What FL for guide scope?

[troypiggo]

I'm using my 600mm ED80 for imaging with DSLR mounted on it. Was recommended this because it has a bigger aperture and I know it's a very popular and highly recommended scope for widefield imaging. So I have used my 900mm ED100 with SPC900NC webcam and PHD for guiding. I've only been able to attempt this setup a couple of outings due to weather.

I was having trouble keeping the guide star in the box on PHD initially. Did a much better alignment last time, and actually did get some decent exposures, but in PHD the star was constantly moving outside the guide "box" and the application was beeping and flashing while it tried to get it back on track.

I am sufficiently inexperienced to know if this is normal or not, but last night I was thinking that it could be that 900mm is too long a focal length and the stars are moving too much, even more than the imaging scope (600mm).

On one hand this could be a good thing. Means that PHD Guiding will be much more sensitive to movement and the resulting images in my DSLR should be better for it.

On the down side, not sure if it's due to the smaller aperture or the narrower field of view, but seems hard to actually find guide stars in the PHD window.

I was thinking that perhaps a wider guide scope with bigger aperture might show more potential guide stars. Something like this Orion (https://www.bintelshop.com.au/Product.aspx?ID=7877)ShortTube 80 Guide Scope I saw for AU$259 (https://www.bintelshop.com.au/Product.aspx?ID=7877).

What is your opinion? Should I get this guide scope, or keep using the ED100? If I get it, should I keep the ED100 or sell it to pay for the guide scope? I do intend to get something like a 9.25" reflector in the future.

[troypiggo]

Whoah, been reading some more about this:

http://www.wilmslowastro.com/tips/autoguiding.htm

Talking about ratios of focal length of imaging scope versus guide scope, pixels of imaging camera versus pixels of guide camera, sub-pixel capabilities of the guiding software, arcseconds (what?)... getting compli-ma-cated...

Summarising my equipment considering the above -

Imaging gear: ED80 600mm f/7.5 with Canon 30D 3504 x 2336 px image size.

Guiding gear: ED100 900mm f/9 with SPC900NC 640 x 460 pixels.

Proposed guide scope: 80mm f/5 400mm FL with above SPC900NC.

I have no idea how or what to calculate arcseconds.

[TrevorW]

"I have no idea how or what to calculate arcseconds"

Nor do I so I use what works, presently i'm using a 10 X 60 Stellarvue finder as my guidescope but have an option of using a 120 f/5 on side by side set up but easier setting up with the finder as a guider.

[Dennis]

Download Ron Wodaski’s free application “CCDCalculator (http://www.newastro.com/book_new/camera_app.php)” which you should find very useful.

For a telescope of Aperture “A” and Focal length “L”, used in conjunction with a sensor measuring “H” mm x “W” mm with a Pixel size of “X” microns x “Y” microns, it will display the “Image Scale” of how many pixels will cover an arc second as well as the “Field Of View” in arc mins.

Cheers

Dennis

[Dennis]

Here are a couple of on-line calculators:

http://www.howardedin.com/articles/fov.html
http://www.tawbaware.com/maxlyons/calc.htm
http://www.mat.uc.pt/~rps/photos/angles.html

Cheers

Dennis

[troypiggo]

Thanks guys.

[leon]

About 500mm and 60 to 80mm apature will be fine, just keep it simple, that rule applies to most things in life.

leon

[troypiggo]

:) Indeed it does. Thanks leon.

[RobF]

Still sounds like might be polar alignment though Troy. Something to eliminate anyway.

I guide with a cheap 2nd hand F10 refractor on my F5 reflector. Phd very reliable, except when:
- star too faint
- haven't forced a recalibration after moving to another object in different part of the sky
- mount or scope bumped

K3CCDtools includes a handy freeware tool for checking polar alignment. 1Ponders has promised a tutorial for us, but I'm sure myself or others here could provide info if you're interested in that.

[turbo_pascale]

My imaging setup (primarily at the moment) is imaging with the 80ED with the Canon 350D and guiding in the 10" LX200 at 2500mm focal length with the ImagingSource DBK31. The disparity between the two focal lengths is not a problem - it is just less tolerant of issues when guiding at the longer focal length. Clearly, if I can guide well at 2500mm, there is nothing at all noticeable wrong with the image in the 80ED. I've even had the guiding stop working for 10-20 seconds during an exposure, and even though the image has moved a bit in the guide cam, can't see anything wrong in the main image because of the difference in scale.

