Problems aligning a Celestron 127 SLT

[SiD the Turtle]

Hi folks, complete beginner here really struggling with the basics! I bought a Celestron 127SLT a while back and every time the clouds part I'm outside trying to get it to align so that the go-to works. I'm using two-star align, the default 'point at any three stars and it works it out' simply fails . I'm at a point where with two-star align the go-to points at least vaguely in the right direction, but near the edge of the eyepiece. Here's what I've tried so far:

• Mount is level using the tripod's bubble.
• Time is set to the second using time.is and I'm not on DST.
• Location matches the coordinates given by both Google and SkySafari and I've made sure I'm using N and W and not mixing lat and long (that's obvious when I do, it's nowhere near accurate).
• Laser pointer and telescope are aligned (and regardless, I try to centre the star by eye through the eyepiece).
• My garden faces due north, so I use Polaris as my first star, then I've tried all the stars in the Plough, or flipping south to Orion's Belt.
• Running off a Celestron PowerTank Li to give it a steady power source.

I wonder if my trying to get the star in the centre during alignment is not as centred as my eyeballs think!

So questions:

• Is that kind of inaccuracy par for the course or am I not aligning with enough precision? If I align, slew from Polaris to say Dubhe (which is then off-centre) and then back again, Polaris will be in a different place in the eye piece than when I started.
• If use the direction buttons to reposition the star to the centre after slewing, it stops tracking. Is that normal?
• Would buying a laser crosshair eyepiece help?

Cheers in advance!

Edited January 7, 2020 by SiD the Turtle

[Davey-T]

Hi and welcome to SGL.

Polaris is not a good thing to use for star aligning, you're better off using a couple of stars around the meridian, you can use Stellarium to find suitable candidates.

To make it easier to centre a star if you defocus it so it fills more of the eyepiece view it's easier to centre.

Dave

[Cosmic Geoff]

I have this exact model, and have used Nexstar on other Celestron outfits.  In my experience, if the GoTo mount is properly aligned, it should place the desired object somewhere within the field of a 25mm eyepiece.  That is as good as it gets.  The theoretical accuracy is =/- several minutes of arc, which is a significant variation if you are using a high-powered eyepiece.

[Anne S]

I had a Nexstar 8se some years ago. For aligning I always used auto two star align, it's quicker than the 3 star align where you point to three bright objects. Mine used to fail as I was using Dubhe as one of the stars and it isnt bright enough!

For auto two star I used Dubhe and Altair if that was visible, if. It something near. The first star you move the scope to manually after choosing it from the list, once centred you choose a second star from the list as far away as possible and ideally higher up. The mount will slew to where it thinks that star should be and you correct with the hand control.

One other thing that could affect your accuracy is backlash. You need to have the final movement in a particularly direction, something like right and down. It makes sure the gears do the final movement in the same direction it's tracking. 
 

It's also useful to calibrate the Mount using the hand control. I suspect that tells the mount the focal length of the attached telescope as it moves the mount away from the star you're pointing at and asks you to move the mount back, if I remember correctly.

Also, if you want to concentrate on one area of the sky you can replace one of the original alignment stars with one placed closer to where you want to look.

There used to be a website called Nexstarsite to help with all the versions of this mount. The owner stopped updating it some time ago but it was still there when I last looked quite a few years ago.

Hope that gives you some ideas.

Anne

edit: I used a cross hair reticle on the 25mm eyepiece to centre the alignment stars.

Edited January 7, 2020 by Anne S

[Mick H]

Hi Sid the (don't get me started 😉) and welcome to SGL

As Geoff (cannot bring myself to use the C word 😉) has said use a high mm eyepiece to bring the star into view then centre star in ep, then as Dave-T said de focus so it looks like a doughnut, and centre again. then choose lower number ep like 10mm and repeat.

Also there is a precise align feature from the menu I believe. 

[joe aguiar]

I agree to doht use Polaris use another star

Joejaguar 

[cletrac1922]

First of all welcome from Land Down Under

This time of the year I use Sirius and Canopus to align my ED80 on EQ5 mount

Also use an App called Ulysse Gizmos for my GPS location and has a compass which you can also set to allow for magnetic variation

Where I am, magnetic variation is 12 deg east

 

Some trivia I posted on here some time ago, is there is actually 3 Norths

 

First been True North, which is line between north and south poles

 

The second is Magnetic North, which is the direction a compass faces north, and can be either east or west of true north depending on where you are

Currently with Magnetic North high latitudes Northern Hemisphere, Magnetic North is tracking further east towards Russia

 

Finally we have Grid North, which is the lines drawn on a map, with Grid North been top of the map

Before we had GPS on our smart devices, we used to use street directly to find our way around our cities

Top of map in a street directly is also Grid North

 

Hope been of assistance

John

 

 

[SiD the Turtle]

Thanks everyone for the friendly welcome and responses! Just waiting for clear skies to try again.

Out of curiosity why is Polaris not a sensible alignment target and something close to the meridian is? Interested in the technical reason.

De-focusing the star to make the star bigger is a good shout too, will give it a go.

[Mick H]

1 hour ago, SiD the Turtle said:

Out of curiosity why is Polaris not a sensible alignment

I have read some people say you need to align to Polaris, and some say you don't, others will hopefully help.

I normally use Vega and Capella with good results.

It depends where you are and how much sky you are able to see.

Good luck next time our

[lenscap]

4 hours ago, SiD the Turtle said:

Out of curiosity why is Polaris not a sensible alignment target

Hi SiD,

When you point the scope at 1st star of a 2 star alignment and press "Align", the software notes the RA & Dec co-ords of that star.

