Real focal length question

[ColHut]

Dear All,
I am lucky enough to own a Bushnell Northstar 4.5" GOTO reflector which (of course) beats-as-it-sweeps-as-it-cleans!:)

My question (only 1) is what is its real focal length. It is described as 900mm long (honest you can check), and its magnification maximum theoretical as being x675 with a 4mm SR and a X3 Barlow. The calculations work out (900/(4/3))=675. and the scope would be a 900/114=f8 ratio. relatively long.

The trouble is the scope itself is umm only 880mm oa between the perpendiculars. The distance between the primary and secondary mirror is only about 700-750mm so really a f ratio of about f6.1. Is there some other part of the light path I should be including?

This matters in determining what eyepieces to buy and in the making of approximate practical magnification limits, and of course in exit pupil calculations.

If someone could set me right that would be great.

cheers

[matt]

Well...

it doesn't appear as though you are including the distance from the secondary to the eyepiece, where the light actually comes to focus???

You should also look to not exceed the maximum useable magnification based on aperture of your scope.

Most people work of 50x per inch of aperture, in very good seeing conditions.

I'd play it conservative and work off 40x.

So, that'd give you 180x ... max magnification.

But I'd say a 6mm eyepiece is about as hard as you might want to push it, in very good conditions. (150x... based on 900mm focal length)

That should help for a start.

others will chime in shortly

[ColHut]

Thanks for that. I hope I can get a definitive answer...
I am well aware (belatedly) of the amazing machinery I have acquired. I now seek to make the bext of it until i acquire a proper telescope.

:lol:

cheers

[RAJAH235]

Matt is spot on as usual...ummm, with all your 'supplied accessories' in use, you are definitely overpowering the t'scope. Even in good conditions I & (most I would say), never go anywhere near their max theoretical mag. eg; 10" dob = 500 x = no way Jose.
I normally view at about 120/180 x .....with good seeing conditions can go to 250/300 x, maybe a tad more... :D L.

[ColHut]

Ahh sorry I did not make my self clear I appreciate that on a very good day (and not many are) a max of 225(50/inch)- ooh at a real pinch 270 (60/inch) is the best. If it is a 700mm fl then this is 3.1-2.59, if 900 then 4mm-3.33mm.

I have actually tried to use the X3 barlow (black plastic ) with the 4mm Sr on the moon and I could possibly have made out a crater or it could have been a water drop or midge. :)

I have since acquired a 9.7 meade and 25mm GSO plossel and bintel x2 bino which all work much better with a couple of filters. I did also try the 4mm SR with its woefull fov and tiny eyepiece in the x2 bintel barlow and the moon looked rather spectacular although focus was diffficult (x450-350). and the moon moves fast! Not really practicable generally though.
I um and arrgh betwen an Vixen LV 2.5/4mm, one of the Orion Stratus/Hyperion 3.5mm clones, or an Epic ED-2/clone 3.7mm.

cheers
Wol

[GrahamL]

hi colhut

All the info above is very true and a must to take on board when
thinking about spending some dollars.

I bought my son a small scope a lot of yearsback and we both got bit by the bug despite the scopes limitations .. a lot of fun was had all the same:)
to be really honest the 4 mm eyepiece supplied never got used much .
fast forward a decade and I buy a 10 " dob and you know what.. the 4mm
eyepiece supplied dosn't get used either... IMO if your going to buy
something a little better in the eyepiece dept .. look at a magnification that is going to let you enjoy the night sky in a variety of conditions and situations ( middle of the road).

your most welcome to gso 4 mm I don't use if you like
I'll even spring for the postage :)

[astro_nutt]

Hi ColHut
I had a peek at Jupiter last night using a #80A blue filter, 7.5mm plossl with a 2x barlow in my 10"/f4.7 dob...this works out to 320x which is right at the limit!!!..I did manage to view a few steady images of the clouds formations though...swap the 7.5mm for a 15mm and the image is smaller of course but allowing the eye to adapt does provide good detail...I too have a...er...sr4mm which I use with the barlow..(600x) at school star nights and ask the kids if they can find the cheese shop on the Moon..I do cop some looks of disbelief though!!!...

