Nexstar alignment problem

[2STAR]

Any suggestions please.

Here is my set up routine, does it look ok as I am having problems with alignment on my Evo 9.25 with Nexstar HC.

Level Tripod--OTA on--Power ok- dew shield --AZ set to Positive--ALT negative (slight front heavy OTA), -- Standard/UT time selected-- Lat & Long correct for my location-- Time correct 24hr clock--

Date set correct USA version-- Telrad and Ota in line-- 2 star auto select (have tried others )-- Vega & Capella selected ( to view Northern Sky)-- Slew to Vega--centre inTelrad press enter-- centre in 20mm reticle ep press Align--select /send to Capella (just visible in ep), centre in Telrad enter, centre in ep align.

Alignment success !!-- test accuracy on Polaris, just on edge of Telrad but not showing in ep-- turn mount off and start again with 2 different stars Polaris and Betelgeuse this time, align success again, send to Vega for test of accuracy, again not in ep, turned off mount and repeated again with different stars  same result.

Removed ep & visual back, fit 6.3 FR and correct spacing attached CCD infinity, carried out procedure only this time alignment on laptop screen via Infinity, all centred with crosshair on screen and align success !!-- sent to open cluster ngc 2169, on the screen, well most of it, waited for backlash etc to settle and sent to Vega, not on screen, sent to Capella same not on screen despite using both auto and FWHM manual on software in case it was a software focus/resolution  problem on the screen, I finished final alignment on stars up & right, then tried down and left, up and left it made no difference.

(Focus is with bhatinov mask}

Any ideas please appreciated

eric

[Radioamateur]

Hi,

A very important aspect for a correct alignment is the direction of movement of the scope when focusing the reference star. To eliminate the backlash, the final movement must be up and right, and must be repeated as many times as necessary, until the reference star is perfectly centered. By the way, these days I'm using Capella and Altair as favorite stars. I hope it works.

Greetings.
 

[2STAR]

2 hours ago, Radioamateur said:

Hi,

A very important aspect for a correct alignment is the direction of movement of the scope when focusing the reference star. To eliminate the backlash, the final movement must be up and right, and must be repeated as many times as necessary, until the reference star is perfectly centered. By the way, these days I'm using Capella and Altair as favorite stars. I hope it works.

Greetings.
 

Thanks & noted for next set up, but I have done tried this, will keep in mind.

[michael8554]

You could chase this problem for a month of Sundays.

Might be worth accepting that at your focal length getting perfect framing on a GoTo is not going to happen.

Instead GoTo a known star close to your target, frame it up with the handset,  and use the SYNC or Realign command or whatever they call it now :-)

Then a GoTo your target should be spot-on.

Michael

[newbie alert]

Maybe more than a 2 star alignment is needed..i use a AVX but it still uses a nexstar+ handset..i use 2 star alignment with 4 calibration stars..seems pretty accurate..but I do use a illuminated 12mm crosshair ep so I know that the target is in the middle of the ep and not just guessed..if I'm out a touch then it's usually human error ..

[alanjgreen]

Eric,

First, read the goto faq

It sounds like you know the routine and have reasonable results, I suggest you choose a different/better PAIR of stars... If you search the internet or App Store, you should be able to find "best pairs" app or similar. These type of apps tell you the best two stars to pick given your time and location.

beyond that, switch to using "auto 2 star" alignment, you pick the first star (by name) and centre it then the scope gives you a list of  second stars based on which you picked as the first (just scroll through a list of good second stars until you see the name of one you can see above - the first one it lists is the "best choice"). Read about this routine in your manual.

- the scope will goto the 2nd star (based on its knowledge of the location of the first ) so you don't need to move to the second star yourself.

- the scope will miss the second star by some small amount. It will be the brightest star in your FINDER SCOPE view

- just centre it and hit enter  

- I had better results with auto 2 star than standard 2 star alignment so used this routine mainly!

