camera lens..... and scopes...... sizes

[garethmob]

this ones been bugging me last few weeks

iv noticed a fair few amazing shots taken, from scopes like the megrez 72, 66 the skywatchers (basacally the widefield scopes) but the pictures have a very similar feel with photos taken with camera lens

which camera lens are similar to which scope sizes

(i no theres alot more "zoom" like 70 - 300 but im just interested in the blocks like 300 or 200 on that lens)

so which lens would be similar to :

megrez 72

130

150

200

gaz

[yeti monster]

I would offer that the 'pro' series lenses are most akin to telescopes, they being of higher quality and larger aperture, hence their ludicrus prices....

[ollypenrice]

To compare like with like in terms of what will be framed, just look at the focal length. With a focal reducer my ZS66 has a focal length of about 330 mm so that would give a similar framing on the sky to a 300mm lens from your list.

Then you look at the aperture, which is not normally given on camera lenses but is easy to work out. My Canon EF200L has a 200mm FL and is F2.8 so its aperture is 200/2.8 or about 71mm. But there's a catch; you can't usually expect to use a camera lens wide open on the stars. You'll get edge distortions. My EF200L is fine down to about F3.5 so that means its effective aperture is reduced to about 57mm. However, that is still very fast. The fastest Apo of which I'm aware is the Takahashi FSQ whch can work at F3.6 or f3.9 depending on the model. But they are seriously (ludicroulsy?) expensive.

Far from being ludicrously expensive, I think camera lenses are quite the opposite, personally. A roughly 60mm f3.5 apochromatic flat field telescope of stunning quality would be likely to cost more that the Canon lens. Of course, very expensive pro lenses do exist but are not needed for astronomy where you will have to stop them down anyway. I very much doubt that there is any point in an astro imager going beyond the L series.

Focus is hyper-critical on any system at f3.5 so manually moving the focus ring is not the way to do it. You need mechanical aid as from a Telescope Service device or electronic control. It may also be best to make a front aperture mask for stopping down because diaphragms create diffraction artefacts.

My Canon lens is well ahead of my ZS66 on optical quality. It produces smaller, tighter, stars relative to its FL.

Olly

[garethmob]

ahhh thanks olly very very helpful answer! thank you! by aperture mask do you mean somthing like a behontiv (i cnt spell it sorry) mask?

thats one thing that i never thought of, spending money on better camera lens instead of a telescope

[ollypenrice]

A front aperture mask would just be a thin black plastic ring in front of the lens with a central hole calculated to give you the f ratio you are trying to stop down to. Steve (Steppenwolf) has made one and says it works. I'm just trying to get round to making my own! On my lens I'll start with a 57mm hole which should be about f3.5. I'll keep trying a slightly biggger hole till the edge quality goes.

Olly

[Psychobilly]

I'm thinking of getting rid of my single use scopes and replacing them with Dual use L series glass...

I will hopefully get a chance to try the new fracs over the winter but if I don't they will be hitting the For Sale section....

Peter...

[Photosbykev]

One issue you have with modern camera lenses is that you can't stop the aperture down if you use a CCD or similar behind them as the DSLR body provides the command signal for the aperture.

There are ways to make the lens stop down i.e put the DSLR on it, select the aperture required, press the DoF preview button and with the button depressed remove the camera body. Some, but not all, lens will then have the aperture stopped down to the selected value and you can fit adaptors etc to the rear mount to take your CCD camera.

This trick is used in timelapse photography to avoid a problem called aperture flicker.

I think I will stay with my Canon prime lens I kinda like my 600mm f4 and 400mm f2.8 glass.

[ollypenrice]

One issue you have with modern camera lenses is that you can't stop the aperture down if you use a CCD or similar behind them as the DSLR body provides the command signal for the aperture.

There are ways to make the lens stop down i.e put the DSLR on it, select the aperture required, press the DoF preview button and with the button depressed remove the camera body. Some, but not all, lens will then have the aperture stopped down to the selected value and you can fit adaptors etc to the rear mount to take your CCD camera.

This trick is used in timelapse photography to avoid a problem called aperture flicker.

I think I will stay with my Canon prime lens I kinda like my 600mm f4 and 400mm f2.8 glass.

Another reason for using a front aperture mask. I don't like snatching out the lens having set the F stop in the camera. Doesn't feel good though it seems to do no harm.

Olly

[garethmob]

i wonder if aunty FLO might see this thread an see about producing the FAMs? by same way the focusing masks are...........

[Psychobilly]

I'll probably be getting mine laser cut...

[Photosbykev]

Another reason for using a front aperture mask. I don't like snatching out the lens having set the F stop in the camera. Doesn't feel good though it seems to do no harm.

Olly

The front aperture mask is a simple option which works well. I've been using the aperture trick for years without any problems for timelapses. One of my 5D MkII bodies has over 800,000 shutter actuations and is still going strong, touch wood.