Guiding Scope

[Ady Alex]

Hi.

I have a 200/1200 goto dob .A Nikon D60 with Pixel size 37.2 µm² and a Microsoft HD-3000 webcam with Pixel size: 3 x 3 µm (i think).
Now I'm looking for a cheap telescope to make it a guiding scope , but i don,t know what to get. I have found a site that calculates the focal length of a guiding scope : http://www.geminitelescope.com/tools/Minimal-Guide-scope-focal-length.html

I don't understand how it calculates or if it is correct.
For me:
Main telescope focal lenght: 1200
Guider CCD pixel size: 3
Main CCD pixel size: 37.2

Result : Minimal Guide scope focal length (mm): 9.67741935483871

By this calculation I can buy any cheap refactor and make it a guiding scope?

Can some wane enlighten me ?

[Cornelius Varley]

5 minutes ago, Ady Alex said:

Hi.

 

I have a 200/1200 goto dob .A Nikon D60 with Pixel size 37.2 µm² and a Microsoft HD-3000 webcam with Pixel size: 3 x 3 µm (i think).

Now I'm looking for a cheap telescope to make it a guiding scope , but i don,t know what to get. I have found a site that calculates the focal length of a guiding scope : http://www.geminitelescope.com/tools/Minimal-Guide-scope-focal-length.html

 

I don't understand how it calculates or if it is correct.

For me:

Main telescope focal lenght: 1200

Guider CCD pixel size: 3

Main CCD pixel size: 37.2

 

Result : Minimal Guide scope focal length (mm): 9.67741935483871

 

By this calculation I can buy any cheap refactor and make it a guiding scope?

 

Can some wane enlighten me ?

Hello and welcome to SGL. Your calculations are incorrect, the pixel size of the main camera is about 6.1microns so the minimum guide scope focal length should be 58mm. The other problem is that you cannot successfully guide a dobsonian mounted telescope, if at all.

[Ady Alex]

thanks for the answer. But at least it will improve it, if i fit my dob with a guiding scope?

[Cornelius Varley]

Dobsonian mounted telescopes are not suitable for DSO imaging and you might not be able to connect an autoguiding interface to the mount.

[steppenwolf]

Quote

But at least it will improve it, if i fit my dob with a guiding scope?

Your experiment will indeed prove it but it won't improve it! Peter above has already given you the correct advice to save you the expense of buying a guide telescope (any guide telescope) that won't achieve what you are aiming for. However, If you enjoy experimentation then have fun, I am all for trying things out for oneself as it is a great way to learn.

The reason that a Dobsonian mount will not work for long exposure deep sky imaging is that your images will suffer from field rotation. Consider the two images below; the first shown what happens with the camera on an Altazimuth mount like a Dobsonian over the period of an imaging session - note that the star images 'rotate' in the field of view. The second images shows the same scenario with an Equatorial mount but here, the stars remain in the same orientation in the field of view throughout the imaging session.

Dobsonian Mount (Altazimuth Mount)

Equatorial Mount

[uhb1966]

Steppenwolf said it all. However, you could still experiment with , maybe, 10 - 30 sec exposures software like DSS would then adjust the single omages correctly. Thers probably  a website to compute the max possible exposure given an acceptable pixel error x.

The following is a finder scope which has several limitations but might be ok for experimenting

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p5295_50mm-MINI-Leitrohr-mit-justierbarer-Halterung---Astrofoto.html

 

[Ady Alex]

A picture is worth a thousand words.Thanks

I have see very beautiful DSO images taken with Dobs on www.astrobin.com

Example: 

ttps://www.astrobin.com/83443/

https://www.astrobin.com/136815/
https://www.astrobin.com/55992/B/

and others

 

[Cornelius Varley]

None of the photographs you linked to used a guided mount. The first image was taken on a dobsonian telescope mounted on an unguided equatorial platform, which will help to reduce the field rotation, but will only allowing for about 1 hour tracking before the mount needs to be reset. The other two would have been taken on unguided mounts using many exposures of 30 seconds maximum to keep field rotation to a minimum.

