Max exposure before star trails form

[SteveBz]

Hi Guys,

I've been trying to take some snaps with a Canon D350 on ISO 1600 through my tracked C8N.  It has some back-focus issues which are resolved by a x2 Barlow lens on the front of the adaptor.

The first time I tried, my polar alignment was shockingly off and it's not always easy to get focus, but even after I've really fixed up the polar alignment to the exact spot on the inner polar-scope circle with Cassiopeia in the right direction.  And although there is no live view on the D350, I can tighten up the focus practising on a bright star like Vega.  Even then, when I do a 30s exposure of a DSO (eg M31) the stars appear "bubble-like" I guess owning to vibration or flickering seeing, and there is still the suggestion of a trail.

If I back up to 15s exposures, M31 is much less visible.

Have I really got to get a guide-scope?  There seem to be a lot of beautiful pictures on this site (eg the No EQ challenge), with much less sophisticated equipment?

What do people suggest.

Regards

Steve.

[spillage]

Could you post up some images so we can see. Also you should not need to use a barlow on this set up. 

 

spill.

[MARS1960]

If your polar align is good then i cant see why this is happening, i had a CG-5 and could happily get 2min exposures without star trails.

Maybe it's because you are using a barlow but being new myself i could be talking rubbish.

I'm sure someone more experienced will help.

Good luck.

[SteveBz]

Hi Mark,

Thanks for this.  Do both your cameras do live view?  If so do you just use live view on the back screen of the camera for focus, or do you use the Bahtinov as well.

Regards

Steve

[SteveBz]

6 minutes ago, spillage said:

 

Could you post up some images so we can see. Also you should not need to use a barlow on this set up. 

 

Here are some unprocessed images of M31 from last night.

Regarding the Barlow, I wasn't expecting to have to use it.  Elsewhere on the site Gina is talking about using a DSLR and she suggests that the C8N will be OK, but that whatever she was using needed a Barlow.  I started out without one, but ran out of focus depth when my focuser hit the end stop.  So then I used the Barlow and it worked.

Thanks,

Steve

[spillage]

As both my cameras have live view I use the bahtinov mask and use apt to get it the best I can. You can just use the mask and take short 5 second exposures to try to manually get it as close as possible. In my c6n I just screwed the t2 to the adaptor and then screwed this directly into the focus tube using the adjuster ring to lock it into the position I wanted....if this makes sense.

[SteveBz]

2 minutes ago, spillage said:

As both my cameras have live view I use the bahtinov mask and use apt to get it the best I can. You can just use the mask and take short 5 second exposures to try to manually get it as close as possible. In my c6n I just screwed the t2 to the adaptor and then screwed this directly into the focus tube using the adjuster ring to lock it into the position I wanted....if this makes sense.

Not sure what you mean by "adjuster ring", I also screw the T2 to the adaptor and then insert it in the focuser holding it in place with the thumbscrews.

Steve.

[SteveBz]

Sorry, I should have said, I only use the nose piece of the Barlow like a filter on the adaptor so it all slides into the focuser.

[spillage]

I am not way an expert at all in this but it looks like your pa is out as all the stars are drifting in one direction and focus is out a bit.

[SteveBz]

Just now, spillage said:

I am not way an expert at all in this but it looks like your pa is out as all the stars are drifting in one direction and focus is out a bit.

Ok, if you think that's all it is, I just need to spend a bit more time and effort on those.

Sunday is looking like my next clear date, I'll try again then.

Thanks.

Steve

[spillage]

I guess the c6n and c8n are not the same.

 

[Cornelius Varley]

Does your C8N have a 2" focuser or 1.25" ? If it is 2" then you should not need a barlow, just a t-ring to attach to the focuser like this.

[SteveBz]

6 minutes ago, Cornelius Varley said:

Does your C8N have a 2" focuser or 1.25" ? If it is 2" then you should not need a barlow, just a t-ring to attach to the focuser like this.

Hi Peter,

Well you are exactly right.  It has both the 2 inch and 1.25, but it seems that if I unscrew the 1.25 barrel it exposes a c-ring on the focuser which does indeed screw onto the EOS t-ring!!!

So, next time out there I'll be:

1. Ditch the Barlow and screw it on directly,
2. Make sure the polar alignment is spot on,
3. Spend more time on focusing.

Thanks a bunch guys, any more suggestions still welcome.

Regards

Steve.

[SteveBz]

12 minutes ago, spillage said:

I guess the c6n and c8n are not the same.

Well, they seem not to be identical, but they are more similar than I understood (see Peter's post above).

Thanks

Steve

[spillage]

If you attach as peter explained and then you use the ring shown in my photo (not attached on the right) to adjust the angle of you camera once locked into the t2. Its not a easy as on my 150pds with a standard 2" nose piece but once set up is fine.

[cjdawson]

Looking at the posts.   I'm getting the impression that when you said about putting effort into getting your PA right.   Would I be right in thinking that what you have done is tried to get Polaris to sit in the little circle on your polar scope?

Doing just that on it's one won't be enough.  In order for it to work, you need to rotate the polar scope to the correct hour angle.  (The hour angle is the hour in RA that is currently at the zenith)  That will tell you how to orientate the polar scope.

Once you do that, line up that little circle with polaris.

I'm guessing that this is what you have done so far.