Firstly, make sure you're well balanced, as well polar aligned as you can be. Then play with the values for the guide movement amount. If your mount has PEC, you might want to try using it if you've eliminated the other issues. You need different settings for different focal lengths. The help suggests changing the value based on the focal length your guiding with - I just went with trial and error - if I did a "force calibration" and the settings were too much that the star image went off the screen, then I knocked it back a bit until I could get it to work.

When I have the rig setup the other way around (imaging at 2500 and guiding at 600) you've got to play with it a bit otherwise the star just wanders off. Generally I've had less success this way, but I just haven't tried as much so it could be just a matter of practice (using the 80ED gets most of the objects I'm interested in at the moment) - at 2500mm, there's not as much pretty stuff to take photos of without lots of work!

I have to say I make the guide box 50 pixels wide, to give it the best chance of not losing track, but then again, my guide cam is 1024x768 pixels, so there's a bit more room to move.

The biggest problems I had were when I was unbalanced. I had a 3D balance system put on (it's fork mount on a wedge), and the immediate improvement to my guiding was massive. It went from haphazzard and mostly unable to last through a 5 minute exposure, to quite capable of going 30-40 minutes without having to readjust PHD.

How do you have the 2 scopes mounted? Side by side plate or piggy backed somehow? Balance could well be an issue, but differential flexture between the two scopes if they are not well secured could explain the movement too.

Turbo

[RobF]

Yes - as to balance, most people tend to have success with slight overbalance to the East in RA - so the mount is pulling "uphill" slightly, not "bumping downhill". Get Dec as close as possible. I don't bother with PEC using PHD - mainly 'cause I haven't had time to play and set it up, but also because many have said not essential (or may even hinder).

[g__day]

I remember once reading guide at about no less than 2/3 of imaging focal length.

That simple equation misses at least two determining factors - differential flex and mirror movement (if you aren't using refractors).

What's worked best for me is an off axis guider and a high sensitivity mono guide cam. Since switching to these two components for my guiding - its been spot on (four imaging periods over several hours) for about three months now. It's rare I can't find guide star (I don't change the orientation of the OAG ever).

[mill]

Troy first of all you are mixing up the terms aparture and focal length.
A ED80 has a smaller aparture than a ED100.
Basically you should get a 0.8 focal reducer and use the ED100 for imaging and use the ED80 for guiding.
To get a wider field of view you can also image with the focal reducer thru the ED80 and guide with the ED100.

And the most important thing to do is, get your polar alignment right.

[troypiggo]

Thanks for all of your added comment, guys. I'm working on improving my drift alignment techniques. Balance I think I'm right on. It's all practise, I guess.

mill (Martin) - I don't think I'm confusing aperture. I realise the diameter of the ED100 lens is larger, but when I talk about aperture I'm referring to the f-stops like in photography terminology. The ED80 is f/7.5 and the ED100 is f/9. The lower number means larger aperture and light gathering ability. Are you saying this is wrong?

[Dennis]

Hi Troy

In astronomy, the accepted meaning and usage of the term “Aperture” is taken to mean the diameter of the objective, be it a lens or a mirror.

As you mention, in photography, aperture is the F1.8, F2, F2.8….F8, F11….etc setting of the iris in the lens, which in photography is variable due to the diaphragm or iris, but in astronomy this is (usually) a single value and fixed. That is, a 102mm F9 refractor, a 10inch F4.5 Dobsonian or an 8inch F10 SCT.

In astronomy, the way you vary the “diaphragm” or “speed” of an optical tube assembly is to use a focal reducer (e.g. x0.63 or x0.8) or a Barlow (e.g. x2, x3, etc).

So, the ED100 f/9 has a larger aperture (100mm) than the ED80 f/7.5 (80mm) although the F7.5 ED80 is a “faster” ‘scope, F7.5 versus F9.

Cheers

Dennis

[troypiggo]

Thanks Dennis for the great explanation and clarification. What you said in the last paragraph is what is important for photography - "faster" meaning it gathers more light quicker.

I'm still coming to terms with astrophotography - the barlows/focal reducers etc. Have been reading about them, just haven't gone down the path of getting any yet.

[RobF]

Paul (1Ponders) also educated me that the light gathering capacity is related to the square of the focal length. That is, if you had say an F6 80mm refractor, and an F9 80mm, the F9 would take about 81mins to gather what the F6 could do in 36mins.

[Dennis]

Hi Rob,

I think you probably meant to write “the square of the focal ratio” and not “the square of the focal length”?

Cheers

Dennis

[RobF]

Oops. Quite right Dennis!

:whistle: