Thoughts on which imaging rigs to concentrate on

[Gina]

There's a strange thing happening with the filter wheel.  In SharpCap the filter is correctly labelled but when I'm in APT the labelling is wrong.  In SharpCap the filters are Ha, L, R, G, B but the R filter is showing as SII in APT and the G filter as L and these are entered into the file name.  These correspond to the labels I had on the filters before when I was doing NB imaging.  APT seems to have retained the old labels and is not reading the new ones from the filter wheel via the driver.  Also, some of the filters are not accessible in APT's filter wheel changing tool - I'm having to shut down APT and use SharpCao to choose filters.

[Gina]

Now onto the Blues.

[Gina]

Here's the correspondence between true filters (shown correctly in Sharpcap) and the filters shown in APT :-

1. Ha - Ha
2. L - OIII
3. R - SII
4. G - L
5. B - R

Try as I may, I can't get APT to show the right filters - tried restarting APT and even rebooting   Also, only positions 1, 2 and 3 work in APT.  Sometimes APT won't go past the end of the first exposure in a run.  CdC keeps disconnecting from EQMOD and a couple of other problems I forget ATM.  I think I may have INDI plus KStars/Ekos working for imaging soon so maybe that will sort out all the problems in one go...

The filter problem makes me wonder about continuing with the longer subs for RGB and M31.  Maybe I'll leave that for another night and set up imaging in Ha on a new target - which I can do from indoors and then leave it running for the rest of the night while I go to bed - I'm feeling tired now.

[Gina]

Here's a 60s blue sub with gain of 139 (nearest I could get to 140) and T=-30°C.

[Gina]

I think that may be a bit better than the 10s at g=148 so I'll compromise and collect 25 subs each of RGB followed by rest of the night on Ha.  Been looking through M31 images and only found one with Ha added to the RGB to show the star forming regions.

[Gina]

100 flat frames captured - gain 49 and exposure of 0.1s - temperature of -26°C.  Next I shall go out and put the lens cap on and grab some darks and biases to calibrate the flats.

Here's a flat and the associated Levels graph.

[Gina]

Darks done and now doing Biases.

[Gina]

Been thinking a bit more about a DIY fork mount.  I have 1 100t MXL timing pulley with 5mm bore so I can buy 2 15t 5mm bore pulleys.  The drive from NEMA stepper motor to mount axes will need a two stage step down belt drive.

1. Motor pulley - 15t PLMXL015AL-05
2. Intermediate pulley 1 - 100t PLMXL100AL-05
3. Ratio = 6.67
4. Intermediate pulley 2 - 15t PLMXL015AL-05
5. Final drive pulley - 224t for 100:1 overall ratio with pitch diameter = 145.6mm (just under 6") which is reasonable for the final drive pulley.

The final drive only needs to rotate 180° so can be a plain pulley with the timing belt fixed to it and therefore not needing teeth.

Edited October 6, 2016 by Gina

[Gina]

Think I'll go over the calculations again...  Longest FL lens I shall use on the no.2 rig will be 200mm and the camera resolution is 3.8microns per pixel.  Pixel resolution will be fine.  So angular resolution is given by tan(θ) = 3.8 / 200x1000 = 0.000019 giving θ as about 0.001°.  NEMA steppers usually have 1.8° per step but using 16x micro-stepping = 1.8 / 16 = 0.1125°.  I think we can call that 0.1° so to get a resolution of 0.001° we need a gear ratio of 100:1  Q.E.D. 

16x micro-stepping is the most obtained by A4988 stepper drivers as used for 3D printers.  There are others that give up to 32x micro-stepping but I think 100:1 is reasonable for the 2 stage belt drive step-down ratio.  The final drive pulley would need just 112t or thereabouts with 32x micro-stepping and 120t timing pulleys are available.  However, I think I would prefer a larger final drive pulley for better accuracy.  Turning a 145mm pulley can be achieved on my lathe as I've done it before.

Edited October 6, 2016 by Gina

[Gina]

The resolution mentioned above should only be needed for the RA axis if the PA is well set up as the DEC axis will only be needed for slewing to target.  With good PA, guiding is not necessary for widefield.  This means that a coarser drive can be used for the DEC axis.  The RA axis needs high resolution for accurate tracking.  I have some 150mm x 22mm Acetal blanks that I can turn for pulleys.  They have marked centres where they were turned off a long cylinder so I can drill a hole with bench drill and then mount the workpiece on an aluminium mandrel to hold in the chuck for turning.

[Gina]

Been out this evening and missed a couple of hours of clear sky.  Then some cloud came over but now it's clear again and I'm gathering more RGB subs of M31.  Here's one - 60s with g=139 and T=-30°C - it's a cold night!!