However... this does not mean that your polar alignment is correct.    It's a rough alignment and should get you close enough for visual work,  however for AP I would suggest that you also perform a drift alignment.

For this you can google the DARV method.  Which means you can get a very accurate drift alignment using your camera to help take measurements, and allow you to make small adjustments to fine tune your setup.

[MARS1960]

57 minutes ago, SteveBz said:

Hi Mark,

Thanks for this.  Do both your cameras do live view?  If so do you just use live view on the back screen of the camera for focus, or do you use the Bahtinov as well.

Regards

Steve

I use live view on both Steve although i do own a bahtinov, i find using X10 live view on a bright star like vega makes focusing very easy.

[newbie alert]

Id say because your stars are drifting in the same direction that your polar alignment is out..not familiar with your setup but for dso try not to use the barlow..you are just increasing your focal length of your scope..

[SteveBz]

13 minutes ago, cjdawson said:

Looking at the posts.   I'm getting the impression that when you said about putting effort into getting your PA right.   Would I be right in thinking that what you have done is tried to get Polaris to sit in the little circle on your polar scope?

Doing just that on it's one won't be enough.  In order for it to work, you need to rotate the polar scope to the correct hour angle.  (The hour angle is the hour in RA that is currently at the zenith)  That will tell you how to orientate the polar scope.

Once you do that, line up that little circle with polaris.

I'm guessing that this is what you have done so far.

However... this does not mean that your polar alignment is correct.    It's a rough alignment and should get you close enough for visual work,  however for AP I would suggest that you also perform a drift alignment.

For this you can google the DARV method.  Which means you can get a very accurate drift alignment using your camera to help take measurements, and allow you to make small adjustments to fine tune your setup.

Hi CJ,

My Polar scope has pictures of Casseopeia and the Plough on it to help with hour alignment.  So I point the diagrams at the constellations and then pop Polaris in the mini circle that is in orbit around the centre of the lens.

However, I haven't done any drift alignment, I'll try this and, as you say, Google DARV.

Regarding focus.  Is it your view that Bahtinov is mandatory?  I don't have a mask and I'm doing it by trial and error.  It's not helped by the fact that it's so cold out there now (-4 last night) that the grease in my focuser appears to have frozen making it so stiff that the electric focusser wont work and I'm focussing manually.  What do you think?  Maybe I should also thin out the grease with some WD40 or something.

Regards

Steve.

[SteveBz]

11 minutes ago, MARS1960 said:

I use live view on both Steve although i do own a bahtinov, i find using X10 live view on a bright star like vega makes focusing very easy.

Hi Mark,

Yes I focussed on Vega yesterday, but not using live view, just trial and error.  When you say 10x live view is there a magnification setting on Live View or are you using a PC?

Regards

Steve.

[SteveBz]

10 minutes ago, newbie alert said:

Id say because your stars are drifting in the same direction that your polar alignment is out..not familiar with your setup but for dso try not to use the barlow..you are just increasing your focal length of your scope..

Hi NA,

As you can see from the photos above, M31 is quite small in the image.  Is there not an advantage to using a Barlow, that the image will be bigger?

Regards

Steve.

[cjdawson]

I wouldn't say that a Bahtinov mask is mandatory.  I would say that it will help you to get that focus spot on easier.

However, what I think will have more of an effect is a change that I have yet to try in the field. (Paying the cloud tax at the moment on new kit)

Instead of struggling with the screen on the back of your camera, use a laptop and Backyard EOS.  There's a great 2 hour presentation on it on Youtube.

This has several advantages.

1. much bigger screen to see the focus.

2. It has tools that use the liveview feed and can help find the sweet spot when you focus.

3. With your electric focusser, if you can get ASCOM working, means you don't have to touch the scope to get focussed properly.

4. If you do get a Bahtinov mask, it has a feature that can read the image and tell you how close your focus is based on that mask. (This is in addition to item 2 above)

 

I have yet to use this in the field myself however my early experiments show me that it will make your life loads easier.    (Of course this assumes that you have a laptop and the money for the licence)

 

[cjdawson]

8 minutes ago, SteveBz said:

Hi NA,

As you can see from the photos above, M31 is quite small in the image.  Is there not an advantage to using a Barlow, that the image will be bigger?

Regards

Steve.

Actually, using the barlow will be a disadvantage.  It will lower the F/ratio of your scope meaning that you need longer exposure times.  It will also magnify any errors, this will make your life a lot harder and frankly give you lots more frustration.

[newbie alert]

If you see it like this..if you look at a target with a 25mm eyepeice..then change it for a 12mm..yes you get a bigger image at the expense of clarity and brightness..dso are normally quite dim..m31 you can only see the core as its brighter..to bring out the backround you need longer exposures ..dont look at it you need to zoom in as you lose the clarity,and assuming its a x2 barlow you will double your focal length needing longer exposure times to bring that detail out..so a barlow isnt the way forward in my opinion

[SteveBz]

30 minutes ago, spillage said:

If you attach as peter explained and then you use the ring shown in my photo (not attached on the right) to adjust the angle of you camera once locked into the t2. Its not a easy as on my 150pds with a standard 2" nose piece but once set up is fine.

Hi Spill,

Well now I've identified what you and Peter are talking about I can't actually see a locking screw. I'll post some photos next.

Steve.