Report: NS8GPS on a wedge

[SteveL]

With many thanks to MartinB for the "loan before you possibly buy", I finally had a clear sky on Saturday night to try out the NexStar 8 GPS on a Celestron HD wedge. A wedge was the way I needed to head for my fork mounted NS8GPS because I was hitting a 20 second sub field rotation limit in AltAz. I thought I`d write a report because there have been quite a few SGL members who have purchased NS8GPS units, and this may be useful to them (or not... )

Earlier in the week, I got out into the garden to work out how to mount the wedge (and the OTA on the wedge), and run through the NS8GPS hand controller menus to get the correct alignment routines. This meant that on Saturday night, I didnt have to worry about that part, and could concentrate on the polar/drift alignment

As mentioned in more than just a few threads on here (and many many more elsewhere on the internet), the Celestron HD wedge is pretty basic, and the adjustor kit for fine tuning is apallingly badly designed. Without the adjustor kit (and extra ~£100 on top of the ~£230 price of the wedge), the wedge is pretty much useless. No adjustor kit means no fine tuning, which would mean having to physically rotate the tripod for Az, and more scarily, undo the wedge plate bolts and raise/lower it by hand with the OTA still attached! This would scare me with my NS8GPS... I hate to think what would happen with a 11" beast!

Anyway, I fired up the scope, and as recommended in the NexStar user book (by Mike Swanson), I did a North EQ align, then a Wedge align, adjusted the mount to point at polaris, then another north EQ align. Using the quick cheat notes about drift align ( http://www.darkskyimages.com/quick.html ) and having previously used the online drift simulator ( http://www.petesastrophotography.com/tutorial/polaralignment.html) I got to work. I think I was pretty lucky to be honest. Two wedge adjustments in Az and 3 in Alt, and the reference stars I was using didnt move off the cross hairs for a good 5 minutes or so.

Just to be sure, I deliberately messed up the wedge settings and started from scratch. Same thing pretty much. Well, that didnt take long, did it. Not as scarey as I had imagined.

One thing that did strike me, and I have no idea if this is just a quirk on my scope, but my GOTOs seemed far more accurate in EQ mode than they usually are in AltAz. Until now, doing the AltAz alignment process and then going off to (example) M13 would require me to be using a 40mm EP to see the object I had slewed to, roughly centre it in that, then recentre with my illuminated EP, then either do visual or slap the camera in and start shooting. The GOTO when I was in EQ mode was spot on, or pretty close to it. The seeing was not fantastic, so my images were poor, but after taking some more M13 pics (for reference) and then telling it to slew to M51, its was there, slap bang in the middle of the camera. Result! M3... dead center.

Now, imaging. The whole point of going down the wedge route was to break the 20 second field rotation limit I currently have in AltAz. I was managing 45 second subs without any noticable tracking errors, and going to 60 seconds only showed fractional movement. The fact that my imaging was done unguided means I cannot tell if this movement on the images is due to polar alignment problems, or due to tracking errors on the NS8GPS mount (No PEC training on the mount has been done as yet).

So, I am looking forward to another clear evening so I get out there and try it again. The more I do it, the faster/easier it will become (I hope!). I might try the polar align software WCS ( http://wcs.ruthner.at/index-en.htm ) next time as well, just to double check (and see if its faster to get aligned). I also have an Off Axis Guider that I am planning on using my DSI-C in, to guide my Canon 350D on prime-focus. As I start to bump up the sub exposure times, then I`ll start to see if the Celestrion HD wedge really is made of bouncey springs It may do me for a while, but only a few nights out using it will be able to tell.

(I hope someone finds this useful... or at the very least, interesting)

[daz]

Interesting and useful Steve - especially the link to the software!

Once download on its way!!

Keep us informed of your tests, I'm sure other NS8 users will be interested!

[ElijahB]

Hi Steve, let us know how things progress.... as someone with similar equipment (CPC 800, 350D, etc) I'm enjoying reading each of your posts.

[Helen]

Ditto! Although I can't really see me with a wedge unless I get to a point where my setup is permanent - I have enough problems carrying it as it is, and I hate polar alignment (which is why I love the GPS Align )

Helen

[MartinB]

Good to hear that you're up and running Steve. The wedge is plenty stiff enough for the for the NS8 despite what you might read. My main gripe about the wedge is the flimsy alt az adjuster but with a bit of care it works ok.

You'd obviously prepared yourself well. Drift aligning really is straight forward and with a bit of practice you can have a very good alignment. I normally do the goto alignment after the drift align since that will be more accurate although you clearly didn't have a problem.

What focal length are you working at Steve? If you are working at F10 2000mm you did very well to get 45 secs esp if you haven't trained the PE.

[beamish]

Now you see this is why I really enjoy being part of this forum.Clearly I don't own the type of scope Steve does but I found his write up and his links enormously helpful and interesting. I'm not at that stage ( doubt I will be for a while ) but forewarned is forearmed. Top piece Steve, loved it , good luck mate!! OOOOh that stella without me tee it's got me gushing!!