[Gina]

Now capturing Green subs - again 60s - G=139 - T=-30°.  Gamma stretched.  Full frame.

Edited October 6, 2016 by Gina

[Gina]

I think a longer focal length lens would be better for M31 as the FOV is a bit too wide it would seem with the 135mm lens.  The 200mm lens is a stop slower so would want twice the exposure for the same brightness.  Anyway,  once I've processed these subs I shall be able to see just how big M31 really is right out to the fainter stuff that tends to get cropped in many renditions.  Then if the 200mm lens really does look better I can do some more imaging of M31 at a later date.  Tonight could be the last time the conditions are right for several weeks due to the moon.

I shall be making up a lens support frame and focus quadrant for this larger lens soon but expect to be concentrating on the RPi imaging and control system first.  There are several DSOs that I want to image with the current 135mm lens and that leaves plenty to get on with when clear night skies permit.

[Gina]

By the time I had collected nearly 50 green subs last night the cloud came in.  Did a meridian flip and set up for blue in case the sky cleared again and went to bed leaving it running.  A scan through the blue subs this morning showed two or three usable blue subs only   I think I have a reasonable collection of red and green subs - next clear sky looks like Sunday but anything could happen before then.  I think I have some blue subs from previous nights.  Maybe do some data processing today...

[Gina]

Been looking into the axle bearings and fixings for the east end fold down flap/window on my observatory.  Originally had 3D printed brackets with plain bearings.  The wind got up the other night and ripped off the polycarbonate window & frame, breaking the brackets.  I have decided that ABS is not strong enough to hold the window (8ft long by 2ft high) against high winds (catches the wind like a sail!!) and have decided to spare no expense and buy a pair of pillow block bearings.  These will bolt through the framework of the observatory.

UCP206-18 Imperial Cast Iron Two Bolt Pillow / Plummer Block Housing with 1-1/8 inch Bore Insert

Using sealed ball bearings and cast iron pillow blocks may seem over engineering (which it is) but thirty quid spent on a pair of brackets is not that much and would save me a lot of work if I were to make something strong enough myself.

[Demonperformer]

Thirty quid will be nothing compared to it being ripped off again, falling into the observatory through the hold and being battered around by winds trying to get back out again ...

 

[Gina]

Quite so   My thought too.

[SilverAstro]

On 03/10/2016 at 19:20, Gina said:

very difficult to achieve in one go without using worm and worm gear and I haven't got the facilities to produce those.

Hi Gina, I'm a bit late on parade here ! dont want to teach eggs, just checking that you know of the ,makeshift ,amateur hob method using a tap - on slow in the lathe chuck and a disk on the top-slide/cross-slide that will become the worm-wheel ?? Depending on dia. of wheel then extensions, and live centre in tail-stock, may be needed whereupon it all becomes a bit iffy

just checking good luck.

[Gina]

Thanks for the suggestion but I have tried this before - unsuccessfully.

[SilverAstro]

2 minutes ago, Gina said:

Thanks for the suggestion but I have tried this before - unsuccessfully.

I guess ,,, the next pass wiped the teeth of the previous pass ? yes all sorts of hazzards await, a good supply of redundant/freebee disks is required it is a bit of a guestimate just how fractionally oversize the blank needs to be.

I'm sure you will overcome, as I said, just checking that you were not missing out on hours of fun

 

[Gina]

Har har...

[Gina]

Clear sky this evening and I'm finishing off my present RGB run on M31.  Moon well out of the way.  Well - 90° off axis

The blue seems brighter than the red and green and I've had to reduce the exposure to 30s to avoid white clipping.  Here a single blue sub 30s gain 0f 140 (14dB) an temperature of -29°C.  Gamma stretched in Photoshop.  Full frame.

[Gina]

Set up for 150 subs of 30s that's 75m total imaging time which will finish quite early so I'll have to think what I want to do for the rest of the night - if anything.  I'll have a look round the sky and see if anything springs to mind (not too close to the half moon).  I could take the rig off the mount and change over to NB.  If the FW held just one more filter I wouldn't have to open it up to change filters.  But I guess we can't have everything...

[Gina]

Decided to go for a goodly set of Ha subs to add to the RGB rather than mess about with filters - feeling too tired for that and to stay up very late capturing.  I think I'll leave all night imaging until I get an automated setup working.  Consequently, I set up for 120s Ha images at -30°C.

[Gina]

Realising I still had a low gain for RGB I reset the gain to 500 and reduced the exposure to 30s and here is the result in a single sub.  Small star forming zones can just be seen.