DIY - OAG

[fwm891]

Hi, I'm a recent convert to OAG's and I bought one to try-out with PHD and it works . Very frustrated though because no matter what I tried re: adjustments, I couldn't get the guide stars to look like stars:mad:

I've had stars looking like straight lines, like a Nike logo, well like anything but stars.

So I have to give a big thumbs up to PHD for finding a centroid somewhere in those images:eek:

I've been building an OAG on my lathe over the last few days, ordered a new prism from Knight Optical with 10 mm faces and the hypotenuse aluminised and black painted.

This OAG will allow me to both tilt and rotate the prism so that incoming light rays are directed to my QHY5 guider without distortions.

The photos are of the build so far. I'm waiting for some PTFE sheeting (1mm) and I still have to make the rotation assembly, so will update when the PTFE arrives and the final machining is completed.

I then need to blacken everything, not sure whether to use chemicals to stain the ali & brass or to anodise...

Hope to up date soon

[foundaplanet]

Nice work.

[steelfixer]

Hi Francis.

That is a nice piece of work.

I assume you are doing this at home on a hobby type of lathe without the aid of power feeds.

I ask as you seem to have a lot of chatter from the boring bar.

Will this surface be optically ok or are you going to have to do some smoothing of the bore.

If it is a manual lathe then I am even more impressed.

Can you post some photos of the brass workings they look very intricate.

Keep up the good work I look foward to seeing the final work of art.

Graham

[Glasspusher]

Very impressive Francis. I look forward to hearing how it performs during am imaging session.

John

[Trull]

Nice work, incidentally if you need to stick the ptfe to anything, make sure you buy etched.

[DIYASTRO]

Francis,

I just love to see this sort of stuff! I make most of my own gear too and I hope when you have it completed it gives you the sort of pleasure that I get from designing/ building our own equipment.

Be sure to post pics of the completed assembly, and report on its obvious success!

Regards

Boyd

[fwm891]

Thanks for the comments.

The flange plate has been made to fit directly to my SX filterwheel.

The inside of the OAG main tube is actually threaded 'V-form'.

Yes it is a small hobby lathe, (Myford Speed 10) with screw-cutting via change wheels and back-gearing.

The boring bar was way over stretched when cutting the thread (70-75mm ext over the cross slide and I was probably running it too fast!!!).

This will all be matt blacked (inside surfaces) so the 'chatter' will be to my advantage there as it will randomly reflect any light falling on the matt black paintwork.

The lathe has a powered axial feed but no powered cross feed.

I tend to feed by hand when turning, only using the power feed when cutting threads.

Next time I take it apart I'll take some photos of the brass pieces. Luckily the lathe came with a vertical slide so it has enabled me to do some milling of square sections as well.

Been looking at 'home anodising' but I get the feeling that obtaining chemicals may be a problem - unless anyone out there knows different...

*** Info*** Chronos Engineering Supplies do a range of Carrs chemical treatments for various metals....

Thanks - Francis

[Gina]

Beautiful job Francis

[fwm891]

Thanks Gina - Your obsy is coming on great guns too...

[Gina]

Thanks Francis

[fwm891]

Hi,

I managed to do some more machining today and worked on the rotation mechanism for the prism.

I decided to make a small worm and wheel (section of) after sketching all sorts of levers and add-ons. Worm is a 3 mm metric standard thread form.

Photos also show some of the brass parts: prism holder etc.

This will now be dis-assembled for degreasing ready for the metal blackening chemicals which should arrive this week.

Francis

[Gina]

Great

[steppenwolf]

What a fantastic project - I'd love to be able to do this sort of machining work - currently, although I often design my own components, I have to have them machined for me.

[steelfixer]

Well Francis what can I say.

I take my hat off to you sir.

That is one first class piece of engineering.

Cannot wait to see the finished results.

[fwm891]

Steve / Steelfixer,

Many thanks for the comments. I have to admit to being quite chuffed myself the way this has evolved.

Decided to put another clamp on the opposite side as the square tube rocks slightly as the worm is rotated, plus a couple of minor alterations.

My PTFE arrived so I can complete the focusing part of the assembly. Big clean-up and blacken everything enclosing the optical path...

Next update will hopefully be the finished article.

[Merlin66]

Francis,

What benefits do you see in being able to rotate the prism on its axis??

I certainly see the tilt of the prism as very handy, as well as being able to move the prism in and out of the light cone; rotation of the whole assembly for better placement of the prism relative to the camera FOV is also great.....

[fwm891]

Hi Merlin66,

Star shape basically. My present OAG gives me 'stars' that bear little resemblance to stars. Lines, blurs, etc., etc!

