Revelation 8 Dobsonian - any tips ?

[John]

To compliment my Megrez 90 I have been on the lookout for an 8 inch dobsonian (hence my Wanted ad). Having considered a 10 inch Revelation that had had a rather hard life as a "project scope", and realising that I did not have enough time to do it justice, I was then offered a Revelation 8 which is virtually "as new" at a very good price so I have gone for that - I'm collecting it on Friday - cue even more clouds across SW England I'm afraid

I know a few folks on SGL use, or have used these scopes - are there any "tips and wrinkles" that I should know to get the best out of the scope ?. It's intended primarily for DSO's but from the reviews I have read I might be pleasantly suprised at the planetary views as well.

Thanks,

John

[tony4563]

Hi John.

Congratulations on the scope. You'll love it! I find it such a comfy height for viewing whether sat in a chair or bending over. My tips are:

* Don't be afraid of collimation.. (It's easier than you think using the right tools...)

* Centre your Primary Mirror

* Leave to cool down at least an hour before use

* Use good EP's

* Get a longer locking base bolt (The standard one unscrews easily because it's too short)

* Flock your tube for around a tenner! Homebase sell great flocking material (you'll need 2 rolls)

* Make some large washers from plastic milk containers. Slip thru centre bolt in between base parts (makes it rotate smoothly)

* Glue a medium size piece of 2" thick foam to the inside of the base. (This stops your Dob banging against the base when facing towards the Zenith)

If I can think of any more... I'll Be Back! 8)

[John]

Thanks Tony4563 (or can I call you Tony ?) - those are really useful tips. The primary mirror is centre marked already thank goodness so that should make collimation easier - is that what you meant by centre your mirror or is that something else ?.

John

[FLO]

The nut that fits the centre/azimuth bolt (lazy-Susan bearing) tends to come undone so look out for a replacement. Also, the roller bearings are 'lumpy' at high magnifications and tend to run too free (just my opinion). I suggest you remove them (assuming they are there) and use regular Teflon pads. Also, cut a few washers (about 3-4" diameter) from some of those large plastic milk containers (the type on offer in the supermarkets) and place them around the centre bolt - keep adding them until you get a smooth resistance.

The collimation screws are a bit weak but that is more of a problem with the larger mirrors - Bob's Knobs offer replacements.

Lets face it - every Dobsonian needs some 'tweaking' and overall, the Revelation Dobs are very good

Oh, and I am not confident that the mirror cooling fan actually has an effect... I left mine permanently unplugged.

Hope that helps.

PS: Just noticed Tony's post but having typed this one, I'll post it anyway

[tony4563]

Yes, that's what I meant about centreing the mirror. Don't be afraid to tinker with anything. It's all pretty robust as long as your careful. Keep an eye on the 3 collimating screws on the secondary. In time they "drill" into the plastic secondary chamber, which is hollow, and eventually drill through. You then find when you need to collimate your secondary that your screw turns without doing anything. This is easily remedied by unscrewing the single middle bolt on the secondary (keep your tube horizontal!) and removing the actual secondary mirror (avoid touching the mirror) On the back of the chamber you'll see the burr holes that have been made whilst collimating. Simply glue a very thin but hard round metal washer over these holes. Leave to dry overnight and paint matt black. When dry, carefully attach your secondary back with the single bolt. You need to collimate again after this, of course. Piece of cake if you need to do it mate

[John]

Steve, Tony - thanks - great stuff !!

I'll look forward to hours of, prudent, tinkering - while the clouds clear !!.

I'm now going to work out how much extra light a 200mm gathers over a 90mm ..... should make M97 easy ?? - I don't think so somehow !!.

John

[GazOC]

I'm now going to work out how much extra light a 200mm gathers over a 90mm .....

John

A couple of (very) rough equations..

The image definition (contrast, sharpness) equivalent to that of an unobstructed instrument with effective aperture D_eff:

D_eff = D - d

The light-gathering power of an unobstructed telescope with aperture D_lg:

D_lg = sqrt( D^2 - d^2 )

[John]

Thanks Gaz,

I've just used the equations that you kindly provided and .....

the computer says ....... lots more

Seriously - thanks - I'll try them "in anger" when I've had some sleep !!.

John

[GazOC]

I forgot to mention, D=aperture and d=obstruction.

[John]

Setting aside the maths, for now, does a central obstruction imact resolution, contrast or light grasp more ?. The 5inch Celestron SCT that I used to own seemed to produce images that revealed more detail than my 4 inch refractor, despite the fact that it had, I believe, one of the largest central obstuctions of any SCT as a % of apeture.

I suspect this question has a lengthy and complex answer .....

John

[geppetto]

Adding setting circles to the DOB is a worthwhile mod John

It's a fine scope, you'll love it 8)

[anthony]

I am sure that this will be pointing out the obvious , ( and so apologies in advance), but........

tinker at the open end with the tube horizontal when you can. Dont want to drop anything on the primary!

Anthony

[FLO]

... does a central obstruction imact resolution, contrast or light grasp more ?.

It mostly effects contrast.

That said, it is not a big issue (unless you have just spent a fortune on an APO refractor).

[John]

I am sure that this will be pointing out the obvious , ( and so apologies in advance), but........

tinker at the open end with the tube horizontal when you can. Dont want to drop anything on the primary!

Anthony

Thanks Anthony - perhaps I'll put a sticker on the scope to remind me of that !!. I dropped an allen key onto the corrector of a SCT when collimating in the dark once - no damage but I felt awful at the time - I ordered a set of "Bobs Knobs" for it the folowing morning !.

John

[daz]

I dropped a screwdriver bit onto my mirror because of not doing this.

I still cry about it............

[Kaptain Klevtsov]

That said, it is not a big issue (unless you have just spent a fortune on an APO refractor).

Captain Chaos

Don't you ever wish that they'd invent an achromatic reflector to level the playing field?

[John]

Don't you ever wish that they'd invent an achromatic reflector to level the playing field?

Maybe those short tube newtonians with a "correcter" lens built into the focusser tube are an attempt to do just that - they never seem to get good reviews !!.

When I get my Revelation 8 inch I guess I'll be the proud owner of 90mm and 200mm APO's :p

John