Wishful thinking? Or has this app got astronomy legs?

[Tim]

I have been looking at ways to put setting circles on my truss tube Dobsonian.

Eventually it would be nice to get an encoder based system, but for sheer simplicity and ease of use I plan to put a printed azimuth circle in the base of the dob, and was going to get a Wixey for the altitude measurement.

Anyway, I started playing about with various compass apps, and clinometer apps, and eventually came across one that has both, combined, and with a night vision always on setting, and dimmable using the brightness settings.

It is called "Field compass".  It was hard to test the compass last night at home, and these things are notoriously unreliable, but I have just had a go at work and think it might just be useful strapped to the back of my dob.

For a start the inclinometer seems very steady and gives repeatable and confirmable readings, so even if only that works it is a big plus. To test the compass, I used Stellarium to get the current Alt/Az position for the sun from my loaction, then set up a thin stake to cast a shadow. The position given by Stellarium was 102°, and the position settled on by the app (using true north rather than magnetic) was 103 - 104°. 

The altitude given for the sun was 23°, and the app got that spot on, I manoeuvred the phone until is cast the smallest shadow and checked the reading.

So what do you think?  Useful as a guide?  I guess only proper testing will say for sure, but having it ready to go, at any location, and without the need for detailed calibrating is definitely in its favour I think.

If I make a little holder or the phone lined up with the dob, it could quite possibly become an all in one digital setting circle, planetarium, and weather station (clearoutside app).

I'll give it as much of a testing as I can before heading off to Kelling, and if it works will shell out the massive expense for the Pro, ad free version (£1.67), after all, it wouldn't be good to suddenly light up the field with an advert for Amazon Local or whatever

Be keen to hear what you guys think of this idea? Has it got legs? Or just wishful thinking?

Cheers

[Tim]

Here's a screenshot of the app in night mode

[Mark at Beaufort]

Tim I have used a field compass in the past to find Mercury. As you say once you obtained the Alt/Az position from Stellarium you just move the scope to that position.

[Stu]

Looks good Tim, my only concerns would be accuracy. Compass apps seem pretty variable when there is any interference (metal I assume), but the inclinometer should work with the accelerometers so if calibrated might be ok. Only way is to try, as you say.

The Wixeys seem to be quite popular and are I assume more accurate, not sure how they function though

[Tim]

I think the wixey is most likely redundant with the app, although they will go to 0.1° accuracy, and the app only 1°.

Hopefully out in the field the app will be accurate enough to get the scope in the right area at least.  I'm having a 20" azimuth circle printed today though which will also help

[Stu]

What do you have in the way of finders Tim? A TelRad is very useful used in conjunction with a 9x50 RACI. The app can get you roughly in the right place then fine tune position with the finders

[Mark at Beaufort]

To be honest Tim although I have used the Field Compass this was only to find Mercury in a bright sky. I find the fun of locating DSOs is with a RDF, Finderscope (correct image) and a quality star atlas (Uranometria).

[Tim]

The dob has a telrad on it, and also an 80mm finder with crosshair EP, although I need to tweak that to make it useful for my eyes. I'm half tempted to use my little Pentax 75 as a finder for a bit more quality, although I have all sorts of finders from various scopes in my obsy somewhere.

I'm hoping to do some outreach and local gatherings for non astronomers with the scope when I get used to it, and really want to be able to nail the positions quite quickly to keep the viewing interesting for them. 

With any luck the combination of the setting circle, digital compass app, and either a Wixey or the readout from the app, I should be able to get within a degree or two of the DSO, and given that  those events will be picking out the more interesting fuzzy blobs, ie, brighter ones, so hopefully even I should be able to pick them out with a wide enough EP!

Just need to figure the best way to align with the telescope base with true north, either that, or figure a way to get close, and then adjust the pointer  itself to 1° once aligned on Polaris etc. Need to have more of a play with the inner workings

[Gazabone]

I have put a circle in the base of my dob and s digi inclinometer on the tube (the inclinometer is magnetic) and it works very well indeed using altitude and azimuth coordinates from Stellarium. The S@N mag (July 2015 page 74) showed how to do it. I didn't follow those plans exactly as my base isn't the original. There is s link to a pdf that you can print for the azimuth circle (I would suggest enlarhing it to as big as your base will take). You'll also need to have some method of levelling the mount, I've put adjustable plinth feet on mine. On the few clear evenings we've had since I did the mod, it works exceptionally well.

[Tim]

Took my dob base to bits last night and had a good look.

Im having a 2ft azimuth circle printed, which I will set on a piece of Dibond. This will be bonded to a new piece of 18mm ply as a new base. The scale will be readable from the eyepiece side of the scope.

Am trying to think of a way to make a levelling base with the ability to rotate the scale and then lock it, easily and in the field. Hmmm, thinking cap time

[Gazabone]

My azimuth circle is mounted on the bolt through the centre of the mount. I've had to put some nut lock glue on it. There are now 2 nuts on the bolt with the circle between them (nut lock only needed for the upper bolt to stop the circle turning the nut as you set it). Rotating the mount doesn't move the circle at all so I can simply plonk the telescope down on the ground, level it, point the scope at a bright object near the faint object I want to find, set the circle to the azimuth Stellarium states and its good to go. I've found the accuracy peters off on the azimuth scale when I stray more than about 90 degrees from my bright reference star. Eg centre the scope on Dubhe and it works really well finding M81/82 but not so good on M27.

Your plan to use a 2ft circle is great, I'd love to hear how you get on as I think I may well upgrade my circle to something a bit bigger. Good luck.

[Bart]

I found the Wixey invaluable. Looking for faint galaxies, the accuracy of 0.1, rather than 1.0, I found, is necessary. Yes, 1.0 is useful for getting you there, but 0.1, reasonably well zeroed, will help find the difficult ones.

I also found that when the alt is accurate, and the Telrad is telling me I'm reasonably close, it's just a matter of sweeping the tube back and forth slowly to locate the object ( no real need for az circle). The base has to reasonably level too, I should say.

Barry

[Tim]

Thank you Barry, that is what I was wondering, whether a fraction of a degree makes a difference. I'll order my Wixey today

[Gazabone]

You won't regret it Tim

[Tim]

It's on it's way. Do you cover the green/LED screen with red acetate or something?

[Bart]

Hi Tim

No my Wixey is black digits on a grey background, no no problem. Actually need a red light to read it. You won't regret it

Barry

[Gazabone]

I'm experimenting with a red cover for my inclinometer, mine illuminates in a green light and it does have an affect on my dark adapted vision. At the moment I'm trying cutting down red poly pockets to see if the helps.

[John]

To be honest Tim although I have used the Field Compass this was only to find Mercury in a bright sky. I find the fun of locating DSOs is with a RDF, Finderscope (correct image) and a quality star atlas (Uranometria).

Thats the way I like to do it Mark (as you know !)