Crosshairing a 35mm Panoptic; crazy idea?

[Atmos (Colin)]

I was driving from one job site to another today and had this probably crazy idea of adding some crosshairs to my 35mm Panoptic so that it could actually be used for alignment.

What I had thought of would be getting a cheap 2" clear filter and gluing a guitar string into a +, this could then be threaded into the bottom of the Panoptic.

Now the rational part of me says that it is too far away from the entry/exit pupil of the lens to be able to see the cross hairs, but is it? Or will I just get nasty/irritating diffraction patterns?

I'd test it but the only clear filter I have is an unmounted 36mm Astrodon L filter which I am not inclined to glue a guitar string onto

[astro744]

The crosshair needs to be located at the field stop which for the 35mm Panoptic is 0.17" below the top of the 2" barrel. See column F at http://www.televue.com/engine/TV3b_page.asp?id=214

[Atmos (Colin)]

Originally Posted by astro744

The crosshair needs to be located at the field stop which for the 35mm Panoptic is 0.17" below the top of the 2" barrel. See column F at http://www.televue.com/engine/TV3b_page.asp?id=214

Thanks for that, so just at the top of the 2" barrel.

[LewisM]

Geez Colin, if it's for imaging, assuming the guidescope is well aligned with the main scope (works OAG no brainer), go into PHD or MaxIM, and in the guide window turn on crosshairs. then zoom if required.

VIOLA - perfect centreing - better than any EP.

If it's for visual, a good compass and a well aligned illuminated reticle finder does the trick.

[Atmos (Colin)]

The idea was purely for Star centering with an EP that has a good FOV

[bojan]

For cross I used optical fibre.
As it is straight, it is easy to place/fix (with superglue)

[Atmos (Colin)]

Doesn't sound like my quick and easy idea is going to work anyway
Was hoping to just be able to screw it in for when I wanted it

[LewisM]

Originally Posted by Atmos

The idea was purely for Star centering with an EP that has a good FOV

Yeah, I know, but if you have the guidecam on, USE IT. I used to use a finder AND a reticle EP until I realised that the guider makes an electronic finder MUCH better than the human eye, and MUCH more precisely centred when using the crosshair overlay (esp. zoomed in - you can nail the centroid using the RA/Dec controls).

[Wavytone]

Colin it would be a bit of a waste of a good eyepiece. Secondly guitar strings are way too coarse - try optical fibre instead. Better still is a laser-engraved glass reticle, or strands of real spider web as used for filar micrometers 200 years ago - the beauty of this is it comes with natural self-adhesive and is elastic, so can be stretched and won't sag. A fork was the traditional way to collect and apply spider silk.

It's also a big help if the eyepiece has a red LED illuminator.

There are some guide eyepieces with 70-degree AFOV at http://www.apm-telescopes.de/en/eyep...eyepieces.html and the prices are OK while the $AU is high.

I've just ordered the 20mm 70 degree beastie to use while calibrating the encoders on my mount.

[Kunama]

Originally Posted by Wavytone

Colin it would be a bit of a waste of a good eyepiece.
There are some guide eyepieces with 70-degree AFOV at http://www.apm-telescopes.de/en/eyep...eyepieces.html and the prices are OK while the $AU is high.

I've just ordered the 20mm 70 degree beastie to use while calibrating the encoders on my mount.

I bought one of those TS reticle eyepieces also sold as APM etc.... after finding that the crosshairs are not centred I dismantled it to find that they were engraved off centre and could not be corrected .......

I kept the caps and illuminator but the rest went into the bin...

My next one will be the Baader Polaris with a T2/2" nosepiece from Telescope Express..

[Wavytone]

Ah thanks Matt.. if it's like that ... well at least it has an illuminator - if it's wonky I'll take the glass out and replace with some optical fibre.

[AndrewJ]

Gday Colin
If its only aligning you want to do, then you will be using relatively bright stars.
As such, just badly defocus until you get a large donut that fills say 1/2 to 3/4 the fov. You can easily centre this as your eye brain can detect any offsets much easier than a point source in the middle.
If you are using multiple stars, dont change focus till done and it will remove any minor focus shift errors.

Andrew

[Atmos (Colin)]

Originally Posted by AndrewJ

Gday Colin
If its only aligning you want to do, then you will be using relatively bright stars.
As such, just badly defocus until you get a large donut that fills say 1/2 to 3/4 the fov. You can easily centre this as your eye brain can detect any offsets much easier than a point source in the middle.
If you are using multiple stars, dont change focus till done and it will remove any minor focus shift errors.

Andrew

[Wavytone]

That method is good for some - certainly if you have low-res encoders and a small scope - and for that I would use a medium power eyepiece with a small field of view - something like a Plossl or Vixen LV.

In my case I have 311,000 step encoders with resolution to 4 arc seconds. So far I have found Sky Safari can put the scope on target to a minute of arc or better if its calibrated to several stars including one near the intended target. I can leave the scope (2700mm focal length) with a high-power 300X eyepiece and accurately put it on target using the encoders.

In a few nights use so far I have also confirmed there is a very simple principle - the accuracy of push-to (or GOTO for that matter) is inherently no better than the errors when you align it on reference stars. And worse, if the mount axes have orthogonal errors.

I have found my AZ8 is good enough to suggest a more precise alignment is warranted, hence cross-hairs.

FWIW - Matt - the APM 20mm 70 degree eyepiece arrived tonight and it has a glass reticle with a laser-engraved cross in the centre - and yes ts a "good-un", i.e. it is well centred. The field however is not flat, far from it

[AndrewJ]

Gday Wavytone
Agree it is not as accurate as a true high power Xhair EP or a computer verified image etc, but its going to be a lot cheaper and more accurate than trying to convert a good EP using home made Xhairs :-)
For quick and dirty 2 star aligning, ( with no "devices" at hand ), it works far better than trying to eyeball if a "focussed" star is centred.
Andrew