Bosma refractor

[adman]

Does anyone have any experience with 80 x 500mm Bosma refractor for use as a guidescope? They have them at Andrews for $299 for the OTA.

If not is this type of scope even suitable for use as a guidescope??

The catch here is that I don't yet own the imaging scope - but am leaning towards something like the WO FLT110 or FLT98.

Adam

[Kal]

An achromat refractor is perfectly fine for autoguiding.

[allan gould]

You can use this for guiding but a check on Andrews has these listed for $349.00

[adman]

Yes - the $349 is with tripod which I don't need. They offered to split it to $299 for the OTA and accessories...not sure whether this will be the case if everyone only wants the OTA though..:).

[allan gould]

Thats a good price for a guidescope. Go for it but remember you will have to fork out for rings etc.

[slippo74]

Hi Adam,

I'm using that BOSMA for the autoguiding... it does its job pretty well.
If you keep your expectations tuned to reality, it is not bad to use even for general view.

Cheers

[adman]

thanks everyone. Another I am looking at as a guidescope is the skywatcher 90mm mak :

http://www.astroshop.com.au/products/details.asp?id=MAS-065A (http://www.astroshop.com.au/products/details.asp?id=MAS-065A)

Are there any considerations that I should be aware of between choosing the mak over the shorter refractor? Is there, for example, any benefit to the longer focal length of the mak? Are the maks heavier, or harder to mount / balance etc?

Adam

[AlexN]

Forget using the Mak for guiding... its got a moving mirror, and will wreak havoc during exposures... the 80x500 achromatic refractor is the way to go for guiding...

The mak's longer focal ratio will make finding a guide star more difficult, as the same star will look much dimmer through the Mak than the refractor...

In my opinion, go the refractor for sure!

What guide camera are you using? have you considered off axis guiders?
Orion make a good OAG for $209.. That way you only have one scope on the mount, no possibility of flexture, guiding at the same focal length as imaging etc.. Provided your guide camera is sensitive enough, that would be the best way to go in most situations...

[adman]

Thanks for that Alex - all makes good sense.

with an OAG, are you always able to find a guide star at the right place in the field of view? Then I have seen those cameras that have imaging and guiding CCDs in the same box - are they any good / better than OAGs?

For me this is all academic at the moment anyway - I have no guide camera and no imaging scope - so my goal in the short term is to get a cheapie scope that is not too shabby to use for some very basic visual stuff, then medium term (once my goto upgrade finally arrives....:mad2:) to use as a guidescope for widefield camera shots, then go the whole hog longer term and get my dream imaging scope, and stick this one on top to guide it....

I kind of know where I am going - but I just want to take it one step at a time so I don't end up with a whole lot of gear that I don't need.

Adam

[AlexN]

Self guided CCD's are the answer.. But they are expensive.. $4000 minimum spend if I recall correctly..

With my QHY5 guider in an OAG in MANY different scopes (11" SCT, 8" RC, 80mm F/6 APO) I've always been able to find a guide star with relative ease. in the refractor, there is always a guide star within the field of view, no matter where I point it... Always...

[allan gould]

Have heard all this before but its not necessarily true.
I use a 90mm Mak for guiding piggy backed on my 8" SCT. Works a treat. Have guided for 4 hr total at 2000mm FL with an SCT and Mak. No flex, no mirror flop. Tried an OAG and gave up as couldnt always find a guide star. Also use an X-Y shifter and makes finding a guide star easy.
Easy 19 min subs and across the meridian. It works for me and I dont see why it shouldnt work for others.
Just my 2 cents and experience.

[adman]

Alex

just reading your post again - isn't brightness a function of aperture rather than focal length?

I am almost the exact opposite of an optical guru, so please humour me if I've got it wrong....

Adam

[AlexN]

focal length and focal ratio are different beasts. The Mak will be F/10~F/12 where as the refractor is F/5. two optical systems of equal aperture, one F/10, one F/5, the F/5 will show a wider field of view, and be brighter... The extra aperture of the mak would be negated by the fact that it has over double the F/ Ratio than the refractor, not to mention the Mak will have a central obstruction due to the secondary mirror...

I can see no possible reason someone would chose a 90mm Mak Cass over a fast 80mm achro refractor for guiding. None whatsoever...

[allan gould]

I do it because it works and I get good guiding. Thats why we are all different.
I mention it because there are a lot of different ways to do the same thing. I also use a 50mm finderscope as a guidescope for my 80mm. Why? because it works, its light and I don't need such accurate guiding for an 80mm scope.
I use a 90mm mak for guiding my 8" and 10" SCT at 2000mm and 2500mm fl because I get more accurate guiding than using my 80mm as a guidescope. I use the 80mm or 90mm scope to guide my 127mm ED refractor.
These systems work well for me and I dont think an 80mm fast refractor will do as good a job as the 90mm mak at 2500mm.
Just my 2 cents with practical experience.
But as I said first the 80mm BOSMA would be a fine guidescope.

[AlexN]

I too have used a 50mm finder to guide a small refractor, and have used OAG's, a nikon 500mm F/8 mirror lens (which is a mak cass), an 80mm F/5 refractor, a 102mm F/7 refractor etc etc.. Horses for courses as you say... Yeah, you probably could use the mak for guiding the SCT, however the best option in an SCT is using an OAG as you don't have to worry about the mirror flop issues, or the chance that your imaging primary will move more than the guiders primary etc... It can be done.. Im not saying it can't. Its just generally easier to eliminate all possible causes of frustration from the get go... All that being said, I used the 50mm F/4 finder + DBK31AU camera to guide my C11 @ F10 and got good results guiding the 2800mm focal length with just 200mm of guider focal length...

[Kal]

Star brightness is only determined by aperture, not focal length, because it is a point source of light. You are confusing star brightness with the brightness of diffuse objects whose surface brightness changes with focal length.

The mak would only show stars slightly dimmer than an equivalent aperture refractor because of the central obstruction, however I agree that the smaller field of view makes finding guide stars harder, and you risk mirror flop messing with your guiding.

[Robbie]

As a guidescope I use a saxon ed80 atop a meade sct about $500 with rings. for the few extra bux you have the advantage of a reasonable secondary imaging scope in the ed80 as well as a guidescope.
Just MHO but I have more fun imaging with the ed80 than I do with the big sct its strapped to so I think choice of guidescope is very important.

[adman]

I have been kind of coming around to that position as well. There will be a bit of time between getting my guidescope and being able to afford a larger imaging refractor, so it would be useful to get something that is not too shabby on its own. I have been thinking about the WO Megrez 90 - great price at Andrews at the moment - great scope on its own, and can do double duty as guidescope later on.

Unfortunately, cheaper and sooner may win out....

Adam

[Robbie]

Yes I see and I seem to have made my point, just FYI and I wish I had thought along these lines when starting out , the number one rule is get the best mount you can afford as a priority, you will always get a better image with an average scope and a good mount than you will with a top scope on a cheap nasty mount.
If I was to do it again in stages I would have first obtained a good solid mount capable of taking the weight of the main ota that you will eventually get for yourself ;but just mount the ed80 or half decent/guide secondary imaging scope on it for now, it may look stupid but your learning curve will be much easier as the setup will be more forgiving providing you with a reward for your efforts rather than endless frustration.:thumbsup:.