Polar alignment and exposure times

[HuN73R666]

Hi all,

I am starting to question whether I am performing a good enough polar alignment of my EQ5 mount. 

The last couple of sessions, I have struggled to take exposures anywhere over 1 minute without getting trails. However, during one session a month or so ago, I was pulling off 2 minute exposure without trails. 

I have no guide scope on the setup, so I'm purely relying on good alignment and equatorial tracking.

My question is, what sort of exposure length should I be able to achieve with this mount, subject to good alignment? I presume that the "zoom" level is a big factor, but I am only using my 1000m (f5) scope and DSLR in prime focus, so the "zoom" is not excessive.

If 1 minute is all I should expect, then I guess I need to start looking at guiding to achieve longer exposure, but I don't want to fork out cash if there are better results to be had with my current setup.

Thanks in advance for any input. 

ATB & Clear Skies,

Paul

[Alien 13]

I can normally get between 90s and 2 mins with a focal length of around 350mm so I think 1 min subs at 1000mm sounds about right.

Alan

[HuN73R666]

Hi Alan,

Many thanks for your reply. Although, I was hoping that maybe I was doing something wrong and it was possible for me to get better results. I must have just been lucky that night I was getting longer exposures.

Can anyone recommend a guiding system which is not going to break the bank?

Paul 

[Tinker1947]

Adding guiding doesn't have to be expensive , a Finder Scope and maybe a webcam should work, ccd's will be better for finding guide stars that are not to bright.....there is a adapter for Finder Scopes that allow in my case a lodestar to connect.....

Adapter Link... http://www.modernastronomy.com/accessories.html#accAdapters

Looks like this....

[Physicist13]

Theres a paper by R.N.Hook, 1989 from BAA that goies into this. Its for film photography but easy to adapt for digital.

P

[HuN73R666]

Thanks for the input guys!

Tinker, am I right to presume that there are no limits to exposure length with a guiding system? Can you recommend a suitable camera which will be sensitive enough to guide whilst viewing nebula. I say that because I am assuming that there is not always a real bright star available when viewing nebula. 

Also, does it matter if you have a cross hair on the spotting scope if you are going to use it for guiding.

ATB

Paul

[Tinker1947]

My guiding set-up now consists of a ST-80 and a Lodestar (Pulse guided with EQMod) these are aligned precisely with the DSLR sensor so once the focus is set i look at the PHD screen to align, i would expect to find a guide star at what ever i choose to image, exposure length will depend on your LP and DSLR/CCD used, somebody a lot wiser than me can comment on this.....

Guide Rings allow for precise alignment, flexture will be mentioned but i don't notice any....

[wxsatuser]

With a DSLR it's a good idea to limit the exposure to somewhere around 25 to 40% of the in camera histogram.

Last night with a Canon 60Da and a Sigma 105mm lens set for f4 and iso 1600 I was doing 300sec subs for a 25% histogram.

This will vary on how bright your sky is, sky glow and LP normally limit the exposure.

You may have to experiment but I used to do 10 and 15minute subs with the 60Da and my 80ED.

An f5 scope will most likey do 10minute subs, just check the histogram that your no more than 40%.

[Knight of Clear Skies]

One thing to bear in mind is you'll get longer exposure times the closer you are pointing to the poles.

[HuN73R666]

wxsatuser - Can you explain further about 20-40% of the histogram? I don't understand what you mean.

[HuN73R666]

Knight, yes that is a good point. Sadly I have had no view of anything decent near polaris recently. I am quite restricted in my view from my garden.

[Nikonshooter]

With a DSLR it's a good idea to limit the exposure to somewhere around 25 to 40% of the in camera histogram.

Last night with a Canon 60Da and a Sigma 105mm lens set for f4 and iso 1600 I was doing 300sec subs for a 25% histogram.

This will vary on how bright your sky is, sky glow and LP normally limit the exposure.

You may have to experiment but I used to do 10 and 15minute subs with the 60Da and my 80ED.

An f5 scope will most likey do 10minute subs, just check the histogram that your no more than 40%.

What do you mean by 25 to 40 percent of the camera's histogram?   As long as all of the data is not clipped - and by shooting raw - then your exposure should be manageable in post....am I missing something.

[themos]

Hello,

if you are curious how good your polar alignment is, you might want to try out the utility I'm testing over at this thread.

[HuN73R666]

Thanks for the link. I'll check it out now.

