First try at DARV and second at M13 (with lots of pics)

[Johnny666666]

Hi to everyone

Following my last (abysmal) attempt at imaging M13 I thought I'd try again, but followed advice from friends on SGL and made a few modifications. Firstly I tightened everything on the tripod, then bought a Bahtinov Mask (what a brilliant piece of kit) and also the Skywatcher Auto Focuser to fit the single speed focusing rack on the 150P to eliminate vibrations when adjusting focus. I set up, levelled the tripod and polar aligned, then thought I'd have a go at drift alignment, as detailed by Robert Vice. Note: all the DARV images below are unprocessed apart from resampling the image size to about 30% and all exposure times were counted by brain with the times below added by checking the EXIF data...

Below is my very first ever result, pointing South as low as I can see (10secs tracking, 60 seconds not tracking, 60 seconds tracking at 2x speed, ISO 800)

Something was obviously amiss because there was no way the non-tracking and 2x tracking star trails could be aligned so I tried again for a longer exposure time, and reduced the ISO to minimum (10 seconds tracking, 130 seconds non-tracking, 130 seconds 2 x tracking, ISO 125)...

Again, not a hint of the star trails drfting out, so I slew the OTA to the West and found a suitably low star. Again, the results looked exactly the same... I even listened to the motors to ensure they were stopping and changing pitch and everything sounded OK to me. Entirely unconvinced that simple PA using the reticle on an EQ3.2 mount could be that accurate, I decided to try something else:

I exposed for much longer '2 x tracking' than I did 'non-tracking,' and the result below shows that the star trail does in fact move in one direction, then overlaps exactly and overshoots the initial 'starting' position of the star...

Still not entirely convinced that simple PA could produce what appeared to be accurate results I decided to go for a very long exposure time! (10 seconds tracking, 260 seconds non-tracking, 260 seconds 2 x tracking, ISO 125) and this was the result:

Somewhat pleased that I had at last encountered a tracking error   I adjusted the Alt setting bolts by 1/4 of a turn (I had no idea what direction, I just wanted to see how much effect it had) and exposed again using the same settings. I also added a Baader Neodymium filter to the optical train to judge the effect it had on light pollution:

The angle of deviation was less but the finishing point was now below the starting position so I had adjusted the bolts too far. I also think the image is more usable with the Neodymium filter (excuse the tree!)

However, the clouds were starting to roll in so I decided to have a go at M13 again. I now have a RACI 6x30 finder (I just couldn't get on with the stock one) but I couldn't see M13 in it, and could barely see it using binoculars but I eventually stumbled across it. Focus was done using the Bahtinov Mask on a star somewhere along the Dec axis (I simply used the fine adjuster until I came across a suitably bright star) then returned to M13. The images I think are an improvement on my previous attempt in respect of both focus and issues with vibration.

This is a 1:1 crop of the central part of a prime focus 10-second ISO 1600 image, with added Baader Neodymium filter and post-processed a bit to bring out more detail (and noise!) There is some coma towards the edges of the uncropped image but the central stars I think are much better:

This is a 1:1 crop of a central part of a 15-second ISO 1600 image, using the Skywatcher 2 x Barlow lens and Baader Neodymium filter. I just wanted to see what it looked like when pushed harder, again I processed the image a bit to bring out some of the fainter stars:

Overall, I am pleased with the results, and any suggestions would be welcome, regarding anything.

John

[wxsatuser]

You don't need to point lowdown to the south.

You need to point to a star on or near the meridian but also near 0 degrees dec.

Zero degrees dec is the celestial equator and will be around 40 degrees up in the sky at the meridian, in the UK.

Same again to the east or west.

A star to the east near the celestial equator, I use a star at least 20degrees above the horizon.

BTW  I don't understand your method.

The way to do it with a dslr.

Find a star near the meridian and 0 dec.

Set cam for long exposure, I use bulb with ISO 800.

You want to use the handset keypad, I use Xbox pad with EQMOD btw.

Trigger exposure, wait approx 5secs and then press the West keypad for at least a minute or more.

