Very Amateur Astronomy

I received an early Christmas present from my wife last weekend in the form of a Baader BCF correction filter for my Canon 1000D, it was purchased from Bernard at Modern Astronomy, not the cheapest supplier of them in the UK, but I was able to ensure I had it in time for the weekend, and support an excellent, helpful and knowledgeable supplier.

I set up my work station to carry out the mod on the dining room table and followed the excellent illustrated guide provided by Gary Honis. The dismantling of the camera went well, other than a struggle with one ribbon cable (marked ‘J’) on the guide, which caused me a bit of a headache until I realised that it was definitely a clipped cable, but that the locking latch was on the opposite side to all the other locking latch type clips in the camera. I’m not sure why that one has to be different but it did cause some confusion.

Replacing the filter itself turned out to be a bit of a challenge, everything came out mostly in one piece, though I did crack the old filter. However once I had put my new filter in and checked it I noticed an unfortunate blob of something right in the middle of the glass. Not good, somehow I had managed to carry some of the silicone glue right to where I didn’t want it even though I made careful effort to only handle the filter by the edges. This meant I needed to carefully remove the new filter and clean it.

It all went back together well enough, though with a few more dust particles getting in that I would have liked. Still, I thought, this is what we flat field for. I had a short moment of panic when it failed to power on again after assembly, but some careful rechecking of all the ribbon cables soon solved that issue.

Some quick snap shots around that house proved that the autofocus was working well still and everything functioned properly.

There was however one last annoyance to be found when I did a flat field for my test images, as it appears I have a small overrun of glue or some other substance slightly intruding on the lower left corner of the image. It’s not enough to affect the picture and the flat field removes it well enough, but I am annoyed with myself for not doing a better job. Enough that I may well dismantle the camera again to see if I can clean it up a bit better.

So some hard lessons learnt, and an interesting challenge, not as hard as the web cam long exposure mod but certainly more delicate in parts.

Thursday night provided a few hours of clear skies for me to get out and test the camera. First I pointed at the heart nebula, which instantly showed the improvement in HA response, but it was a little large to fit in the FOV of the 250mm reflector, so I’ll save that one for a good night and the 66mm. Instead I took a small stack of five minute exposures on M33, here is the result :

And for reference here it is again from September last year

You can see the affect of both a years worth of experience working with Pixinsight and the greater colour response of the camera, especially showing up the regions of nebulosity within the galaxy.

Guiding for me continues to be an exciting adventure in getting things right, or at least right enough, so that reliable guiding can be achieved. I am left needing to balance the capacity limits of the mount vs my requirement for having the right scopes available. Typically I have used the Vixen A70LF as a guide scope which appears to match my needs, but realistically I only want two scopes on the mount, so if I want to use the William Optics ZS66SD I have to change around the entire setup, not ideal.

Now I can use the ZS66 as a guide scope instead but I have found the ideal focal length for my guiding is around half the focal length of the imaging scope, which given my camera is about 2 arcsec per pixel which tends to give the best results in practice. The ZS66 give a wider field of view but always at the cost of more guiding error.

The aim then is to find the right combination of scope and barlow to allow me to use the ZS66 as a guidescope, and therefore have it available as a widefield scope when required. Lets take a look at the available options, all calculations worked out using the excellent (and free) CCDcalc :

Guide Camera

Philips SPC900NC – 5.6 x 5.6 pixel size

Current Guide Scope Configuration

• Vixen A70Lf – 70mm F12.9
• 0.6 Focal reducer
• 17.5 x 24.4 arcmin – f/l 542mm – 2.13 arcsec/pix

So we can see that the combination of the A70LF and focal reducer actually gives us close to the ideal, it’s a wide enough field of view that I rarely have to hunt for a guide star and the arcsec per pixel seems to allow for decent reliable guiding. Now I just need to match that with the ZS66.