Karlo

[SteveL]

Thanks all. I`m glad someone found it useful. Martin, yes f10 2000mm. i`ll keep up the report and get feedback on WCS when I get a clear night at that, as well as a full report when I get the Off Axis Guider and PHD/GuideDog/K3CCD running. Then I can start pushing the exposure times up.

MartinB - i think i`ll keep this wedge, I`ll PM you soon about payment (nearer payday )

[SteveL]

And so, after what seemed like a lifetime, the skies cleared

And all the kit out and assembled early to let the OTA cool and to get polar alignment done in the twilight (finding the align stars by looking through my scope mounted telrad with binos... yes, that was uncomfortable)

Polar alignment is fairly painless now, if a little long winded. I only did a basic alignment due to the shortness of the dark hours, and all the things I needed to play with, but it seemed pretty much spot on. I still havent used the WCS software package to do alignment, will get around to doing that at some point.

Anyway, the night was primarily to get my Off Access Guider (OAG) installed and working. The idea is that your primary camera is on prime focus, and a small prism deflects a small part of light up at 90 degrees and into a second "guide" camera. This guide camera is being run by an application that can spot any movement and send corrective commands to the mount to move back to where it should be.

Nice and simple... well, it should have been, but there were a few issues, some I had discovered last week, some I found out in the dark (ooeer!!). Some may have looked at the above image and wondered why there is a huge spacer between the OAG and the DSLR body. The issue here is the flash unit on the Canon overhanging the front of the body, as well as the 1.25" guide eyepiece vertical tube sticking out over the end of the OAG. You cannot actually attach the Canon to the OAG because its a T-thread, and the vertical tube and the flash unit knock each other. Hence the gap. In this case its a T-Thread to 1.25" adapter which is a little long, but the only thing I had at the time (my T-Thread extenders arrived from Steve@FLO this morning).

So, that was the problem I found last week.... now on to the problem I discovered last night.

The process for the OAG is as follows:

• 
  [li]Point at object[/li]
  [li]Focus primary camera using the scope focuser[/li]
  [li]Slide the guide camera up/down the vertical tube until the dim image is also in focus, and lock it off there with a parfocal ring[/li]
  [li]move the guide scope adjuster to align on a decent guide star[/li]
  [li]run the guide application[/li]
  [li]start shooting[/li]
  

good... lets do this. I focus the Canon, then move the guide camera (Meade DSI in this case) up and down... and up... and down... and up slowly... and down more slowly. 10 minutes later I realise that because I have had to add an extension tub to the canon to allow it to connect to the back of the OAG, the focal point is now further away from the OAG body. No problem, I have some 1" long 1.25" extension tubes, I`ll use them. Well, as luck would have it, the focal point was JUST shorter than two of the extensions, but slightly more than 1 of them out as far as it would go. If I held the guide camera so it was only just in the vertical tube (above the thumb screws) I could see stars in focus.

Oh well, at least I know I can reach focus, and I just need to get some different length 1.25" tubes for emergencies. Off came the OAG, on went the T-Thread and Canon directly into the SCT thread, and I started imaging (jsut dont say anything about me cursing and swearing at DSLRFocus because it wouldnt do bulb mode.... Note to self: I really must check that the bulb cable is inserted ALL the way into the socket on the Canon).

More news as I go along. Now I`m off to priocess last nighst image sequences.

[ElijahB]

Excellent post Steve, keep 'em coming as I'm keeping a close eye on the progress your making. One main reason is that you started with a similar set up to me and therefore am hoping to pick up on any issues you have to try and avoid them myself (when I have the money to buy the other equipment that is).

[Kaptain Klevtsov]

Great report Steve.

BTW don't you just turn the camera to avoid the impact?

Kaptain Klevtsov

[Nessy]

Hi Steve - Great post, only just managed to read it.

REALLY useful and VERY easy reading. Thanks and keep em coming cos I will be doing this soon!

Of course, that means questions coming your way!!!

[SteveL]

REALLY useful and VERY easy reading. Thanks and keep em coming cos I will be doing this soon!

Of course, that means questions coming your way!!!

Questions I can deal with, its the answers that might be tricky

[barkis]

It's a credit to the professionalism shown on this forum, that guys like Steve, whilst coming to grips with a new setup, can pause and write meaningful accounts of what is happening on site. It is like he has a huge audience hanging on every word.

Of course he will get everything to work perfectly, and then we will all reap the reward of the images that will ensue.

Great piece of instructional reporting Steve. And your patience is commendable.

Ron.

[Nessy]

OK - this is going to seem like a stupid question but which bit do you align to Polaris?

Is it with the OTA on the Wedge, pointing straight up at Polaris, or is it an angle on the wedge?

Also - those flight cases are far too tidy!

