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Will I need an extention tube??

NIGHTBOY

By NIGHTBOY
in Getting Started With Imaging

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smudgeball

smudgeball

Posted
Posted

Hi

They mention the extension tube as you can sometimes struggle to achieve focus dependent on type of scope / focal length.

A common problem for some people with refractors is getting enough back focus for their dslr.

Even with a field flattener fitted in my imaging train, my focuser is still three quarters of the way out to obtain focus.

I guess they supply it just in case?

Looking at the photo, If you go for it, maybe check if the dslr adapter is metal or plastic?

Could be wrong, but looks a bit "plasticky" to me?

A good quality t-adapter will be around £20 itself. You want a good quality connection if you are hanging hundreds of pounds of camera off your scope.

Neil

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Twisted Lip

Twisted Lip

Posted
Posted

Worth looking around. I have a couple of barlows one of which is threaded and one isn't. Smudgeball is right too, you may have the same back focus issue with a Dob - I certainly do with the Orion (on the few occasions I've used it for imaging).

Will

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Lonestar70

Lonestar70

Posted
Posted

Hi Nightboy,

When you attach a DSLR camera directly to the focuser on a lot of Newtonian telescopes a problem often occurs where you cannot wind the focuser IN far enough to bring the camera to focus... This is caused by the problem known as BACK FOCUS.

When you take the lens off of the DSLR you will notice that the distance from the lens mounting face on the camera to the front face of the image sensor is about 45mm or so (it varies slightly from one make to another). This is the BACK FOCUS distance of the camera.

The problem is that the image has to come to focus on the image sensor... which means the focuser must be able to travel far enough INWARD to achieve this... most Newtonians don't (unless you have a PDS scope).

By using a Barlow lens, the focal point of the telescope is moved out from the focuser further which is often enough to allow the camera to come to focus within the available INWARD travel of the focuser.

The extension tube mentioned in the link you posted refers to refractor type telescopes only... with this type of telescope a 45deg or 90deg diagonal mirror unit is used between the telescope focuser and the eyepiece... it makes viewing easier since you can look down into the eyepiece rather than having to get low down to look through a horizontal eyepiece.

If you take the diagonal off of this type of telescope and fit a camera directly onto the focuser draw tube you will not be able to focus the camera as it will now be too close to the focuser and the OUTWARD TRAVEL will now be the problem (exactly the opposite to a Newtonian)... the solution in this case would be to add an extension tube of suitable length to match the length of the removed diagonal (typically 2" or more)... you need not worry about this on your scope.

A suitable barlow, with T thread would be the Skywatcher one.

http://www.harrisontelescopes.co.uk/acatalog/SkyWatcher_2x_De-Luxe_Achromatic_Barlow_Lens_.html

Normally, FLO carry the same model but it appears they are currently out of stock, hence the link above.

This should bring the focus point far enough out from your focuser to eneable you to achieve focus.

What Camera are you using?

Also, remember you will need to increase your length of exposure when using a barlow to take account of the lower brightness of the image produced.

Hope this helps you to understand the issue.

Best regards.

Sandy. :grin:

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NIGHTBOY

NIGHTBOY

Posted
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Fantastic sandy thank you very much. My scope is a 200p dob and my camera is a canon 700d. I did try a sort of mock up with a Barlow and my camera and found I couldn't wind the focuser far enough out. I have just bought an astara 3x Barlow and tring from eBay, this all slots into the 1.25 tube so I have everything crossed!

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Lonestar70

Lonestar70

Posted
Posted

Hi Nightboy,

X3 maye be to much especially with a 1.25" fitting... you will almost certainly get 'vignetting' on your shots.

X 2 would be better.

Nice camera but what are you trying the test focus on... you need to focus on something at least 2 miles away if trying it in daylight.

Keep happy.

Best regards.

Sandy. :grin:

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Lonestar70

Lonestar70

Posted
Posted

Hi Nightboy,

Not meaning to dampen your enthusiasm... but... does your DOB have motorised tracking... if not then you are going to find it impossible to take any planetary pictures with a DSLR... you may just manage some moon shots, since it is large and very bright... planets are not; and you will find it impossible to keep the image centred ,and steady, whilst manually tracking the OTA.

Even with a tracking DOB mount you will struggle to take a picture using a DSLR directly connected to the focuser without any Barlow fitted... the required exposure time will be far too great and you will get star streaking mainly due to field rotation.

Add a barlow and you will need to make the exposure time even longer and your field of view will also be a lot smaller... making it very much more difficult to keep the target centred.

For DSLR work you really need an equitorial tracking mount... and exposures between 60 seconds - several 10's  of minutes.

You could get some half decent shots of a couple of the brighter planets (Jupiter, Venus and possibly Saturn) or the moon with the DOB by using a webcam which will allow you to capture a short video of a few hundred frames length.

You can then stack the individual video frames using some free software (registax) which will produce a single picture.

You will find it much easier to use... then you can save up for an equitorial mount for use with your DSLR.

Before you spend any more money on equipment  I would suggest you purchase the book 'Making Every Photon Count' by Steve Richards (AKA Steppenwolf on this forum)... it is the astrophotography bible and will explain all that is necessary and, more importantly, what not to do and why.

It is available from 'FLO' (First Light Optics) in the books section.

Best regards.

Sandy. :grin:

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NIGHTBOY

NIGHTBOY

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Yes I'm well aware of this sandy. I took a pic of Jupiter the other night through my dob (that doesn't track) and I was surprised how fast I had to set the shutter due to its brightness. I'm not expecting miracles believe me!! My camera boasts a ISO of 12800 (tho I know less iso the better). I'm only doing it on a shoe string really. I won't be spending thousands.

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Lonestar70

Lonestar70

Posted
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Hi Nightboy,

Having a high iso capability does not matter much for astrophotography.

Take 2 shots of say 10seconds... one at iso 800 and one at around say iso 3000... approx the same number of photons will strike the camera sensor during each.

The only difference between them will be the percieved brightness, due to the higher ISO version having been amplified by the camera before output.

The only trouble is... the base readout noise of the sensor will also be amplified for the higher iso version... hence a noisier picture.

ISO only really matters for daylight photography where the very much higher photon count in a given space of time will quickly saturate the sensor... hence you can expose for much less time and achieve a good result... the signal/noise ratio will be much better balanced for daylight pictures.

Does your camera have a video mode?... if so, you could try taking a short video of Jupiter at say 800iso.

Stack the resulting frames using registax... you might be surprised with the result.

It's all good fun though.

Keep happy.

Sandy. :grin:

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NIGHTBOY

NIGHTBOY

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Yes sandy it has video mode, I'm now quite familiar with pipp, registax and dss. I had tried Jupiter before but it was too bright but to be fair it was that small in the viewfinder you couldn't really see if it was in focus or not, hence why I've just bought the Barlow :)

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