Periodic error

[Ken82]

So what is reasonable unguided periodic error for imaging ? I currently have 20arcsec but celestron claim the cge pro to have only  -+5arcsec so I should send it back ?? Am I missing something or being unreasonable ? 

[theastrodragon]

Depends on your resolution.

Celestrons claims are often a little - optimistic.

Try the free version of PemPro, it can give a big improvement to some mounts, and it will show you if any software based improvement is possible.

[Pompey Monkey]

Guiding will sort out 20" of PE no problem. Even if the mount was working at 5", you'd want to guide that out anyway.

Guiding.

[dph1nm]

If that is +/- 20" I would speak to your dealer. The blurb says "typically +/-5" before PPEC so I would be pretty unimpressed with +/-20 at that price. In fact I have just noticed that Green Witch claim +/-3" for it.

NigelM

[Ken82]

Yep Nigel well said , I thought I was being unreasonable but I'm sending it back . Useless 

[Ken82]

I interpret the -/+5arcsec as a maximum error of 10arc sec peak to peak . Is that correct ? 

[ollypenrice]

The value doesn't depend on your resolution, it is an absolute description of error. I would consider the +/- 5 to be very optimistic as well. Takahashi claim +/- 3 for the EM200. As Pompey Monkey says, though, for most applications you'll want to guide and these budget Chinese mounts do tend to autoguide out very well.

Olly

[steppenwolf]

The +/- 5 figure does seem very optimistic indeed to be honest. Assuming that your 20" is peak to peak, this represents +/- 10 arcseconds which although not as low as the blurb you quoted, is more realistic. However, this will guide out very easily as has already been pointed out.

[kens]

Quote

The typical tracking accuracy for the CGE Pro is +/- 3 arcseconds without the use of guiding or PEC (periodic error correction). 

This is not a guaranteed specification, but it’s quite common for the CGE Pro mounts actually in use. Performance can vary a bit from this typical value; however, small differences will not affect performance in any way.

http://www.celestron.com/support/knowledgebase/articles/what-is-the-correct-value-for-the-tracking-error-of-the-cge-pro-mount

Interpret that as you will.... (my bold)

[Ken82]

Thanks Ken , its not guaranteed but quite common and yet mine is over 4 times worse!  laughable ! the not guaranteed bit is not commonly written in the cge pro documents. Im now considering just getting a refund and getting something else, fed up with chinese Rubbish.

[kens]

I agree. "Not guaranteed" to me means "in your dreams". 

[theastrodragon]

I must admit, having had rather better built mounts with uncorrected errors in the range +/- 3-4", the Celestron figure does seem a trifle...overachieving?

Trying to think of a polite way of saying not a snowballs chance in hell, here

[Pompey Monkey]

27 minutes ago, Ken82 said:

Thanks Ken , its not guaranteed but quite common and yet mine is over 4 times worse!  laughable ! the not guaranteed bit is not commonly written in the cge pro documents. Im now considering just getting a refund and getting something else, fed up with chinese Rubbish.

Best get the Amex card out then

What do you realistically expect for a mount that will carry a 40 kilo payload for £4k?  It took me six months to get my £1.3k AZ-EQ6 to do guided subs at below 1" rms. If this hobby was easy and/or affordable, it wouldn't be worth doing

[Ken82]

What do I realistically expect ?? I expect the mount to be at least near to what has been advertised simple as that ! Quote "Typical unguided periodic error of -/+3arc sec" so mine is just "typically useless " sorry just [removed word] of with David hinds and celestron . Martin is on the case from FLO 

[Ken82]

Might I add that periodic error isn't the only issue if I'm sounding frustrated and annoyed . I also have a lot of backlash so unable to calibrate a guide scope ! 

[dph1nm]

15 hours ago, Pompey Monkey said:

What do you realistically expect for a mount that will carry a 40 kilo payload for £4k?

Well all the reports I have seen suggest the somewhat cheaper EQ8 has a periodic error around +/- 3" ...

NigelM

[Ken82]

I would seriously doubt that myself but that's just an opinion. The eq8 comes with no statistics at all a bit like the eq6 or cgem as synta can't afford to claim anything on these widely differing mounts . Just look at the statistics for the eq6 mount in this link , some have periodic error of 120arcsec shocking ! The statistics for the cge pro here are taken when it was first released (at that point celestron claimed -/+9arcsec ) so the readings were within what was claimed . 

http://lambermont.dyndns.org/astro/pe.html

 

 

[theastrodragon]

Given that the error (without software correction) of the bare Paramount is over 3", the suggestion that a £1k Chinese mount can achieve this is rather...optimistic.

Even the top-end mounds have a fair bit of mechanical PE, the difference with them is its smooth and easily improved significantly with software like PemPro.

The table here http://www.ccdware.com/products/pempro/examples.cfm  may make interesting reading - its real results sent in by real users to CCDWare, not their own figures.

[kens]

Isn't the CGE Pro over GBP 3500? At that price, and with the manufacturer's claims, one would surely expect better than a GBP 1000 mount. Interestingly, when I looked up the price the retailer's website I found makes a more reasonable claim of +/-9"

[theastrodragon]

+/- 9% I would likely believe.

Also the figures given by the manufacturers are often misleading.

First, they are using a new mount, set up by people who understand it intimately and how to tweak it for the best performance (that's assuming they haven't picked a best-of-batch performer as well), so they are going to be the best figures possible.

Some of the mounts have PEC available, and its rare to see the manufacturer say whether or not this was in use.

When it comes down to it, you get what you pay for. There's a reason the high precision mounts are expensive - not just the overall accuracy, but proper quality control and a performance figure that they expect to exceed.

[Ken82]

Reply from celestron -

 

 "... Concerning tracking, we state +/- 5 arc seconds peak to peak periodic error which is exclusively a tracking error figure. There is often mount flexure which is non-periodic and exceeds the total PE the longer tracking takes place. It's another good reason to autoguide..."
 

[Ken82]

This is part of an email response from  the PhD software developers -

 

  If we see a sine-like wave of 20 arc-secs peak-to-peak, that’s not flexure.

 

celestron  have sent me a scrap heap challenge and I'm not impressed ! 

[theastrodragon]

So basically it isn't tracking error because they have blamed it on something else?

Colour me unimpressed.

If it IS the mount bending and swaying, as it were, its even WORSE than PEC, as correction software such as PemPro is unlikely to help (it MIGHT be possible to get a decent correction, but it will only apply at a particular mound position)

[Ken82]

Yea typically trying to blame it on something else ! I'm disgusted !! Not accepting it simple as that . 

[theastrodragon]

It should be fairly straightforward to determine if its PEC or mount flexure.

PEC is (broadly) sine like, and while it differs slightly with the mount orientation, it doesn't vary that much.

If its mount flexure, it will vary with the mount orientation.

So if you can measure the total error with the telescope in various attitudes, you can with luck see which one is the main culprit.