When you select the 2nd star the mount calculates the difference in RA & Dec co-ords between the 2 stars & drives the RA & Dec axes by the required amounts.

Since Polaris is very close to the North Celestial Pole you could make large errors in RA and the scope might still be pointing close enough to the Pole Star for you to accept the alignment, but the mount would be using the wrong RA co-ord and subsequent slews would be innaccurate.

 

 

 

[Cosmic Geoff]

A while ago I frequently used Polaris as an alignment star for the 127mm Mak SLT, and I can't say the results were any worse than when using other elignment stars.  Results may differ if the mount is an equatorial GoTo.

[SiD the Turtle]

Thanks again. Had a break in the clouds last night long enough to give it another punt with different stars, chose one close to the meridian (Capella) then over to Dubhe, the results were A LOT more accurate, so much so I was actually able to do some stargazing before the clouds made a return 😎. I think with a bit more practice I'll get it, but I'm so much closer than when I was using Polaris.

Cheers all!

[joe aguiar]

On 11/01/2020 at 06:48, SiD the Turtle said:

Thanks everyone for the friendly welcome and responses! Just waiting for clear skies to try again.

Out of curiosity why is Polaris not a sensible alignment target and something close to the meridian is? Interested in the technical reason.

De-focusing the star to make the star bigger is a good shout too, will give it a go.

cause the longitude lines go from pole to pole from a point to being spread out at center then to a point at the other pole.

Does that make sence? so your alignment could be off cause that

let me see if I can post a pic

also do you know its common for alignment stars to be off 5 to 15 degrees? and u have to move the scope to put it in the ep and push align?Just wondering in case you are just hitting align when the scope stops moving then you are not on the star it wants. Goto are not that good. only after it says success then it will put objects in the ep

joejaguar

Edited January 13, 2020 by joe aguiar

[ollypenrice]

On 11/01/2020 at 12:48, SiD the Turtle said:

 

Out of curiosity why is Polaris not a sensible alignment target and something close to the meridian is? Interested in the technical reason.

 

Ah, a nice easy question to answer, for a change! Polaris is very close to the celestial pole. If you watched it all day it would describe a very tiny circle around the celestial pole. It's only about three quarters of a degree off the celestial pole (the centre of the Earth's rotation) so its apparent movement describes a tiny circle about 1.5 degrees across. This means that 1.5 degrees is all that distinguishes its movement in right ascension during one full rotation. Imagine trying to distinguish between twelve o'clock and six o'clock on a watch face a mile away. Tricky. But fixing the position of the watch itself relative to the horizon would be easy. So Polaris gives perfectly good information in deciding declination but terrible information in deciding right ascension.

Olly

[GraemeH]

On ‎11‎/‎01‎/‎2020 at 11:48, SiD the Turtle said:

Out of curiosity why is Polaris not a sensible alignment target and something close to the meridian is? Interested in the technical reason.

Others have already given perfect explanations of why Polaris isn't suitable for aligning an equatorial mount, but it is perfectly good for alignment of a Goto Alt-Az mount.  The same arguments can be used to explain why a star near the zenith is bad for Alt-Az alignment (Alt=90°, Az=anything you like), but perfectly good for equatorial alignment.

[DaveHKent]

I have the smaller SLT90 and usually use the 3 star alignment. I tend to pick three stars spread far out as possible in the sky, which takes a while to slew the scope around but gives accurate results.

The other thing I find makes a difference is to watch how the target star moves when you to a goto move to it. For me, the last movement is up and right. I always make sure I have the star slightly off centre below and left, then move it up and right to take the slack out of the cogs before hitting align. This tends to make it more accurate and hold the tracking better.

[SiD the Turtle]

Hi all, OP here. Just wanted to conclude by saying I've had several nights of play now and got this down fairly well. Keys seemed to be both choosing stars near the meridian and also accounting for backlash. I had no idea the mount had so much play and making sure I focused on ending up 'up and right' helped immensely.

Thanks again!

[jefrs]

I have a 130 SLT which uses the same mount and tracking. Backlash. If you dismantle the mount, fairly simple, you can tighten the two clutch bolts which takes some of the back lash out. Don't over-do it as we want them to slip if they have to. The drive plate for the EQ ( battery box holder out) is held in place by two rather small screws, these can become loose and let the left/right become very sloppy. The screws are really too small for the job but do not overtighten as they are into plastic. The sprockets are metal and like a good grease, not too much but congealed yuck is not a good lube.

[jefrs]

On 29/01/2020 at 12:56, DaveHKent said:

I have the smaller SLT90 and usually use the 3 star alignment. I tend to pick three stars spread far out as possible in the sky, which takes a while to slew the scope around but gives accurate results.

The other thing I find makes a difference is to watch how the target star moves when you to a goto move to it. For me, the last movement is up and right. I always make sure I have the star slightly off centre below and left, then move it up and right to take the slack out of the cogs before hitting align. This tends to make it more accurate and hold the tracking better.

This can be programmed into the handset Settings menu, process is in the manual, advanced settings, as is cord wrap limit to 360°, and many other things.

Slew rate can be adjusted faster too.

Edited February 22, 2020 by jefrs
addendum

[JOC]

I use an application called Synscan Init 2.0 on my Goto unit.  Most of these things seem to need similar data and I find that the Synscan app. on my mobile helps me to ensure that I have the correct information in the correct format to enter into the system.  Even though you seem to have it sorted you might still find the app. useful?