[Roger Davis]

Take the supplied 4mm eyepiece, a centre punch and a claw hammer.
Set the eyepiece on a solid block of wood.
Place the sharp end of the centre punch on the exit lens of the eyepiece.
Tap abruptly with the hammer on the centre punch to break the lenses.
Or unscrew the 1.25" sleeve and remove the retaining ring and then the lenses.
First one is much more fun!
Now you have a collimating tool.
At least with the barlow and an eyepiece you will retain the apparant field and eye relief.
When you are allowing 2x mag per mm of aperture don't forget to allow for the secondary mirror.
About that 3x barlow .... Keep the dustcaps, they will come in handy!

[astro_nutt]

Good one Roger!!..why didn't I think of that??

[ColHut]

Sorry I do not follow here? Are Newtonians restricted by the size of their secondary mirror?????

Thanks all for your helpful comments

decisions decisions and more cloud

BTW on my scope the motor drive moves the scope via some kid of friction nut, so when it sets itself up it can hit the stops and then sort of knows it has gone past and resets itself (clear as mud?) Of course it is so slack that near the horizon with a weighty eyepiece it just slides down! So I go the chance to pull the housing off and tighten up the friction bolt which cures the problem. Of course this is a worst case scenario for the telescope as the balance is out. Anyhow the battery voltage has dropped after weeks of use to 8 volts (an alkaline) and cannot elevate now against the added friction! Some twaeking and in goes one of my new NiMH batteries and we are away.. Success at last. Now if only the cloud will go away.

I will certainly consider that Orion acufocuser too.

[ausastronomer]

ColHut,

Everyone else has given you sound advice already in terms of limiting your magnification. Roger has also given you sound advice as to what that 4mm eyepiece is suitable for.

I will add my .02c worth.

While your Bushnell Northstar might beat-as-it-sweeps-as-it-cleans, I think and don't hold me to this, it uses the same Optical Tube Assembly as the Tasco 4.5"/f8 which is made by Synta in China. This has a SPHERICAL primary mirror, usually of FAIR quality. Regardless of whether it uses the Synta OTA it will have a spherical primary.

I will try not to get too technical. While at F8, the scope exceeds Raleigh's Criterion for parabolisation of a 4.5" mirror, really it comes up a bit short. Many experienced opticians believe Raleigh's Criterion to be about 25% conservative at this aperture, which means to push really high magnifications with a 4.5" scope it needs to be F10 or longer if the mirror is spherical, or it needs a parabolic mirror at F8. What does all this mean? Probably that regardless of how good the seeing is your best views will be at low to medium power. In other words under about 140X. The 675X advertised on the box? Breaks all the rules about buying a telescope. Never buy a telescope that's advertised as having a maximum magnification, it is always and I mean always, from a practical point of view, a load of cocky sh_t. In fact it usually implies by convention the contents of the box to contain a CBTT. (For those wondering CBTT = Christmas or Birthday TRASH Telescope)

Don't be disheartened. Get some slightly better eyepieces (the scope isn't worth eyepieces costing over $100 each) stay at low to medium power and you will get some really nice views.

CS-John B

[ColHut]

Ahh I tried to be easy on the sarcasm. - no point in being bitter - The scope is okay at a pinch - and we live and learn Tasco are owned by Bushnell BTW.

cheers
WOl

and thanks

chin up:D

[johnno]

COL,

JohnB, is right,
You can get some VERY nice views,with this scope at the lower powers.

I have something similar,an Andrews 114x900,
works very nicely,at the lower powers,
As JohnB, Said.

Basically,
If the smaller size scopes,provided views compareable to the bigger ones,
why bother with bigger in the first place?.

Simply,

The bigger the Aperture,the more light is collected,the Brighter the image to the Eyes,
Thus,the more Detail to the Obsrver.

Regards.
John