Problems with current Setup

[w00h]

Hello everyone,

I started out Astrophotography with a buddy a few weeks ago and ran into some problems, which I will try to describe to my very best of abilities. Hours of searching the internet didn’t yield any useful results, so maybe you can help us out.

 

Our setup is: Skywatcher EQ3 (with sturdy legs) with Synscan GoTo, Skywatcher Maksutov 150/1800 and a Canon 50D (unmodified).

 

In the first session, we wanted to try to capture M13, which in hindsight yielded quite positive results. We did careful polar alignment with the Lüthen-Kahlhofer method and a three-star alignment. Exposure was 60 seconds for each of the light frames at ISO 3200. About 50% of the shots didn’t make the cut into the final image and they all showed trailed stars corresponding to movement of the RA axis. The other 50% were okay-ish. I figured it was maybe a balancing problem (backlash) and we decided to balance the mount a little bit east-heavy next time.

 

A few weeks later, we tried the same setup, checked the polar alignment and again did a three-star alignment. This time we went for double the exposure time and half the ISO, just to have it a go. The resulting images were not at all impressive. For analysis reasons I did a timelapse of the 2 hours of light frames and observed the object jittering around periodically (both axis) and slowly moving out of frame.

 

Now there are some open questions:

1) What is the probable reason the jittering movement was caused in the first place? The rather slow periodic movement for me rules out wind, ground vibrations and cable tug.

2) Is the object expected to be drifting out of the frame in the course of one night? (3-4 hours later M13 wouldn’t have been in the frame anymore)

3) What exposure times should be possible with the setup with your experience?

4) We did the 3-star alignment with live view and placed the stars at the exact center of the sensor with APT. Could it be that the center of the sensor was not exactly in the center of the optical train and thus the Alignment gets thrown off?

 

I hope you have some suggestions for us, any help would be very much appreciated

[newbie alert]

1800mm fl scope will highlight any imperfections in the setup.. Polar alignment maybe worth a look at  ..star alignment less so, as that's for pointing accuracy..

A friend had awful probs with his eq5..he was getting drift in 10 sec subs..so he stripped it down,relubed.. changed the bearings etc and as far as I'm aware it's far far better..

 

[w00h]

Thank you, we already have in mind to re-grease the gears in the mount since the mount sat a while unused in the closet. 
We do the polar alignment detailed in this video (german): 

 which should be comparable to drift alignment, according to the author. 
Maybe it has been thrown off polar alignment at some point in the process.

[newbie alert]

Drift alignment is highly accurate..but sorry I can't understand German ..you say it's similar..how does it differ?

[w00h]

It's quite different but deemed quite accurate (the authors state 1 arcminute accuracy). You mount a DSLR with a prime (e.g. 50mm) to the mount and take an image of star trails while moving the mount along the RA axis. You then measure the X and Y coordinates of the start and end points of three named stars in the JPEG and put them in a spreadsheet, along with their RA and Dec for the current observing location and the coordinates of the observing location itself. After that you see in the spreadsheet how far off you are with your polar alignment in both axes.

For us starting out it's nice to be able to quantify the accuracy of the polar alignment and to have a quick polar alignment process. But maybe we should have a look at drift alignment, too.

Here's a document in english detailing the process: https://www.sternwarte-nms.de/ext-links/downloads/

[PeterCPC]

I think that you are asking a lot of an EQ3 mount with that scope on it. I would try to get your hands on a small refractor like an ED80 and try that.

Peter

[w00h]

4 minutes ago, PeterCPC said:

I think that you are asking a lot of an EQ3 mount with that scope on it. I would try to get your hands on a small refractor like an ED80 and try that.

Peter

That was one of our thoughts, too. Next time we want to just use my 200mm f/2.8 lens on the DSLR instead of the scope and go for larger objects. Thank you for your reply!

[steppenwolf]

I agree with Peter, such a long focal length and relatively heavy telescope on the EQ3 really is asking a lot. Your 200mm camera lens is at the opposite end of the scale but should yield some great Milky Way images. 

Even though you are going to use a much shorter focal length for your next experiment, still carry out your careful polar alignment.

Good luck!

[MarsG76]

If you're just starting out than 1800mm is quite a high reach... hard but possible.... by your description it looks like a combination of drift due to slight polar misalignment/periodic error combo.

Are you autoguiding?

1800mm will show any setup imperfections very quickly... and for such a long focal length autoguiding is a must, especially on a light weigh mount like the EQ3.

 

[w00h]

19 minutes ago, MarsG76 said:

If you're just starting out than 1800mm is quite a high reach... hard but possible.... by your description it looks like a combination of drift due to slight polar misalignment/periodic error combo.

Are you autoguiding?

1800mm will show any setup imperfections very quickly... and for such a long focal length autoguiding is a must, especially on a light weigh mount like the EQ3.

 

No, so far no autoguiding, just the GoTo and have it running.
From what I gather so far, it's probably best to go for a shorter focal length first, get really to grips with the setup, then at some point swap out the mount and work our way up to 1800 mm.
Maybe we have to be proud of our first attempt to capture M13, which seems to be already on the verge of what's possible with our equipment and without autoguiding. 

[MarsG76]

1 hour ago, w00h said:

No, so far no autoguiding, just the GoTo and have it running.
From what I gather so far, it's probably best to go for a shorter focal length first, get really to grips with the setup, then at some point swap out the mount and work our way up to 1800 mm.
Maybe we have to be proud of our first attempt to capture M13, which seems to be already on the verge of what's possible with our equipment and without autoguiding. 

Thats very good for a first image.. VERY GOOD... the fact that it's 1800mm FL makes it even better.

That said yes, shorter FL will help you get more comfortabel with the gear andprocedure and without autoguiding getting longer subs than what you already have will be hard, most subs will be throw aways.

Autoguiding is a definite must... unless you're ready to spend $10,000 on a mount, than it will be quite accurate for unguided imaging.... but autoguiding will enable your subs to be virtually unlimited if properly polar aligned.