[SteveL]

OK, wedge alignment requires the NS8GPS to do a wedge align procedure (under the Utilities menu) to start with, but I cant work out how to do that without doing a EQ North Align first. Then I g back to the "wedge align" bit.... rotate the OTA so the two markers on the OTA and the fork arm line up, undo Az clutch and rotate the OTA so its pointing south (both of these items are descibed on the hand controller). It will then rotate north and point the OTA straight "up" (in an EQ sense of the word "up"). AT that point, I then adjust the *wedge* to make the scope point at Polarisso that it is centered in the eyepiece. Thats the important bit. The scope doesnt actually know where its pointing rigt now.... it thinks its pointing at celestial north pole, so you must not move the OTA... you MUST move the wedge to make the OTA move.

See how the scope looks in that image with the neat flight cases . That is just after I had done the wedge align and adjust, but waiting for the skies to darken a bit more before I start any align/drift stuff.

[Nessy]

ok - thanks, this is becoming clearer in my mind now!!!

[SteveL]

Think of it as trying to align the imaginary vertical line that runs up from the base of the NS8GPS and through the forks (which in AltAz oints at the Zenith), and pointing it at Polaris instead by tilting the whole unit over. That way, the base is aligned with the ecliptic and the sky no longer "rotates" in the eyepiece in long exposures.

Another way to think of it: If you were standing up in the garden watching the moon come up in the east and travel across the sky and go down in the west, the moon would have rotated slowly clockwise as you watched it, and followed an arc as it travelled.

Now imagine doing the same thing but sitting in a deckchair, where the back of the deckchair was pointing at Polaris. As the moon travelled across the sky, you only have to move your head left to right. The moon appears to travel in a straight line when you sit in the polar aligned chair.

(Maybe that didnt help much... not sure)

[Nessy]

It helps! Now if someone can invent that deckchair!!!!!

[SteveL]

OK, here is something for you NS8GPS users to remember. In AltAz mode, the mount does *NOT* support the PulseGuide command that PHD uses (and maybe requires). Unsure if other guiding apps use the same ASCOM Command, but I would imagine they probably do. When I put the mount into EQ North, PHD reports no problems.

[SteveL]

OK, two clear nights in a row... it must be summer! (I am so tired...)

Last night's job was to get autoguiding working. First off, I had to get my DSI-C on the OAG parfocal with the Canon on the prime focus (but also on the OAG). Finally found that focus point, and locked a parfocal ring on the extension tubes to make sue I can just drop it in and go. Its also near enough the right length to use on my 40mm EP so I can visually find a suitable guidestar, then swap to the DSI-C.

Fired up PHD, configured the camera (DSI-C), configred the scope connection (ASCOM), clicked on the guide star on screen and hit" go". On its first run, PHD does a few calibration moves to work out how the scope/mount behaves. It probably took a minute or so I guess. It took into account my backlash amounts, DSI-C orientation, and was done. I had it updating the guide "nudges" every 5 seconds to start with, but with my dodgy polar align and untrained PEC, this was too slow to stop it wandering. I dropped the nudge update down to 1 second, and took a 1 minute sub... no visible movement in the image. 2 minutes... no visible movement in the image. 3 minutes... still no movement. Good enough for me

Swung the scope round to M51, something I`ve been after for a while now. Found a guide star, set up PHD and set it guiding. Configured DSLRFocus to take 10 frames of 3 minutes and left it to do its job. I kept an eye on PHD to ensure it kept its lock on the guide star, but I neednt have worried. I was so intent on PHD that I didnt really look at the images the DSLR was taking until this morning.

OTA: Celestron NexStar 8 GPS + Wedge

Images: Canon 350D Unmodded, DSLR Focus, 10 x 3min @ ISO1600, 5 x darks

Guiding: Off Axis Guider, PHD + Meade DSI-C

Processing: DeepSkyStacker, Photoshop CS

Please note: As I had reinstalled DSLRFocus in the last few days, dumbass here didnt notice that I managed to take the sequence as normal/small jpegs, and not large/raw... doh! So not only are the originals small, they are also all 8 bit data... double doh!

So, finally, I`m getting close to where I wanted to be. Its taken a lot of time, a lot of reading and learning, and a fair amount of money, but when I see the images pop out of DSS or Registax, it makes it all worth while.

[ElijahB]

You're progressing at a great rate of knots Steve, I'll be travelling in your wake (if a little slower due to a cash trickle rather than a cash flow). Great image

How long does it take you to get set up now you have all that gear to sort out?

[MikeP]

Beautiful.

Mike

[SteveL]

How long does it take you to get set up now you ahve all that gear to sort out?

Not sure... i`ll have to time it next time. I`m getting faster at it, thats for sure.

[Kaptain Klevtsov]

That setup is working really well Steve, the results speak for themselves. Bet you can't wait for more dark now.

Kaptain Klevtsov

[moondog]

You're really motoring now Steve. You seem to have cracked auto guiding - excellent image of M51.

Look forward to some more images from you !

8)

MD