Periodic error

[ollypenrice]

I have long argued that the claims made by, above all, some American manufacturers or resellers are sufficiently close to fraudulent to be worthy of investigation by trading standards. If they sold in the volume that car manufucturers sell then this would have happened, with no doubt at all. In another life I had some involvement with trading standards law. Take this phrase:

You’ll be able to track through long exposures using permanently programmable periodic error correction. Image across the meridian without doing a meridian flip, so you can seamlessly image the best part of night sky.

In my opinion a man on a Clapham omnibus would take this to mean that 'permanently programmable periodic error correction' would allow you to take 'long exposures' and this is what? Gentlefolks, this is a downright, bare faced LIE. And eveybody in astrophotography knows it.

Once you get into real amateur astronomy, and away from cheap hype, you soon move into a different world from that of (insert American rebranders' names) and into the world of Mesu, 10 Micron, Astro Physics, Paramount etc where claims of unguided capability are dealt with realistically. To run unguided you need a mount managed by absolute encoders and a planetarium model of the sky. A few manufacturers claim it and I know from experience tha 10 Micron can deliver it. The others I mention don't claim that, they claim to make mounts which, autoguided, can image at very long focal lengths and they, too, can deliver that. They do claim a +/- PE error and they meet it.

In a bigger market there would be more court cases.

That said, the budget mounts under autoguiding do cash-strapped amateur astrophotographers a huge favour. I would be the last person to say otherwise. My argument is not with that, it is with the blurbs in their marketing.

Olly

[Ken82]

Very well said olly ! I would absolutely agree, a couple of my favourite nonsense from celestron - the edge HD has an imaging circle of 42mm (lucky if it's half that!) and yet they supply a standard t thread adapter which is only 38mm . The Cpc is advertised as a "serious Astrophotography tool" don't make me laugh what utter Rubbish !!! 

[theastrodragon]

You may find this site of interest http://demeautis.christophe.free.fr/ep/pe.htm

It's in French, but he's measured quite a few commercial mounts (unfortunately not the most recent ones), but it gives an idea of what a lot of them are actually like.

 

[ollypenrice]

8 hours ago, theastrodragon said:

You may find this site of interest http://demeautis.christophe.free.fr/ep/pe.htm

It's in French, but he's measured quite a few commercial mounts (unfortunately not the most recent ones), but it gives an idea of what a lot of them are actually like.

 

That is a VERY interesting link. Non French speakers will be able to understand it easily. Thanks for posting this.

Olly

[ollypenrice]

11 hours ago, Ken82 said:

Very well said olly ! I would absolutely agree, a couple of my favourite nonsense from celestron - the edge HD has an imaging circle of 42mm (lucky if it's half that!) and yet they supply a standard t thread adapter which is only 38mm . The Cpc is advertised as a "serious Astrophotography tool" don't make me laugh what utter Rubbish !!! 

Could you tell us a bit more about this?

Olly

[Ken82]

16 minutes ago, ollypenrice said:

Could you tell us a bit more about this?

Olly

The edge HD white paper claims the imaging circle of the scopes is 42mm (which would be a full frame 5d etc) I've done some testing and it doesn't even illuminate an aps-c- sensor . It is a lot better than a standard sct but not what is being claimed . Even with flats you can't image at this size of sensor . 

I mean they claim this in the marketing paper and yet still supply a standard 38mm t adapter for the scope . At least takahashi who claim 42 with the fsq have a wider t adapter . I've tested that as well and perfect all the way to the edges on my full frame canon 6d 

[Ken82]

I was speaking to Ian King about this and he was just laughing about the celestron claims. My wife has a master in media science (marketing) and she's surprised someone hasn't had a day in court with celestron. You know olly if I had more time on my hands as a matter of principle I would take them to court . I would have them under section 75 "misrepresentation of a false statement of fact"! Luckily we are well looked after at FLO so it won't be needed. 

[Waldemar]

For the RASA Celestron claimed a 70mm optimized image circle... they corrected that laterly:

Today, with CCD sensor sizes as large as film—or larger—the Schmidt Astrograph offers a 43.3mm optimized image circle to capture pinpoint stars on the largest imaging chips. (Note: We previously published 70mm which is actually the aperture of the last lens in the telescope. The optical design has not changed). Combine this large image circle with a focal length of just 620 mm and you have an instrument suitable for wide-field imaging, creating huge mosaics of the night sky, surveying, and even comet hunting. 

So, I am totally not surprised if everything else is vastly exaggerated also...

[alacant]

On 07/03/2016 at 23:00, 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.

Hi

Yeah, it came as a surprise to me too. Having rebuilt a used mount, I thought that if I spent €1200 on a new mount, it would a lot better. It was in fact a lot worse. No grease, screws missing and a loose motor. To their credit the manufacturer was very good in sending replacement parts (very much in the style of it being expected!) but wtf. It took weeks to get it tracking properly. The truth is it seems that at any price as soon as you get something new, you have to pull it apart to make it work as it should.

[libraryman]

I am surprised that no one has pointed out that periodic error is not in itself a major issue, it is the shape of the pe curve that is more important. 

Of course 20 arcsec is quite a bit....but... If the pe curve is fairly smooth and slow; then the corrections will control your mount very well and quite possibly better than a mount with 5 arc sec pe but steeply shaped curve.

if you can guide at a fairly low rate.. I.e. 0.5x sidereal or less then you should be fine but if you need say 1.0 x sidereal to keep up with a steep curve then that's a potential problem.

Ray

[Ken82]

Yes a good point Ray smoothness is what we're looking for! I appreciate a smooth periodic error of 20 is better than say 5 that is spikey. 

[theastrodragon]

Somehow I suspect that if you have 20 arcsec error it ain't that smooth...