Experiment with a 4inch Tasco

[paulF (Paul)]

Hey everyone,
i'm planning on buying a new 8 or 10 inch in the near future so this will leave me with a 4 inch reflector and a 2.3 inch refractor to play with

I'm thinking of changing the focal length of the 4inch from 900mm to a minimum of 2000mm. i know this will mean a lot smaller field of view but a lot more magnification but not quiet sure what to expect.

Can someone can shed more light on what should i expect with such a set up, a 4inc reflector with a 2000 focal length?

[renormalised (Carl)]

Trying to increase the FL of a 4" to 2000mm will leave the scope practically useless. You'll have such a small FoV and dim view through it. It'll be like looking through a 1" piece of pipe. What extra mag you'd get out of your ep's would be a waste of time. You'd be far better off just sticking with what you've got till you get the bigger scope.

[paulF (Paul)]

Thanks for your reply Renormalised
I thought that aperture controls how dim the image is and not the focal length. Could you please shed some light on why would the image be more dim?

Thanks

[AstralTraveller (David)]

AFAIK the light gathering and theoritical resolution won't change. A longer fl will allow you to use a smaller secondary which will improve contrast but that will only be detectable if the original obstruction was >20%. You will of course get a higher magnification for a given eyepiece but with a scope that small you will hit the upper mag limit pretty quickly. The very big downside I see is that a 2m tube is difficult to transport, move and, with a small diameter, keep rigid (trust me, I've had a scope that long).

[Terry B]

Originally Posted by paulF

Hey everyone,
i'm planning on buying a new 8 or 10 inch in the near future so this will leave me with a 4 inch reflector and a 2.3 inch refractor to play with

I'm thinking of changing the focal length of the 4inch from 900mm to a minimum of 2000mm. i know this will mean a lot smaller field of view but a lot more magnification but not quiet sure what to expect.

Can someone can shed more light on what should i expect with such a set up, a 4inc reflector with a 2000 focal length?

How were you going to change the focal length?
You would need a new mirror.

[Ian Robinson]

Those old 60mm Tasco refractors (if it's the one I think you are talking about (I've still got one of them)) came with a 3x barlow.

OK with 20mm and 12.5mm oculars , which is how I used it , useless with the 6mm and 4mm oculars ....

My old 60mm Tasco has been rebirthed as a guiderscope for my CG5 (d/axis stepper motor version) based camera platform that I plan on enhancing to enable autoguiding (ShoestringAstronomy style).

[Ian Robinson]

Originally Posted by Terry B

How were you going to change the focal length?
You would need a new mirror.

A barlow lens will do that ....

[AstralTraveller (David)]

Originally Posted by Ian Robinson

A barlow lens will do that ....

But unless you use a barlow that is worth more than the scope wouldn't you actually go backwards in image quality?

[paulF (Paul)]

Hey guys,
the plan was to keep the same setup(main/secondary mirrors ) and only change the OTA to be longer. Again, this is just to experiment, nothing serious. I just want to see how much i can push the longer focal length

[DavidU (Dave)]

Originally Posted by paulF

Hey guys,
the plan was to keep the same setup(main/secondary mirrors ) and only change the OTA to be longer. Again, this is just to experiment, nothing serious. I just want to see how much i can push the longer focal length

Your mirror is ground & polished to a focal length. To do a 4" 2000mm FL scope would require a new mirror, tube and secondary, in other words make a whole new scope.

[paulF (Paul)]

Hmm Ok thanks for that David. I thought it would be nice to play around with the 4 inch reflector but if my little experiment will only break it, i prefer to keep it and use it more wisely in the future.
Thanks everyone

[renormalised (Carl)]

As the others have said, your resolution and light gathering capacity will stay the same, but having such a long focal length means your FoV will change and with so little light gathering capacity in the first place, with such a restricted FoV, the images will be dim. As some have said, best to use a barlow to do the trick, but for a good quality one, you'd be paying more than the scope's worth. To do it the other way means your tube is going to be 1 metre long, or larger. You'd have to make it out of carbon fibre to keep it rigid enough not to obstruct your viewing.

[AstralTraveller (David)]

Excuse me if I seem a pedant Carl but I don't quite agree. Ignoring vignetting, the true FoV = the apparent FoV of the eyepiece / magnification. So, yes, if you keep the eyepiece the same the FoV will decrease in proportion to the increase in the scopes fl. However if you lengthen the fl of the eyepice by the same ratio you increase the fl of the scope the mag and FoV remain the same. The dimming then is just the normal dimming associated with increased magnification. What you lose with longer fl is the ability to achieve low magnification. However at 2000mm fl you can still get 65 mag and a true FoV of 1.3 degrees. On the other hand you can get to higher mag more 'comfortably' in that you do not have to use such small eyepieces and you don't need a coma corrector.

You see, when I was young and impressionable I ordered a mirror just like the one the club had - a 10" f/8. I built a pretty poor scope around it which is now retired due to accidents and breakdowns. But I have good reason to believe that the mirror is very good, and the small (2") secondary doesn't cause significant vignetting while not affecting much. So I have plans to build a great visual scope around the mirror. In general I am a fan of long f ratio newts and think the 'conventional' wisdom to use long ratio scopes for visual observing is quite sound. So I am keen not to have misunderstandings about the pros and cons of different f ratios.