ZS66SD Possible Guide Configurations

1. Standard (no reducer or barlow) – 24.4 x 32.5 arcmin – f/l 389mm – 2.96 arcsec/pix
2. Barlow 2x  - 12.2 x 16.3 arcmin – f/l 779mm – 1.48 arcsec/pix
3. barlow 1.5x – 16.3 x 21.7 arcmin – f/l 584mm – 1.98 arcsec/pix

As standard with the webcam directly into the scope we get a lovely wide field of view and no issues finding guide stars, however testing last night using PHD Guiding was a disappointing, out of 13 x 6 minute exposures not one showed good round stars compared to guiding results with the A70LF setup.

The second configuration with a two times barlow was also tested last night, but at the expense of my sanity I could not live with that, finding a guide star in that field of view away from the milky way was soul destroying.

The third option looks like a winner, and not only that but I appear to have an old soligor 1.5x screw on barlow lens sat at home doing nothing, in fact I’ve been using the tube extension, normally to convert it into a 2x barlow, just as a useful tube extension for webcam imaging anyway.

I’ll test this setup tonight and report back on the results.

As I recently identified and fixed a major issue with mirror slop in my reflector I decided to take some time out to have another run at capturing periodic error, as my previous results along with my guiding were badly effected. Here is the PEC recorded over 8 worm cycles :

The data shows a periodic error of +8.4/-5.7 uncorrected, much better than the +14.2/-13.0 I had previously recorded. I will get out on the next clear night to record the corrected results.

I thought I’d publish my results from measuring and training PEC on my CGEM, other users may find the results useful as might potential buyers. I used PEMPro after researching the subject as it is generally thought to give the best results, though I had a bunch of issues getting it to work properly with then SPC900NC webcam, see my earlier post on that subject if you are having problems.

PEMPro offers full support for Celestron mounts as well which is handy, allows you to directly upload a PEC curve to the mount. Anyway, on to the results, the first screens show the uncorrected curve written after 5 worm cycles.

Individual worm cycles :

PEC Curve :

The reported periodic error is +14.2/-13.0, certainly enough to give me problems with extended stars on the RA axis at my image scale, though guiding corrected a reasonable amount of this it was not perfect by any means.

The PEC curve created from this data was then uploaded directly to the mount and I took another set of two worm cycles with PEC running to get an idea of the improvement.

Individual corrected worm cycles :

The data shows corrected periodic error of +5.5/-5.1, a great improvement and a 5 minute guided exposure afterwards showed much better stars, and also showed up how badly out of collimation the scope had become during the recent pier installation. There are probably better results to be had with more worm cycles of data, and I’ve not looked into smoothing the curve or adjusting the time shift, but I’ll report back when I do.

If you have been looking to get accurate periodic error correction then you have probably found your way to PemPro by now. If you have an SPC900NC webcam, like many of us do as it is the default model for long exposure modification, and you have found PemPro, then you have likely found out that the two of them together makes for rather a mess. Pempro crashes every time you try to set the exposure levels for the cam.

Here is a quick way round that, you will need some software that works with the webcam, I use the free version of K3CCD Tools and WcCtrl (web cam control) to manage the webcam settings.

1. Start K3CCD, centre a star and set the correct exposure and gain to get the image.
2. Start WcCtrl, connect it to the webcam so that it inherits the settings, I’ve found WcCtrl won’t let you switch to manual exposure by itself.
3. Stop K3CCD, leave WcCtrl open and start PemPro.

PemPro will now run with the webcam settings defined in WcCtrl, and you can now use the software to sort out your PEC.

I seem to have had some issues with the permalink structure which I had not noticed. Apparently any of my updates not on the front page had vanished and were returning a page not found error. I have reset the site back to standard permalinks now and the archives have sprung back to life.

However if for some reason you were linking to an update in the old format then your link is going to fail. Hopefully you’ll follow the link on the error page and find your way here instead.

Update!