Finally, 3 star alignment is even more accurate as long as you pick 3 stars in a nice wide imaginary triangle, you want the imaginary triangle to have plenty of area inside it. 

As said above always finish RIGHT AND UP, it's very important. Keep going around the final centre in the eyepiece until right and up is achieved and centre is spot on.

Use a high power eyepiece to increase centre accuracy.

Defocus the star so it becomes a large unfocused blob, it's much easier to get in the dead centre like this.

Alan

[PeterCPC]

You say that you use Vega and Capella - Vega would be in the West but Capella is in the East. Your first two alignment stars should both be in the West and any Calibration stars in the East. The use of an illuminated reticule eyepiece is really beneficial and try not to swap eyepieces during the process.

Peter

[impactcrater]

I find goto improves as the observing nite goes on . just accept the initial inaccuracies.....

[Luna-tic]

17 hours ago, PeterCPC said:

You say that you use Vega and Capella - Vega would be in the West but Capella is in the East. Your first two alignment stars should both be in the West and any Calibration stars in the East. The use of an illuminated reticule eyepiece is really beneficial and try not to swap eyepieces during the process.

Peter

Agreed with above. Your process is precisely the same as mine (Edge HD 800 / AVX mount), but I use Vega and Altair as my 2-stars, and Capella as my calibration star, and Aldebaran as a second calibration star. I don't have a reticle EP (yet), but centering as best I can using a 13mm EP, I usually get my chosen object in the FOV with a 25mm Plossl the remainder of the night. If I decide to take any pictures (just single image stuff), I use my flip mirror and a 25mm Plossl, it is parfocal with my camera in the flip mirror. I focus using the EP, the open the light path to the camera and shoot.

[alanjgreen]

Eric,

One last thing. Read the manual entry for "precise goto". This is a great feature for finding the faint fuzzies. The scope will slew to a bright star (of its own choice) near your chosen target, you then centre the star and hit enter, the scope completes the slew to your target and it will be dead centre.

http://www.nexstarsite.com/Book/Updates/PreciseGoTo.htm

Alan

[michael8554]

On 12/1/2017 at 20:27, michael8554 said:

use the SYNC or Realign command or whatever they call it now :-)

 

13 hours ago, alanjgreen said:

Read the manual entry for "precise goto". This is a great feature for finding the faint fuzzies. The scope will slew to a bright star (of its own choice) near your chosen target, you then centre the star and hit enter, the scope completes the slew to your target and it will be dead centre.

So that's what they call it now.....

Michael

[Olsin]

I'm not that experienced yet and have only recently ironed out the quirks in my own setup routine.
What does strike though is you using a telrad with an Evo 9.25 scope. You give no reference to a reducer so that's a long focal length you're working with.
I've never used a telrad but i understand they are just 1x. Maybe i'm wrong but i don't really see how that combination of FL and finder could be anywhere near precise.
I spent a lot of time aligning my 6x30 finder with my C8 and i can usually put a star about halfway from the middle to the edge of FOV in a 20mm EP. Never get it spot on though despite the apparent sync between finder and OTA....there's to much focal length.
You have more FL with a finder that's only 1x. A miniscule variance in head/eye/telrad position could give you wildly varying error margins when you align first with finder then EP..........or not. i don't know. Just my thoughts.

I'm sure i've read a rule of thumb that said telrads and red dot finders are fine for shorter FL scopes, not so much so for the longer.

Alternatively, and again not that i have a huge understanding of it, but have you ruled out cone error?

[michael8554]

21 hours ago, Olsin said:

Maybe i'm wrong but i don't really see how that combination of FL and finder could be anywhere near precise.

You missed the bit about him centreing in a 20mm reticle eyepiece.....

[Olsin]

1 hour ago, michael8554 said:

You missed the bit about him centreing in a 20mm reticle eyepiece.....

No, i got that. I was mainly referring to the potential difference in coordinates between center in finder=enter, center in EP=align. Centering a star in a 1x telrad could still leave it out of FOV in a 20mm EP at FL 2350mm.

