Tracking seems affected when using a Barlow

[StarGazingSiouxsie]

Hi  

Tonight I was looking at Jupiter and got some nice views with a range of eyepeices. The best image detail wise and for clarity of image was a 15mm, which on my 8" Evolution gives me appx 135X 

I tried using my Televue 2X Barlow which normally works fine. I can't remember exactly which eyepiece this happened with, maybe the 15mm or a 9mm, but when I used the barlow the planetary image would soon disappear out of the eyepiece. The mount seemed to otherwise be tracking OK although notice the planets seem to move out of the eyepiece view after a while. So did I have too much magnification with the Barlow? I think I did manage to get focus, the image just wouldn't stay put.

Thanks for any insights or advice 

 

Siouxsie 

[Knighty2112]

With a barlow you are getting more magnification, so in general the more you magnify things then the quicker they'll zoom out of site. With this model goto you won’t be able to hold high mag views in site as easy as lower magnifications. You would only get that tracking stability over longer periods with much more expensive goto mounts.

[vlaiv]

I don't think that using x2 barlow should produce that effect.

I regularly use dob at x200+ magnification and planets don't just zoom out of the view. Even with regular eyepieces of 50 degrees, if you center your target, at sidereal of 15"/s and at magnification of x270 (that is two times x135) you'll still get (25 * 60 * 60 / 270) / 15 = 22 seconds for planet to exit FOV. I would not call that zooming out of the FOV. That is without any tracking.

With my cheap AzGti I used magnifications of x236 at one time and image happily stayed in center of the FOV. This is $300 mount head we are talking about. You pretty much can't get cheaper than that.

What could possibly happen would be following:

- Barlow produces unacceptable level of imbalance that causes some sort of slip in mount / mount drive?

- Barlow moves focal point outward and focusing mechanism causes image shift at that point (this is with SCT scope, right?)

- Adding barlow makes rear end of SCT too heavy and mirror tilt happens?

- Wrong tracking mode engaged - lunar or solar rate instead of sidereal?

[Geoff Barnes]

10 minutes ago, vlaiv said:

Wrong tracking mode engaged - lunar or solar rate instead of sidereal?

Agreed, I find this is the most common cause of the problem for me with my Dobsonian. 

Sometimes though, using my Goto mode I find once I've centred on a target it drifts quickly out of view when it should stay centred. I usually find if I re-centre with the handset (or smartphone) back to where it should be it then stays centred (usually).

Low battery power can also cause the drift as well.

[vlaiv]

6 minutes ago, Geoff Barnes said:

Agreed, I find this is the most common cause of the problem for me with my Dobsonian. 

Sometimes though, using my Goto mode I find once I've centred on a target it drifts quickly out of view when it should stay centred. I usually find if I re-centre with the handset (or smartphone) back to where it should be it then stays centred (usually).

Low battery power can also cause the drift as well.

This reminded me of one more cause of target drift - one that happens exclusively after slewing.

Backlash. If there is enough backlash in the system, this can happen - you center target with controller and then target moves from the center - sometimes small amount and sometimes it can even leave FOV  (high mag, large backlash). This happens if you slew to target in one direction but not when doing it in opposite direction (usually in one axis only, but can be in both with AltAz mounts).

In this case, Mount is tracking properly, or rather drive system is tracking but backlash in gear system means that this motion is used up to clear backlash instead of tracking. Once backlash is cleared up - mount continues to track as it should.

[Hallingskies]

Alignment needs to be good at higher magnifications.  

[StarGazingSiouxsie]

Thanks all for your replies and insights, I appreciate them. 

My tracking is set to sidereal. I did the usual Starsense (SS) alignment and followed up with at least 6 additional alignment reference points. 

When I first calibrated the SS camera to the OTA I'm thinking I could have centered the star a little better and I'm wondering if that's affected my goto accuracy? All my goto targets are in a 32mm eyepiece but most appear at the 7 o'clock position in the eyepiece. I might do a new calibration and see if that helps. People say it's a one off thing but in my mind if it was not done accuaretly then surely it will affect alignments?

Planets move differently through the sky relative to stars, right? So does that mean that tracking rates for planets are different than for stars? IE does it follow that planets will stay centered in the eyepiece for a shorter time than stars and DSOs will?? 

[vlaiv]

4 minutes ago, StarGazingSiouxsie said:

Planets move differently through the sky relative to stars, right? So does that mean that tracking rates for planets are different than for stars? IE does it follow that planets will stay centered in the eyepiece for a shorter time than stars and DSOs will?? 

You can tell the difference in 24h - planet will move compared to where it was yesterday but you won't be able to see it at eyepiece - night is too short to notice any real movement.

For example, Mars does something like 17' per day at the moment (speed will depend on relative position of earth and planet in their orbits).  That is 42.5"/hour. Well, if you have background star in your FOV and you observe for an hour or two - you should be able to see those two move relative to each other, but Mars certainly won't leave FOV due to that in said hour or two.

[Geoff Barnes]

Please be sure to let us know how you get on @StarGazingSiouxsie. Waiting with baited breath! 😀

Edited August 18, 2020 by Geoff Barnes

[Starwatcher2001]

This is a useful article. It reads like a small novel, but the TL;DR is that:

- his problem was backlash, which is a known problem with the NexStar, particularly in Azimuth. (I've got lots of play in mine).

- finishing each alignment with UP and RIGHT is vital

- The UP and RIGHT refers to what the OTA is doing, not the view through the eyepiece

- The buttons on the controller can be set to swap RIGHT/LEFT and/or UP/DOWN, depending on the motorspeed. This can confuse matters somewhat and may lead a person to think they are finishing the alignment movement on UP RIGHT, when in fact they aren't.  (Telescope | Setup | Direction Buttons).

- He doesn't mention it, but newer handset software also has an option called "Goto direction", which further muddies the water.

Hope this helps.

Mark

https://polywogg.ca/solving-alignment-problems-with-the-celestron-nexstar-8se/

 

 

[StarGazingSiouxsie]

@Starwatcher2001

Thanks, Mark, I'll take a look at that later