Just a question so I can get it straight in my head. If it were possible for me to get a perfect polar alignment with my Equatorial mount, then should I not, in principle be able to perform longer exposures than 1 minute? Surely if everything is aligned correctly, the mount should be capable of tracking better than this

[themos]

In principle, yes. But the problem with budget mounts is that the gears are not perfect so when a wonky gear tooth makes contact too early, the tracking speeds up a bit and then drops off. This is known as periodic error because it repeats with every revolution of the gear. For some mounts you can train the electronics so that they know where the bad teeth are and adjusts the speed of the motor to compensate. That is known as PEC, periodic error correction. 

A quick way of finding out if your polar alignment is out is to check which way the star trails are aligned. If east-west, it's more to do with periodic error. If it's north-south, it's more the polar alignment. But I've seen that quick "rule" violated, in practice.

[Alien 13]

Themos is correct, you can push the unguided subs but what will happen is the ratio of bad to good ones will increase if I run my mount for 3-4 minutes at 400mm FL then perhaps 1 in 4 subs will have good star shapes 2 will show tracking errors and one will have very bad trails over the entire image.

It is worth trying to push the limits though good balance with a very slight east heavy bias and a good regulated power supply to the mount can sometimes help.

Alan

[Thalestris24]

I think this is relevant:

http://canburytech.net/DriftAlign/index.html

Handy also for the mathematically inclined

Louise

[wxsatuser]

wxsatuser - Can you explain further about 20-40% of the histogram? I don't understand what you mean.

This is the histogram on my 60Da.

I'm presuming you know how to display it on your camera?

The white one at the bottom is the luminance histogram, this is the one you want to use.

See that big peak, like a mountain, try to get it somewhere between those first two lines.

Each line on a Canon is 20%, so you can expose to get that mountain as far as the second line, at

a pinch you could go to 50%.

[wxsatuser]

What do you mean by 25 to 40 percent of the camera's histogram?   As long as all of the data is not clipped - and by shooting raw - then your exposure should be manageable in post....am I missing something.

See my post above.

Not shown on that histogram is a long thin line of pixels to the right of the 'mountain' that stretchs nearly all the way to the far right.

If we go much more than 40 to 50% with the mountain a lot of those pixels will get clipped.

You can of course experiment with your camera to see how far you can push it.

For anyone starting out this exposure area will give good results.

It's up to the individuals choice but I almost always use ISO 1600 with the histogram around 25%.

I do the same wether I'm imaging with the Borg 67mm f3.8, ED 80 or any of my camera lenses.

BTW

I have moderate LP and always use an LP filter, Hutech IDAS P2 or an Astronomik clip.

[HuN73R666]

On the guiding topic. Do you guys think that this kit will do the job for me?

http://www.amazon.co.uk/gp/product/B0069W17R8/ref=pd_lpo_sbs_dp_ss_2?pf_rd_p=556244467&pf_rd_s=lpo-top-stripe&pf_rd_t=201&pf_rd_i=B001EPND18&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_r=12NX91XKRG1DZHKEV8RJ

Failing that, there seems to be a beefier option. 

http://www.amazon.co.uk/Orion-Awesome-AutoGuider-Refractor-Telescope/dp/B001IWXBS2/ref=pd_sim_sbs_ph_1?ie=UTF8&refRID=0CHVFGYK010QYTK67QZ5

[HuN73R666]

You guys (and girls) which guide, how do you get on with having your laptop outside when dew starts to form? Is there guiding software which could be run on a raspberry pi and a small screen? Thinking maybe I could build it in to a flight case or something.

Paul

[themos]

A bit overpriced, you can do the same with an adapted finderscope and a long-exposure enabled webcam for around £100.

[themos]

You guys (and girls) which guide, how do you get on with having your laptop outside when dew starts to form? Is there guiding software which could be run on a raspberry pi and a small screen? Thinking maybe I could build it in to a flight case or something.

Paul

Let it form.

Some people are trying to use the raspberry pi   http://www.raspberrypi.org/forums/viewtopic.php?f=41&t=15723

[HuN73R666]

A bit overpriced, you can do the same with an adapted finderscope and a long-exposure enabled webcam for around £100.

Can you provide me with any links themos? Sorry to be a pain, but im worried if I try and piece something together myself, ill end up with a pile of junk. 

[themos]

http://www.modernastronomy.com/camerasGuider.html sells finders and adapters, and you can modify a webcam yourself (google "lx webcam")