Focuser upgade?

[Mr Flibble]

I have a rack & pinion focuser on my 6 inch scope which is really getting on my nerves so I'd like to upgrade. Ideally I'd like to get a Crayford focuser but I'm encountering two problems :

(1) All Crayfords seem to accommodate 2 inch eyepieces which means I'd have to enlarge the hole on my OTA to accommodate the bigger drawtube.

(2) Most Crayfords seem to be designed for scopes of 8 inch apertures and above (Although there is a Skywatcher Crayford suitable for 6 inches; and Moonlite do a 6 inch mounting plate but their Crayfords are VERY pricey and problem (1) still applies).

I'd be gratefull for any advice or suggestions .

[johninderby]

I don't know if you've seen the thread I started on fitting a replacement crayford to a 6" Newt, but if not here's the link.

http://stargazerslounge.com/diy-astronomer/119323-fitting-2-focuser-skywatcher-150pl-newt-ota.html

It's actually not a difficult job and there are a wide range of focusers that will fit.

John

[Mr Flibble]

John, thats fantastic. Thats EXACTLY the info I was after and you've also got a set of instructions with pics as well

Before I go ahead and order the baseplate and focuser, the only part I'm unsure of is exactly how the focuser attaches to the baseplate?

When you say this :

"Step 3

Fit the focuser by tightening the grub screw on each of the curved sides of the base."

Does this mean the grub screws on the focuser itself? If so, I'm assuming the focuser clamps to the baseplate? I just want to check to make sure so that I order the correct focuser to fit the baseplate.

Thanks John, that guide is really helpfull.

[johninderby]

Hi

There are tiny holes in two sides of the baseplate with hex bolts inside. These hex bolts tighten up against the focuser body and clamp it in place. Here's a photo (upside down) showing the tapered bit on the bottom of the focuser body that fits into the baseplate.

BTW While the standard GSO type crayford on TS's website is good the TS Monorail focuser is much better and will handle twice the load and isn't that much more expensive really. I had one on a scope I used to have and that's why I can recommend it.

Telescope House has their Revelation crayford that is also very good and is very keenly priced as well and fits the TS baseplate.

http://www.telescopehouse.com/acatalog/Revelation_Newtonian_Crayford_Focuser_Dual_Speed.html

John

[Mr Flibble]

Excellent!

I've just ordered the baseplate from Telescope Service and I'll now look around for a crayford to order. If I go for that Revelation it will cost me a grand total of £110 all in (apart from the bolts!) which is pretty good value.

Thanks again for the help John, its much appreciated. The project is underway

[Mr Flibble]

One thing I've found out about these crayfords is that they're much shorter in height than my rack & pinion focuser. I've measured the focus point of my R & P focuser and eyepieces to be 145mm from the surface of the OTA. Therefore I understand that I'd need a focus extension tube on a crayford to achieve the 145mm, the length of which will depend on which crayford I buy?

My main query however regards DSLR imaging. I understand that lower profile crayfords are designed so that focus can be reached with a DSLR on a reflector. I don't own a DSLR but to future proof the purchase of my crayford in case I want to get into imaging is there anyway to work out what height from the OTA the focus point will be? The reason I ask is because the crayfords all have different heights and I don't want to purchase one based on trial and error. My scope details are :

Optical Design : Reflector

Aperture : 150 mm (5.91 in)

Focal Length : 750 mm (29.53 in)

Focal Ratio : 5

Mount : CG-5 Equatorial

Limiting Stellar Magnitude : 13.4

Resolution (Rayleigh) : 0.93 arcsec

Resolution (Dawes) : 0.77 arcsec

Photographic Resolution : 400 line/mm

Light Gathering Power : 459 x

Angular Field of View : 1.3 °

Linear Field of View (@1000 yds) : 68 ft (20.73 m)

Optical Coatings : Aluminum

Secondary Mirror Obstruction : 1.75 in (44.45 mm)

Secondary Mirror Obstruction by Area : 8.8 %

Secondary Mirror Obstruction by Diameter : 29.6 %

Optical Tube Length : 27 in (685.8 mm)

Telescope Weight : 35 lbs (15.88 kg)

Any help is much appreciated.