Rotation and tilt of the prism will allow me to exactly square-up the prism to the incoming light rays. That way I get virtually all the light from even faint stars to guide with. At present I can only use brighter stars as their light is spread into whatever shape the misaligned prism whats me to see (sorry wants PHD to see - and a good job it does tooo!)

[Merlin66]

Hmmm

I think you'll find the tilt and being able to slide the prism in towards the centre of the field helpfull to give some corrections but unfortunately the rest of the distortions come from the coma/ aberrations out towards the edge of the usable field. Unless you are using some specific flattener/ correctors before the OAG - there will always be an issue......sorry.

[Gina]

This is very interesting I'm a sheer novice on this, having only done some rather cursory imaging. I presume the idea is to save on a guide scope and use the main imaging scope for guiding.

[fwm891]

Hmmm

I think you'll find the tilt and being able to slide the prism in towards the centre of the field helpfull to give some corrections but unfortunately the rest of the distortions come from the coma/ aberrations out towards the edge of the usable field. Unless you are using some specific flattener/ correctors before the OAG - there will always be an issue......sorry.

I'm using an MN190 which has a very wide well corrected flat field.

Misplacement and or misalignment of optics cause image distortions right across the FoV but manifest themselves most at corners and edges (ie, radially away from the centre).

The prism I'm using in the above OAG has large entry/exit faces and aluminised hypotenuse partly to collect more of the incoming light bundle but also a brighter image. This unit has been designed so that the prism doesn't require movement into or out of the light beam. I sits neatly between the broad edge of my 8300 chip and the side wall of the focuser tube.

Being able to both tilt and rotate the front prism face allows me to reduce image distortions induced by the prism to a minimum, and also reduce unwanted reflections within the prism because light enters the prism normal to its entry/exit faces.

I've imaged with a full frame DSLR using the MN190 and only get minor vignetting at the corners but stars that are still sharp everywhere across the frame.

It was this superior sharpness over a wide field that made me want to give up on the current OAG and design something to exploit the optics of the MN190.

If this sounds like an advert for the MN190 so be it! but the optics are outstanding after my 8-inch f4 newt.

Francis

[fwm891]

This is very interesting I'm a sheer novice on this, having only done some rather cursory imaging. I presume the idea is to save on a guide scope and use the main imaging scope for guiding.

Hi Gina,

Yes it removes the need for a guide scope as it collects a portion of the light from the telescopes primary optics and relays it to a small guide camera (typically Loadstar/QHY5 etc).

Because an optical assembly can distort during any observing run i.e. twisting, bending etc., as it traverses across the sky, what also happens sometimes where you use a separate guide scope is that the two OTA's flex differently. Your guider software then follows the flex of the guide scope not the flex of the imaging scope leading to stars that are not pinpoints.

The OAG because it uses light from the imaging scope it corrects for a flex of the imaging scope which in turn means your stars remain as pinpoints.

The added benefit is that without the guide scope's extra weight less counter weights are needed on the mounting.

Francis

[Gina]

Hi Gina,

Yes it removes the need for a guide scope as it collects a portion of the light from the telescopes primary optics and relays it to a small guide camera (typically Loadstar/QHY5 etc).

Because an optical assembly can distort during any observing run i.e. twisting, bending etc., as it traverses across the sky, what also happens sometimes where you use a separate guide scope is that the two OTA's flex differently. Your guider software then follows the flex of the guide scope not the flex of the imaging scope leading to stars that are not pinpoints.

The OAG because it uses light from the imaging scope it corrects for a flex of the imaging scope which in turn means your stars remain as pinpoints.

The added benefit is that without the guide scope's extra weight less counter weights are needed on the mounting.

Francis

Thank you for confirming that I thought of the alignment problems of using a separate guide scope even if the guide scope were quite small and light. I might have a go at an OAG when I get into serious imaging. It won't be as well engineered as yours as I haven't got the facilities (or skill) but I might be able to rig something up. (Mrs Heath Robinson ).

BTW... I looked at the MN190 on FLO site - beautiful scope

[fwm891]

Hi Gina,

I'm certain you can 'knock' something up to suit:rolleyes:

I got my MN190 secondhand but (probably bias) its easily the best scope for imaging in that size range and price.

They come up occasionally on UKAB&S and other forums as well just a case of prowling on a fairly regular basis.

Francis

[Gina]

Thanks Francis

[fwm891]

Well the chemical blacking of the aluminium parts turned out very patchy so where possible I have removed it, leaving a brush aluminium finish. Other areas have been painted with matt black enamel. The brass parts which were supposed to be the most difficult to coat are fine!

The prism has also been installed so hopefully I can give it a trial soon to check it and the recent belt mod to the NEQ6 drive....