Cone error

[DaveT39]

A very simple question - should I be able to see Polaris in my main scope in the Home position once it is polar aligned?

 

I seem to have a lot of cone error. I have adjusted the polar scope reticule and that looks ok. I centre Polaris in the fov of the polarscope but in the home position Polaris is way out of the fov of both the finder and the main scope. 

I am planning a daytime cone error correction but the question I asked is simply to make sure this is what I should be expecting.

Scope is 8” f5.6 Newtonian, guide scope is 60mm and the mount an HEQ5 pro.

I’m struggling as I want to be able to use sharpcap polar alignment routine with my ZWO 120 colour camera.

Any advice welcomed.

 

 

 

 

[Davey-T]

I can generally see Polaris in the scope with the mount in the home position, it's at about the spot shown in a polar app except in a different spot on the clock face depending on the type of scope.

Dave

 

[Gerry Casa Christiana]

I seem to have this problem as well. I will be interested to see what people say because mine seems excessive too and I have already adjusted the vixen connection by lifting one end up but it doesn't seem enough. Also worth saying do your collimation before you start changing things because that can also move the pole star position. 

Gerry

[IanL]

Not necessarily a problem so worth giving it a try. My setup has over a degree of cone error and I have successfully used the SharpCap polar alignment routine. Polaris does not appear in the camera's field of view and the SharpCap display shows the pole as being well outside the display. Even so I am able to dial in good alignment just by aiming to get the numbers down as close to zero as possible.

[Gerry Casa Christiana]

Cone error adjustment video. 

[vlaiv]

Not necessarily a cone error.

You polar align non moving part of your mount - RA axis (axis itself does not move and stays fixed). OTA on the other hand moves in both DEC and RA.

What you call home position should be such that both RA and DEC are at 0 value, but are they? Using clutches to balance the scope can cause small variations in RA and DEC even if you "return to home position" - mount thinks it is pointing at home, but clutches were tightened in such way that OTA is pointing to the side. Once you set home position correctly - DEC and RA 0, have scope aligned, and don't move the scope on its support (no fiddling with clutches or rings) then you should be able to see Polaris each time you move to home.

Here is hint how you can check if this is issue. Balance scope, polar align, prepare everything. Don't use any star alignment, or sky model, just slew to selected bright star. Now carefully release clutches and "by hand" make that star in center of the eyepiece. Tighten clutches again. It is a bit of fiddly process since scope will shake a lot, and tightening clutches tends to move scope a bit, but with a bit of practice you will be able to keep the star in center (or close to) of eyepiece.

Now go to home position - Polaris should be visible in eyepiece (it should not be centered, but it might be close to center - this does not mean bad polar alignment, it just means that you did not put scope in perfect alignment to selected star).

Cone error will be visible by: polar align, make sky model or star alignment, go to star near meridian (just before meridian), at DEC close to 0. Center scope on star and add that star to star alignment / sky model. Park scope and wait for star to cross meridian. Slew to that star again. If it is not in center of FOV, and provided you have good star alignment / sky model - you have Cone error. Amount of shift of star from center indicates how large Cone error is.

[DaveT39]

Really helpful comments - I plan to go through the steps from reticule alignment to the Astronomyshed video steps and finally use Conesharp at night picking up the pints raised above - I’ll feed back on progress in case it helps anyone else.

 

 

[DaveT39]

OK - I spent a couple of hours today going through the steps.

1. Reticule alignment - this is as good as I can get it and the object I am locking onto does not move out of position whilst rotating the mount.

2. I checked alignment of the guide and main scope and that is smack on. Scope bubble shows level after some adjustment 

3. Started to go through the steps in the Astronomyshed video and it all began quite well. As  I'm using an object on the horizon it does mean screwing the altitude bolt in until I can swing the scope to the other side then lowering it to the same point - pretty easy as the object is still in the centre of the polar scope.

4. I am assuming that the aim is to align the main scope with the centre of the polar scope (al least that makes sense to me). Following the steps led to a problem though. In the video the instruction is to move to one side and use DEC and AZ bolts to centre to object in the main scope - fine so far. Step two is then to swing the RA around 180 degrees to the opposite side (means jacking up the AL bolts until the counter weights miss the leg - then back checking the polar scope fix but that seemed fine)

The next step is to adjust the bolts on the mounting bar to tilt the school half way towards the object no problem here

Its at this point I lost the plot!!

I swung back to the other side and according to the video (at least me reading of it) you can only use DEC and AZ to centre the object in the FOV of the main scope but that means the polar scope is no longer centred on the remote object. If you do this and then swing back you seem to be shifting the target point further away from the cente point of the Polarscope - have I missed something?

Then the sky opened and hail and rain came down in a torrent - rapid covering and dismantling required!!

When I try again I would like to be really clear how this works!!