Orion Optics 300mm f6 updates

[Stu]

I'm in the middle of some updates to this scope so thought I would document them here.

I bought the scope used some time ago, it was shipped down from Scotland which was a bit hairy but it arrived basically in one piece. The only damage was to the secondary support which had a broken tensioning thread on one of the vanes which was easily repaired.

The scope was sold as 1/12th wave. I confess I always had doubts about this spec because it is above the normal Ultragrade OO spec. I always intended to get this checked out though.

Once I received it, I tried the scope on an AYODigi mount, seated on short pier which worked to a degree but was not so stable. The first upgrade them was to buy an OD mount from OO. Ultimately this turned out to be very good, but it needed some minor modding in the form of spacers to

prevent the trunion blocks rubbing on the uprights. Frustrating as a minor design change could easily avoid this.

Having used the scope successfully for a while, I recently embarked on further updates.

The primary and secondary were sent away to OO for testing and recoating with HiLux coatings. Barry at OO remembered the mirror (amazingly), as a special order back in 2004 making it 11 years old. He was concerned that it may have shifted in figure over the years, but ultimately it came out at 1/11th wave, 0.993 Strehl so is still a cracking mirror. The secondary is of similarly high standard.

With the mirrors out, I decided to upgrade the focuser to a Baader SteelTrack which was intended for another scope but will suit this one very well. I have a Steeldrive unit I can use on it too to get fine focus without vibrating the OTA.

This evening I completely flocked the tube, a fairly major undertaking but it all went very well and I'm happy with the result.

The scope was supplied to me with two secondary mirrors, a 50mm and a 60mm. The 50mm is best for ultimate contrast but won't fully illuminate a 2" eyepiece. For the moment I am going to refit the 60mm on the 4 vane spider and have ordered a curved vane secondary support from Protostar for the 50mm.

Once received and fitted, I hope to have a top notch planetary newt. 300mm f6, 1/11th wave optics, sub 3% obstruction by area, flocked tube and curved vane spider for eliminating diffraction spikes.

The mirrors are due back this week, probably won't fit them until next week some time but I will update this thread with pics and further info as I proceed.

Stu

[dweller25]

Thats going to be one big planet killer .......

[jetstream]

I second Dwellers statement. Very Nice scope Stu.

[John]

It's going to be a cracker Stu. Another cracker of course - you already have a few !

[Ajohn]

Sounds great but I would check the 2ndry sizes you have. It's % diameter that matters not area. As the % diameter goes up contrast drops. Both mirrors are pretty good in that respect even the 60mm which comes out at 20%, a figure that some one might just choose to use as it will give acceptable results with very little difference to the smaller one which comes out at 17%. People sometimes choose a size that just covers the moon, bit more than 1/2 degree and use that for planets as well. For wider fields the main limitation is the quality of the image further and further off axis. Your scope for instance will show 0.1 mm radial fanning on stars at the edge of a field of 1.35in covering just over a degree.

There is a decent article and little program for working out 2ndry mirror sizes at the bottom of this page   not sure I would get too excited by the bent mirror program unless some one wants to try doing that for fun.

http://www.skyandtelescope.com/astronomy-resources/freeware-from-sky-telescope/6/?c=y

The program works out 2ndry sizes that will dim the edge of the field by 30%, a sort of pro standard.  An even bigger 2ndry wont dim at all but will drop contrast more so it's a good compromise. It also helps the off axis image problems a little as well because the light at the end of the fanning is very weak. The drop off in light levels isn't too important visually as we look round big fields and our eyes adjust. I suspect some manufacturers take liberties at time due to that. I'm thinking of a certain Meade newtonian that I might sort out one day. The 2ndry is so small that it has problems covering the whole mirror even on axis. It's an F4.5 scope. I've no idea how common this is. I'd guess that the approach can mask mirror figure problems to some extent.

Maximum sizes on compound designs such as SC's often go up to over 30%. Specified as an area they come down to ideal sizes % diameter wise which I feel is misleading. Major observatory scopes too. This size  is another compromise. The airy disk centre has around  82% of the light it should have in it. The missing bits go into the rings spoiling contrast. It's noticeable visually on some objects especially weak low contrast detail in planets.

John

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[Stu]

Thanks John, interesting article. This scope will be primarily for lunar, planetary and small DSO viewing, definitely not widefield so I am less concerned about vignetting in 2" eyepieces.

