wedges and the performance of the Amercian forks on such wedges varies. I'm squarely located in the 'not impressed' camp. Ken, on the other hand, has made them work.

I find them a pain to polar align and my LX200 never got anywhere near being able to image without trailing. The nearest I got was piggybacking a short FL refractor on it and that did work well enough under guiding.

Long FL scopes need good mounts and I don't think I'd call either Meade or Celestron mounts 'good' in photographic terms. I wish I had never ever gone anywhere near a wedge and I was glad to be rid of it. I loved the LX200 in alt az for visual use and should have left well alone.

Olly

My Meade LX10 (8") was on a wedge. The OTA is the same as the more expensive goto models of that era, and is still sound (apart from no mirror lock), but the mount was the cheapest thing Meade could make. No goto, DC motors for tracking, no electronics and the fork was about as thin as the one you eat your dinner with. The wedge didn't even have an alt or az adjuster to speak of (bought an after-marker alt adjuster years later, but never sorted out az).

The most I ever managed unguided was 60s (see below), which is not much exposure time on an f/10.

M42

I did build an autoguider using an arduino and a relay board. It just about worked if you used a webcam through the OTA as the guidecam and imaged with a piggy-back DSLR and lens. I think I could just about get to ten mins on a good night with no trailing if the plastic Dec gear didn't bend too much. No way that set-up would have guided the main OTA though.

It was a 'formative' experience, and I did learn a few things about forks and wedges:

- Getting anywhere near polar aligned is hard with a wedge, due to no polar scope. if you had a permanent set up I think you'd be able to drift align your way there, but otherwise it is hard work, whereas getting close to good PA with a GEM and polarscope is a five minute job at most. There are polarscopes that can sit on wedges in place of the OTA (e.g. the polarmate), but I think it unlikely that you would get a repeatable and spot-on alignment between the device and the wedge each time.

- If you need to break everything down to set up and pack away, weight is an issue. Getting the relatively small 8" with its featherweight forks and drive base on to the wedge was hard enough. I wouldn't like to have to do the same on my own with a bigger OTA and heavy goto forks/base. Wherease mounting on a GEM is easier since the two heavy parts (OTA and mount head) can be lifted on separately.

- The fork is an issue, e.g. if one arm is slightly lower than the other then you get all sorts of interesting pointing/tracking errors. If you trust the factory to get everything spot on, you're kidding yourself and adjusting to fix is not for the faint hearted.

- The fork/wedge combo is a source of all sorts of instability/vibration. The main weight of the OTA is a long way up and out from the RA bearing, unlike a GEM where it is about as close as you can get it. I know mine was puny, but lots of other people with LX200's seemed to have issues with vibration and flex. The lower the latitude, the more of an issue it becomes.

- Think about the limited clearance between the back of the OTA and the fork base when you are imaging high up. There isn't much space and you are limited as to the length of the accessory train that you can put on the back of it.

I bit the bullet and bought an NEQ6 when I got interested in AP (after dabbling with the LX10 for a while first). I don't think I would go back to a wedge/fork mount again, but go for the same OTA on a GEM if I was in the market for a bigger SCT.

On the other hand, if you already have a big investment in the SCT and can pick up a wedge at a reasonable price (second hand maybe), it would be cheaper than a whole new mount, so might be worth it if you have no other choice.

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I think you sum the issues up very well IanL.

I don't doubt that a wedge, in expert hands can be made to work very well or the skills to drive it optimally can be learned with experience. But for those without a permanent set up who have to break down in the fickle UK weather, with all that implies, I am not at all sure this is the way to go to further your AP career. There is a need for square pegs most definitely. There is likewise a need for round holes. But you don't put square pegs in round holes and from my research that is what you do with mounting a fork mounted scope on a wedge.

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I have a wedge with my CPC11'' ; along with auto-guider it a single best investment. This turns your scope into fully-fledged astro-imaging system without the mess with meridian flip and balancing.

I have two systems sitting side by side and Alt-Az on a wedge is a lot easy to work with. You start it and leave for whole night from East to West.

If you have Hyperstar you can try 20-30 seconds sub exposure. With full F/10 it sounds like no go - may M42 or M27 only.

Mark

Mark, looking at your signature suggests that you use the fork and wedge at about F2 with the Hyperstar. I can see that working, but do you find that the CPC and wedge can consistently deliver round stars at native FL or even at F6.3? And if so, over what kind of exposure time?

A fork that works is a great idea and if that's the CPC then fine. However, literally everone I know personally who has tried imaging on the American fork and wedges has given up and switched to GEM.

Olly

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Olly,

I did not try it at f/10, you need Arizona sky for this. I did use it with AP 0,67 reducer for galaxies and for M13. Exposures were 5 to 8 minutes with OSC camera which required very long integration time.

I did not discard any single sub but I used it with PEC training and of course did Polar alignment. On the other hand I was guiding it with a finder at 1800mm focal length.

This was a standard wedge from Celestron.

Mark

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