short focal ratio scopes

Hey All

Looking at purchasing a new scope over the next few weeks. So i've started the research into some areas that i have previously avoided. Started with diffraction and working my way towards ensuring i get the right scope-camera combo.

I previously had a long focal ratio scope with a EQ6. This combination lead to alot of difficulties for me while trying to learn to take astro shots. These were primarily due to lack of experience in what i was getting myself into.
1. approx 18Kg OTA on EQ6 (wont be doing that again)
2. Mechanically poor OTA
3. standard drives in EQ6
4. Temporary setup (valuable time wasted setting up)
and just not a stable enough arrangement.
I guess some people may have got decent pictures under the same conditions but it i feel much on all as i enjoyed the learning experience and figuring this stuff out i want to spend this winter learning new stuff. The reason i highlight long focal ratio is because the length + weight of the scope was 2 much for the EQ6 and coupled with narrower field of views (Canon350D) it was always going to be a battle to get decent tracking. (Correct me if iam wrong or not making sense).

I want to reduce as many of these issue as i can starting with a much lighter scope 11kg (Orion Optics 250mm f4.8 SPX Range) with a shorted focal ratio F4.8. However i've being somewhat stopped by an article i read which goes roughly as follow regarding short focal ratio scopes:
-- hard to test accuracy to high degree
-- even if optically perfect + collimated will produce wild aberrations
--ultimately lead to images of less then diffraction limited

The second and third point stick out alot, how can a professionally figured mirror say 1/10th PV produce wild aberrations or worse lead to images of less then diffraction limited?
Will a smaller F/ratio not have a smaller airy disk?
In reality is F/4.8 considered to be short focal ratio?


PS: Any comments on the Orion Optics UK 250mm f4.8 SPX Range appreciated

Thanks
Mike

for anyone that might stumble on this thread i found my
answer!! (same website in fact)


www.rfroyce.com/mirror_performance.htm

I in fact ordered the F/4.8 this week!!!!

Mike

Mike You wont regret it, it really is a 1st rate scope.

I have a rare version of the 250SPX its an f/4.0 unit which I got specially made and has been modded slightly for photography (Oversized secondary). Couple of points regarding photography with an instrument like this.

a: Collimation is critical at such short focal ratios. A laser collimator is just not good enough. I normally get it roughly collimated with a laser then use a cheshire eyepiece to finalise before doing a final star test.

b: There is generally ample back focus, however if you need to put a lot of bits between the focuser and camera you just *might* run into problems. e.g. in my case I have an adaptor-filterwheel-baader MPCC-55mm spacing-camera. With all of this I needed to bring the focus back about an inch. How I solved this was to drill three new holes an inch further up the tube and move the mirror. 20 minute job.

c: The coma corrector is *essential*. I use the Baader MPCC. With the correct 55mm spacing if is first rate and quite a good price comparatively.

Also consider dew heaters or your heart will be broken!

Hey Mike / Dave, just looking at this here now, thanks for both your inputs. Mike was your eq6 the pro mount? I too would like to get this mount, and am seriously thinking of going down the same route of getting a really good set up, saving the pennies at the moment

If that mount can handle 11kg then great.

Keith.

Keith

I use the EQ6 Pro. It is documented to take 40lbs (18Kg) max payload. My setup with max payload comes approx 15Kg and typical setup of about 10-11kgs.

At my max payload I might be pusing it but feel comfortable with 10-11kgs.

To achieve the higher payload I needed to purchase an extra counterweight as the counterweight bar is not very long (it is totally accommodated within the mount head when kit is being put away).

The mount head is 16kgs and that is always a mini-challenge to get of the floor. If you decide to get the mount I highly recommend getting replacement ALT bolts such as those provided by astrodevelopments in UK (I got mine for £19 from ModernAstronomy in UK) Even unloaded I really struggled to get any upward ALT movement for polar aligment with the supplied bolts. With the replacement ALT bolts I can get all the movement I need with the head fully loaded without too much struggle.

This is a common gripe with this mount.

Mark

Thanks Dave
Yes iam looking forward to getting it, although i have quiet a bit of work to do in setting up the rest of the gear (mount, permanent home for the scope etc)!!
I take your first point on collimation this kept coming up every-time but i have to admit i thought laser collimator was the job so i will definitely look at the cheshire eyepiece.

In fact Dave you've made me a great deal more confident of my decision with you comments on back focus, even if it is a problem its easily sorted, I tried to get some dimensional information of Orion so that i could at least try and preempt some of these problems, unfortunately i wasn't able to, either way i guess it would have been very difficult to cater for all the items that could be attached. (looking at your list i am not sure i would have!!!!) And as for the coma corrector i was going to put it of for a while but i reckon can't avoid it and should start with it.


Keith my EQ6 was the basic version with just standard keypad/motor drives and gearbox, not good enough for astrophotography but ok for visual.
11KG will be a piece of cake for the EQ6, however i will need to upgrade my drive electronics and possibly the motor gearbox. I might go so far as to doing an upgrade i brought up in another thread
www.irishastronomy.org/forum?func=view&c...id=81263#forumbottom

for the electronics upgrade i will use
eq6.update.rajiva.de/#Update
appears the items are being sold here now:
www.deepsky-solutions.com/e-main.htm

The cost of these roughly falls under the cost of upgrading to the skyscan (EQ6 Pro) and fall well under the cost of the Boxdoefer upgrades (1000euro).
I believe with these low cost upgrades the mount will perform brilliantly for astro-photography.
I do like the skyscan upgrade for its fast slewing rate (800x) but i am currently trying to see if i can do this with the rajiva upgrade. If you look at the comparison chart for the rajiva versus other commercial controllers, its quite impressive. ( astronomie.rajiva.de/astronomie/smstv.html )

With the EQ6 pro i am not sure how much if any upgrades are required for astro-photography, i know anyone with the skyscan should be interested in:
eq-mod.sourceforge.net/tutindex.html

I assume your planning on astro-photography Keith?

Iam trying to pack all my upgrades(i.e. spending) into a few weeks so the pennies are being free'd for ever, all of them


Mike