My thought was a wondering if large inconsistent discrepancies in a stars position between pressing enter and align on the handset, could mess up goto accuracy..

 

[2STAR]

Thanks to you all for your input, I have exchanged chat with Michael Swanson Author of Celestron Alignment etc, he is very knowledgeable in this area.

What I am doing is basically ok, but a couple of things like choosing stars on the night and their position . My OTA is slightly front heavy hence the RIGHT & UP  final movement when centering in eyepiece .Quote below from Michael:

avoid stars above 70 degrees in altitude or below 20 degrees.  Then select two stars with a separation of between 90 and 135 degrees.  Polaris is a good selection for your location as long as you are not using the scope on a wedge.  When centering the stars in the eyepiece, your current GoTo Approach settings dictate that final motions must be with the RIGHT and UP arrow buttons in order to eliminate the effects of gear backlash.  Other than that, your description has all other critical items accounted for: level the tripod (exact not required), enter the correct location/date/time/time zone and DST setting.

[2STAR]

On 03/12/2017 at 10:55, alanjgreen said:

Eric,

First, read the goto faq

It sounds like you know the routine and have reasonable results, I suggest you choose a different/better PAIR of stars... If you search the internet or App Store, you should be able to find "best pairs" app or similar. These type of apps tell you the best two stars to pick given your time and location.

beyond that, switch to using "auto 2 star" alignment, you pick the first star (by name) and centre it then the scope gives you a list of  second stars based on which you picked as the first (just scroll through a list of good second stars until you see the name of one you can see above - the first one it lists is the "best choice"). Read about this routine in your manual.

- the scope will goto the 2nd star (based on its knowledge of the location of the first ) so you don't need to move to the second star yourself.

- the scope will miss the second star by some small amount. It will be the brightest star in your FINDER SCOPE view

- just centre it and hit enter  

- I had better results with auto 2 star than standard 2 star alignment so used this routine mainly!

Finally, 3 star alignment is even more accurate as long as you pick 3 stars in a nice wide imaginary triangle, you want the imaginary triangle to have plenty of area inside it. 

As said above always finish RIGHT AND UP, it's very important. Keep going around the final centre in the eyepiece until right and up is achieved and centre is spot on.

Use a high power eyepiece to increase centre accuracy.

Defocus the star so it becomes a large unfocused blob, it's much easier to get in the dead centre like this.

Alan

Cheers Alan, similar to what Michael Swanson has told me.

[2STAR]

On 12/4/2017 at 21:59, michael8554 said:

 

So that's what they call it now.....

Michael

Precise goto only works if your initial alignment is correct though, doesn't it ??

[michael8554]

2 hours ago, 2STAR said:

Precise goto only works if your initial alignment is correct though, doesn't it ??

If you've used "precise GoTo", then the target should be close enough that initial alignment errors won't cause much of an error - that's the whole point.

Michael

[2STAR]

9 hours ago, michael8554 said:

If you've used "precise GoTo", then the target should be close enough that initial alignment errors won't cause much of an error - that's the whole point.

Michael

I have used precise goto, if my initial alignment is out like it has been, then Precise goto does not correct the initial misalignment and I have to search around to find and centre the star that Precise goto shows in the HC.   

Regards Eric

[2STAR]

Just now, 2STAR said:

I have used precise goto, if my initial alignment is out like it has been, then Precise goto does not correct the initial misalignment and I have to search around to find and centre the star that Precise goto shows in the HC, and that is not easy if the star is average size and surrounded by other average stars !!   

Regards Eric

 

[michael8554]

Hi Eric

My initial post said to Goto and centre a KNOWN star before SYNCing or whatever they call it.

I too would have trouble sorting out any random star chosen by the scope.

Once you've done that then any alignment errors should be small enough over a short distance to get your target in view.

My point was to stop agonising over super precise alignment.

Michael