Newbie with DSLR and unguided scope

[daniyyel]

Hi guys!

I recently bought a 450D and last night I tested it on M42. And the results were awful! Why I'm saying that ? Well ....if I set exposure over 1.6 sec, I start to see star trails... I know it's a normal thing for an unguided telescope, but after 1.6 sec ? I saw on this forum, some decent pictures of galaxies and other DSO nebulas with exposures of 30s and unguided telescope. How did they do it ? Any help would be really appreciated!

P.S I own a Sky-Watcher 150/750 and no, I didn't use a barlow along with my camera. Just the camera connected directly to the focuser with no extension tube/nose!

The photo is a single frame 1.6s, iso800 and already starting to see star trails.

[brrttpaul]

total beginner myself but that would be about right with a telescope wouldnt it? depending on the focal length, I have a 300mm lens which i divide by 600 equals 2 so 2 secs maximum with that, a 20 mm lens  would be divided by 600 equals 30 secs , as I said though I am a total beginner sure someone will come along and explain

[D4N]

What mount do you have?

Is it motorised?

Unless you have a motor for the RA axis and a good polar alignment you will always get star trails.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[ronin]

I suspect you have mixed the term "unguided" and "tracked".

With out tracking I would say that 1-2 seconds is likely to be the best with what amounts to a 750mm lens on the camera (your scope).

You need an equitorial mount with RA+Dec motors (preferably goto) and a polar scope, then carefully align the mount then take an image with the mount tracking the object, then you should get 30-60 seconds out of it, depending on the accuracy of the polar alignment.

Guiding has a second scope and camera attached that identifies an independant target (star) then calculates corrections to maintain that target star at a fixed position.

[Lonestar70]

Hi Daniyyel,

What mount are you using?   Is it an equitorial motorised mount?

There is a difference between a motorised mount which follows (TRACKS) the stars accross the sky and a Guided mount.

A  motorised mount can TRACK stars but due to inacuracies in the mount gearing etc it can still produce star trails at times, albeit these are generally only apparent after 30 - 60 seconds if the mount is properly polar aligned.

A guided mount has motors for TRACKING but also has a separate guide scope and camera attached... this guide camera is used in conjunction with some software (PHD) which issues additional command to the mount when the guide camera detects any drift (the main cause of star trails).

If your mount is  non-motorised then the 1.5 seconds you are getting is about right for the focal length.

If the mount is motorised then it is more than likely that you have not got the polar alignment close enough... or you forgot to switch the tracking on (an easy mistake most of us made when we started out).

If you let us have some further information on your mount and set-up then we can give you better advice.

Keep happy.

Best regards.

Sandy.  

[daniyyel]

Thanks for the explanation Sandy! Very thorough! I have it motorised but only on RA, I turned the tracking on , so I guess the only explanation is that it's not properly alligned on Polaris. Since I use my telescope inside, I allign it by using a compass to show me the North, where Polaris is. The mount ALT is locked on Polaris though (didn't touch those knobs since I alligned it on Polaris last time - a while ago)

[D4N]

Beware compass magnetic north isn't the same as true north.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[daniyyel]

Yes, that I know. But it works pretty well visually (not pinpoint accuracy) and the object is centered in the eyepiece... But it seems it's not good enough for photography...

[SkySurfer]

Looking at the image the RA axis has no trailing, where you have motor drive, it is the DEC that is the problem, so it must be very poorly aligned on the DEC axis.

In my opinion.....

SS

[Lonestar70]

Hi Daniyyel,

I am a little puzzled by your last post... you say you use your scope inside... does this mean that you have it in an observatory? or do you mean inside your house?... if inside your house then are you trying to take photos through a window?

How exactly are you doing your polar alignment?... a compass can only give a very roughly direction for North unless you know the correct magnetic deviation for your location... even then it could still be inaccurate since it can be effected by a lot of things around it including the mount.

Can you actually see Polaris? and does your mount have a polarscope fitted?

Do you leave the scope and mount set up all the time or do you dismantle it all and put it away?... if so, then you need to polar align every time you set it up.

I agree with Skysurfer in that the drift seems to be mostly in DECLINATION but that could be mainly due to poor polar alignment.

More information really required for a better answer to your problem.

Best regards.

Sandy.

[daniyyel]

I have it inside my house for now, since it's quite heavy, I'm skinny and it's very cold outside. I don't shoot photos through windows Like I said, I did a precise polar alignment outside (in October I think). After that I didn't touch the mount knobs (the alt/az ones). And given the fact that after this, my mount has Polaris alligned on ALT, I only need to position it on the AZ using a compass. But now I learned that gives me a very poor alignment for astrophotography. It's fairly good on visual - it keeps the object in the eyepiece for a few minutes.

If you guys know a better way to allign my mount to Polaris from inside my house with no view to the star let me know! I would appreciate it!

[glowingturnip]

If you guys know a better way to allign my mount to Polaris from inside my house with no view to the star let me know! I would appreciate it!

well if you're sure your alt bolts are correct, then you could drift align on a star you can see out of the window, and adjust the az bolts only until you get no more drift.

For adjusting the az bolts, you should be looking at a star that is on the meridian, and as close to the celestial equator as you can get.  I presume you've got a south-facing window (?) since you can see Orion so finding a suitable star should be easy enough.  Actually any star could work, but a star on the meridian and equator is most directly affected by the az bolts so gives you the quickest feedback.  Google the DARV alignment technique.

Are you sure your alt bolts are accurate though ?  Your living room floor is likely to be perfectly flat, but if where you did your last full polar align wasn't entirely flat, then your alt bolts won't be right for the living room either.

Edit:  I do think you're not really going to get very good results shooting from inside though.  Not only is this alignment going to be difficult to achieve well, but also your're going to have steep temperature gradients between inside and outside through the open window, which will give you localised 'seeing' problems, and also your target is going to move out of view too quickly.  I'm afriad that there's nothing for it but to put on a thicj coat, lug it down the stairs and get out there !

[Thalestris24]

Yeah, you can't polar align in one place and then move the mount expecting it to remain polar aligned - if only! For AP, the aim is to achieve PA to within less than a minute of arc which is tiny! Az should be done first - ideally on the meridian but you can get away with slightly south east or south west. Then check/adjust alt in the east or west. Then re-check az. There are a variety of software tools to make this procedure semi-automated. PHD2 is good for drift aligning. I actually use Astrotortilla since I use it to centre on targets anyway. It just gives the az and, separately, the alt error. Once you get the errors within your camera fov, you can enter the error values into a utility called polefocus which enables you to adjust alt/az on a synced star. Get polefocus1004.zip from http://www.scopefocus.info/polar-alignment (link at the bottom of that page). Also, the focus in the image you posted looks off. Download APT http://www.ideiki.com/astro/Download.aspx and use the inbuilt focus tool else get a Bahtinov mask 

Louise

ps I'm not sure whether you can download polefocus right now as I got an error when I just checked it. That may be temporary. Hopefully!