Celestron 8" EdgeHD

[John Gill]

Hi,

I am currently running a Celestron 8" EdgeHD scope on an AVX mount.  I also use a StarSenseAlignment gadget. 

I do an auto alignment and after 2 minutes the scope understands the Universe.  I then select a well known star, Alpha Centauri or Antares and hit the button.

The scope slew to the object but I am visually off center by 3-4 inches.  Anyway, I then do a ASPA do the necessary adjustment and all is good.   I then do another 

auto-alignment and I am still off by 3-4 inches.  I have gone as far as alignment, ASAP to Alpha Centauri, power off, another alignment, ASAP to Antares and

a further alignment but still off by the dreaded 3-4 inches. 

The scope is level, is pointing south (cos I am a Southener), balanced in Dec and RA, time and location is 100% etc etc.  The only positive item is that when I attach my Canon d600

camera I seem to be closer to the target or perhaps the FOV is bigger

I have contacted Tech Support at Celestron but none of their suggestions helped.

So any ideas, suggestion would be most welcome.

Regards

John

[CodnorPaul]

To be honest with a mount of that cost (no offense intended as I run an neq6 which is not an expensive one) as long as it is in the field of view then that is all you need, it is rarely bang on centre all the time.

If you want real accuracy then you need to spend real money (multiple thousands) but you don't really need that.  Go to is to find things, the handset can then be used to finetune.

[PeterCPC]

I find that not turning the power off between alignments gets me a better result. Each time you power off you need to set the alignment marks and this introduces a possible error.

Peter

[John Gill]

Thanks guys, maybe the AVX is not the most accurate mount, but it should be better than 3-4 inches off target.  I have done alignments with the power always on and also by powering off between alignments.

I have also done 2 ASPA using two different stars and the second ASPA is only off by a tiny tweak on the ALT/AZ adjustment screws.  Once I am on target, the tracking is great and I am able to do 30/60 second exposures.

So, I guess I will just have to fiddle around or hit it with a big hammer.

Regards

John

[PeterCPC]

Have you updated to the latest firmware? http://www.celestron.com/support/manuals-software

Peter

[Cjg]

John,

What are you using to power the mount with? If a portable unit, it may be checking that, as a start.

Chris

[ollypenrice]

To what do the 'inches' apply? I'm not sure what you are looking at in saying 3-4 inches off target.

Olly

[John Gill]

Hi,

To power the scope I  am using a 12.4 A/Hr battery, so I should have plenty of juice and power.  The scope is an 8" and after the alignment the target object appears at the edge of the FOV, so hence 3-4" off center.

Regards

John

[ollypenrice]

I'm still not sure how you estimate 'inches' through the eyepiece but if the object is in the EP then hasn't the GoTo done its job?

(On the estimation of inches, I'm not being deliberately obtuse. If I hold a ruler close to my eye I can look at three inches. If I move the ruler further away the same three inches subtend a smaller angle. To me, three inches through the EP could mean anything.)

Olly

[Stu]

Hi,

To power the scope I am using a 12.4 A/Hr battery, so I should have plenty of juice and power. The scope is an 8" and after the alignment the target object appears at the edge of the FOV, so hence 3-4" off center.

Regards

John

Which eyepiece are you using when finding things John? This would give us an idea of how accurate the Goto is in degrees

[John Gill]

So about the 3-4 inches off.....I am using the standard 40mm plossl eyepiece.  If the object I am looking at is half way between dead centre and the edge of my view thru the eyepiece, then that would be about 2 inches off center cos it is an 8" scope.  I could live with that variance.  But in my case the object is right on the edge of the FOV so I miss the target completely.  I have since updated all the firmware and calibrated the SSA but the clouds rolled in and I have not been able to test the system again.....maybe this weekend will be better.

Thanks

[CodnorPaul]

By no means the right solution but a great excuse to get a 2" diagonal...

[YKSE]

So about the 3-4 inches off.....I am using the standard 40mm plossl eyepiece.  

That's much clearer discription

A 40mm plossl has about 27mm field stop, the Apparent Field Of View (AFOV) in degree( you see through a telescope can be calculated with

(field stop of eyepiece)/(focal length of the scope) * 57.3

For your 8" SCT,

it'll be 27/2032*57.3=0.76degree, or about 46 arcminutes.

You should be able to get better alignment by repeating alignment process. My simpler EQ3 can be aligned to less than 20 arcminutes with only the build-in firmware.

[Chris]

Hi John, if your Goto slews put the object anywhere in the field of view of a 40mm Plossl using a C8 then I would say your Goto's are very good and the mount has done it's job.

Also I see what you're trying to say with 3-4" out, but the aperture of your scope doesn't have any bearing on how far out an object is in the eyepiece. The focal length of the scope and the eyepiece used will though, and if you use say 40mm 2"  68 degree wide angle eyepeice with a 2" diagonal the objects will appear closer to the centre of the field of view during the slews because the field of view wil be bigger.

One way to show that the aperture of the scope doesn't relate is that you could use the same eyepiece in a 16" scope with the same focal length and the object would appear in the same place as in your 8" scope.

[knobby]

The SSA calibration should improve things, mine was cack til I did it a few times

[Jimtom]

As a matter of interest [i've got the same mount], have you checked  the Polar Align error after you've done the 2nd ASPA?

[John Gill]

Thanks for the updates and suggestions.  Clouds, clouds and more clouds .... so I will have to wait a few days and then do some more tests.