[steppenwolf]

OK, let's put this into perspective - yes, of course, it is physically possible to capture images of bright deep sky objects if you keep the exposures to 30 seconds or less (to hide the field rotation) but the key here is the exposure length. The majority of deep sky objects require long exposures to capture the detail and this is where the Dobsonian falls short. 

If you are happy with the image quality obtained by using short exposures on bright deep sky objects then that is fine and I have no doubt that you will have lots of fun doing so. I wish you luck and look forward to seeing your images posted on SGL, perhaps with two versions, guided and unguided - I'm always happy to learn from other people's experiences!

[Ady Alex]

Thank you.

I have found many AZ mounts good pics  and they all have a common exposure time under 30 seconds .

For the near future i will purchase a telescope for DSO astrofoto  that i can carry with me when i go to vacations in the mountains. Can i get something decent for 1000 Euro? Can you give me an example please?

[Ady Alex]

I have found  Alt-Azimuth Mount Tracking Movement and Field Rotation http://daltonskygazer.com/alt-azimuth-mount-tracking-movement-and-field-rotation/

Is the formula correct?

T = 30Cos(A)/[Cos(L)Cos(Z)]  where

T = maximum exposure time

A = the altitude angle of the object (0 degrees horizon, 90 degrees at the zenith)

Z = the azimuth angle of the object (0 degrees is due North, 90 degrees due East)

L = the latitude of the telescope/camera location

 

 

[uhb1966]

To be honest, i cant falsify the formula, but its based on the assumption that 0,125 degrees are an acceptable error. According to the tables on the website, for an observer at 40 deg latitude and an object at 50 deg elevation, you will have a max exposure time, based on the objects az angle, between 26 and (almost) 300 seconds. So 30 secs seems a good rule of thumb.

[ollypenrice]

M42 and the core of M31 are about as bright as deep sky objects ever get. The Network Nebula is not so bright but it is still an easy DS target. Don't extrapolate from these images that many DS targets can be captured in very short subs. But as Steve says, have a go!

Olly

[cjdawson]

Actually, it "IS Possible" to take DSO images with a dob.

With an Alt-Az mount, you have image rotation, as exaplained above.

There are two ways to work around that problem.

1. Check out this thread for changing the dob over to an Alt-Az mount.  (Strictly speaking it's no longer a Dob)

2. You'll need to derotate the image.

https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=field+derotator+dobsonian

I know that Meade came out with a product for the LX-200 that did just this, it was very expensive and the results were always a compromise.  It also complicated matters as it is yet another drive that needs to be calibrated.

 

 

Personally, I'd stick to very short exposures.  So guiding would not even be an issue anyway.

The engineering problems alone really warrant getting a completely different mount rather than a Dob for imaging.

Think of it as trying to crack a soft boiled egg with a sledge hammer.  You can do it, but it would be better to use a spoon instead ;-)

 

[alexbb]

Hi,

First of all, welcome to SGL!

Second, I've never heard of a GoTo dob to be able to guide itself and I really doubt that there is a stock one of that kind.
Also, there's an older topic about this:

Moreover, I doubt that the drives and gears are accurate as the ones of a more suitable EQ mount. Putting the D60 on a 1200mm focal length would result in a 1.05" per pixel which is pretty tight.

I'd say to take your time and do some more research before you spend your money. Most probably you'll end up with buying other equipment.

 

[alexbb]

As for your second question in this topic, maybe you could find something used in this forum: http://www.astronomy.ro/forum/Vinde-(privat)-f10.html
Usually people take care of their gear and sometimes they also modify it to a better version.

As a minimum reliable mount for astrophotography in terms of accuracy and payload, you'd need at least a HEQ5 or, a newer version, the AZ-EQ5 . This is also one of the lightest mounts that you could carry with you. An EQ6 it's heavier and an AZ-EQ6 even heavier. Others are much more expensive.
A very good scope for the money would be the SW 130P-DS and you would need a coma corrector too for taking pictures.
The refractor way, the SW 80ED is one of the cheapest that delivers. You'd also need a field flattner for this.
Also, you could find some long or long-ish FL old lenses that are cheap and could be used directly with your camera, though, for Nikon, they are not easy to find since they don't usually focus at infinite and you have to modify them.

Then you could start thinking at guiding.

Regards,

Alex