The site changes have been live for a bit and I’m seeing a bunch of errors for misdirected links from google, and apparently I’m getting traffic coming in from a link on the overclockers.co.uk forums which I’ve now broken, so sorry about that if you are visiting from there.

Everything should returns to normal over the next few days as the search engine bots update and at least now the last year of updates is available again!

I had a go with getting autoguiding working the other night with some success and some issues. The first problem was that my wireless access point down the shed wasn’t communicating back to the house very well resulting in dropped packets and the starbook being unable to get an IP via DHCP. In the end I directly connected it to the laptop via a crossover cable. That worked but it’s not quite what I was after.

Next I slewed my now dual mounted scopes round to Arcturus and spent some some getting the secondary scope lined up on it. I need to get my old spare red dot sight mounted to it to make things quicker. Once there I hooked up the webcam and ran through the configuration for PHD Guiding having a play with the webcams gain and exposure settings at the same time. It was a quick and easy process without errors and looks like it will work OK.

Next I spent some time on alignment, trying out the CCD drift align method. This was new to me and quickly showed that the mount was out, pointing to far west. I was short on time so didn’t spend to much time on this. I’ll have another go when the weather allows.

The last thing I tried before calling it a night was to test the sensitivity of the webcam for a guide camera. I slewed the mount up to the ring nebula and then tried to locate a suitably bright guide star in the second scope. There was not quite enough movement to reach Vega and the standard webcam was incapable of picking up anything else in that region. So we can say that a standard Philips SPC900NC webcam is not suitable as a guide cam for astrophotography.

Last night however I took a trip to Maplin and then took my soldering iron to the webcam. I’ll write about the result soon.

This is not the month for astronomy, why the weather men ever put anything up that isn’t ‘It’s April, what do you want?’ is beyond me. I’ve not done any new imaging or observing as when I have expected clear skies the clouds have rolled in the minute I get the shed roof off, or I’ve been convinced to get settled in in front of the TV with my wife and a glass of wine and it’s turned into a crystal clear night.

I have however not been entirely idle. I’m currently constructing a light box for the scope, it’s going reasonably well but I ran out of glue for the hot glue gun last night so it’s on hold for now. I’m following a selection of plans available on the net and the idea is that it will just slot in over the top of the scope.

I should have it finished for the weekend so we’ll see how well it performs then.

I’ve been looking a bit more into image processing and I cannot believe how blue my colour image is, really, my eyes must have been tired as I didn’t spot that at all. I need to go back to that and start again, being a little more careful to achieve a proper back ground black level. I know I’ve got a great amount of detail in that image but pulling it out is the real challenge.

I’m hoping the skies will clear for the weekend as it’s the first in a long time where I have nothing else I should be doing, but well, it’s April.

It looks like the issues last night were caused by the RA motor pushing itself away from the RA drive shaft. The motor and shaft run parallel and are connected by a pair of gears wheels. The RA motor is held in place against it’s mounting plate by four screws but there is a small amount of play to allow the two gears to be properly meshed.

It seems these screws were to loose and possibly the RA axis was to tight which has allowed the torque of the motor to push itself away from the RA shaft. A real pain to be honest which would explain the dotting effect seen in the second picture below where the gear wheels are not in contact all the time so you end up with dots where it’s tracking and a dash where it’s not.

This morning I have removed the RA motor and re-adjusted the the RA shaft worm mesh, I’ve reduced backlash on the mesh and it now turns by hand. It still feels a little tight so maybe I need to dismantle it and replace the greece at some point. I’ve replaced the screws holding the RA motor in place and hopefully have the gears meshed a little better now. 

The MET office tells me it should be clear tonight, so we shall find out then if my work have made the required difference.

Not much to report recently, I’m still getting used to the new mount.

BAS have been running a moon watch in a local shopping centre so I’ve been along there the last couple of nights with the scope giving the public a chance to take a look. It’s been a lot of fun and we’ve had all sorts coming through to take look. This being the international year of astronomy you should be able to find similar events in your local area.