After that minute or more let go of the west key, wait approx 5secs and then hold the east key for

approx a minute or more.........you don't want 2x tracking.

The longer you make each exposure the better your PA will be.

Here is a 3minute each way star to the south.

Here is a 3minute each way star to the east......notice it's slightly out.

[Johnny666666]

I think the stars I chose were pretty near to your suggestions, surrounding houses, fences etc means that 'low down' to the South is about 40 degrees for me, and West I am able to get a bit lower, probably quite close to 20 degrees above the horizon. Perhaps I should've made that clearer.

I also think the two methods we are using are the same, though I stand to be corrected if I'm wrong. Bear in mind I don't use a computer or any software. I use the camera in Bulb mode, with the telescope tracking at single rate for a few seconds in order to create the blob of the starting position. When the motor controller is set to '2x' mode, pressing the 'W' keypad causes the RA motor to stop running (single rate minus single rate = stopped) creating the first star trail across the image, akin to not having any tracking. Then after a further minute I press the 'E' keypad, which causes the RA motor to run at 2x speed and the scope to slew East (single rate + single rate = 2x speed). This then causes the same length of star trail on the image but in the opposite direction. You need 2x speed (on my set up) to slew the scope East, single rate will just hold the star at the same point on the image, as it should do. If you look at my last DARV images, they are the same as yours, although not as accurate!

The only thing I don't do is wait for 5 seconds before reversing direction, but only because this wasn't mentioned in the DARV literature I have.

Hope this makes things clearer?

[wxsatuser]

Ok.

I don't know your mount so you could well be correct on what you say.

Fine on the stars you use, quite a few people look for stars near the horizon and of course thats not really correct.

The proof is if you get nice round stars on long exposures, which by the look of your M13 they are quite good.

You should be able to get some nice images, well done.

[Johnny666666]

Thanks, though you have give me more information and further things to try! In truth my exposures of 30 seconds are beginning to show elongated stars, and I was quite surprised at this when I thought the drift aligning was more accurate. The images I posted are 10 and 15 seconds, beyond this they do show signs of inaccurate tracking. If I can get longer exposures I can ease off with the ISO settings and get less noisy images.

I'm not sure what you mean by: "A star to the east near the celestial equator, I use a star at least 20degrees above the horizon." Isn't the celestial equator exactly on the horizon due East or West, or have I got it wrong? Therefore would I be better to try and drift align on a star closest to the horizon or is it not that important?

[wxsatuser]

Now I understand, brain was a bit dead last night, your using stars the same as me.

For the east star I go a few degrees to the southeast and above the equator and yes

the equator is at the horizon due east/west.

The equator is used because thats where the quickest/best results will be.

Longer exposures will be needed as you move away from the equator.

There is a formula somewhere but cannot find it at the mo.

It may be just your mount cannot track much longer than 30secs, even with good PA.

My HEQ5 goes about 90secs before stars go awol, even with very good PA.

I have checked my PA with various methods and I believe it's as good as it can get.

You can get less noisey images with short exposures but you need a lot more total exposure.

[Johnny666666]

I think I've read several times that the EQ3.2 can just about manage 30 second exposures when set up well, so I don't think I'm far off maximum. I'll wait for another clear night and have a second bash at DARV, just to confirm I'm doing it correctly. Unfortunately I have no permanent position for the telescope so I have to put it all together and tackle down after the end of a session. It's no real hardship, just a bit of a pain.

Once I've got the 15 - 30 second exposures cracked I'll start experimenting with stacking software, presumably detail can be teased out of faint images in a similar way that Registax does for planetary? Getting a decent exposure time for planetary images is not a problem but if I'm restricted to 15 - 30 seconds for DSO's at low ISO then *I* won't be able to see much DSO detail on screen, but will the software?

[wxsatuser]

Stacking will make a big difference even with 30sec exposures.

Here are a couple on Noel Carboni's site that are stacks of 30secs.

http://noel.prodigitalsoftware.com/Astroimages/M13_Globular_Cluster.jpg

http://noel.prodigitalsoftware.com/Astroimages/M104_Sombrero_Galaxy_2_Small.jpg