I'm not sure I follow your point about diameter vs area, surely both are relevant as they are totally related?

I understand that faster imaging scopes often have larger secondary obstructions but as this is for visual I want to keep contrast to a max plus star shapes with max light in the airy disk.

I have these two secondaries and won't be buying another, they were specced with the scope originally so should be fine. I will try the 60mm on the 4 vane and see how I get on

Thanks for your input, very useful :-)

Cheers,

Stu

[Ajohn]

I believe once in days of old when knights were bold scopes usually specified the % diameter of the 2ndry mirror. People were or often are aware that increasing diameter degrades contrast so when compound scope started appearing they started specifying it area wise which oddly enough on most designs brings the number down into the 20 - 25 % range which in terms of the diameter way makes the number look better.

Like most things though it's swings and roundabouts. Some one might say I don't want to have an SC for instance because calculated the right way the central obstruction will come out at about 30% and a bit.  All it means in practice is that the scope will have less contrast than one without or with a significantly smaller central obstruction. The resolving power will be more or less the same as a scope without an obstruction. That's fine for splitting stars but resolving power expressed that way isn't so good on things like planets. At Rayliegh's limit contrast is only about 7 1/2 % of what enters the scope. What is more important for that sort of thing is when the contrast has dropped by say 50% or some such figure and that is where central obstructions have more effect, Mirror errors too up to a point as well. There is a decent page here that elaborates on the effects. It needs MTF plots to show it. The MTF curve is the same for all perfect scopes and shows resolution across the bottom and contrast on the vertical axis. That way fractions (coarser to the left) of any maximum  resolution can be shown on the bottom scale for any scope. So the 1 for instance relates to when contrast is zero and the actual resolution that this implies is set by the diameter of the scope.

http://www.damianpeach.com/simulation.htm

When looking at pages like that it's best to bear in mind that people use equipment that on that basis is NVG but they still get decent or even spectacular results. For that reason I wont try to find one showing what happens off axis on a Newtonian. It's pretty bad and gets worse as the size goes up and as the F ratio goes down.  A compound scope can do something about that - more swings and roundabouts.  Looses out in one area and gains in another. That's life with telescopes.

John

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[Stu]

Well the scope is all back together, wanted to get the mirror safely back in as soon as possible. It looks brilliant, and centre spotted too (at my request), which saves me some stress!

Now comes the challenging part of trying to collimate from scratch. Some reading over the weekend then will try next week I think.

[jetstream]

Your 60mm is a fantastic size for this scope Stu offering a very small CO for this aperture. I like OOUK's extension adapter ( focal point) as it allows for a bit smaller sec size, the 60mm will be the best of both world in this scope IMHO. Below a certain size of CO contrast loss is minimal. I use the 63mm sec on my 10" and this scope has VG contrast if I may say so myself.

Bigmak your newly coated high spec mirrors are going to be "wicked" on the moon and the planets...

[faulksy]

I'm in the middle of some updates to this scope so thought I would document them here.

I bought the scope used some time ago, it was shipped down from Scotland which was a bit hairy but it arrived basically in one piece. The only damage was to the secondary support which had a broken tensioning thread on one of the vanes which was easily repaired.

The scope was sold as 1/12th wave. I confess I always had doubts about this spec because it is above the normal Ultragrade OO spec. I always intended to get this checked out though.

Once I received it, I tried the scope on an AYODigi mount, seated on short pier which worked to a degree but was not so stable. The first upgrade them was to buy an OD mount from OO. Ultimately this turned out to be very good, but it needed some minor modding in the form of spacers to

prevent the trunion blocks rubbing on the uprights. Frustrating as a minor design change could easily avoid this.

Having used the scope successfully for a while, I recently embarked on further updates.

The primary and secondary were sent away to OO for testing and recoating with HiLux coatings. Barry at OO remembered the mirror (amazingly), as a special order back in 2004 making it 11 years old. He was concerned that it may have shifted in figure over the years, but ultimately it came out at 1/11th wave, 0.993 Strehl so is still a cracking mirror. The secondary is of similarly high standard.

With the mirrors out, I decided to upgrade the focuser to a Baader SteelTrack which was intended for another scope but will suit this one very well. I have a Steeldrive unit I can use on it too to get fine focus without vibrating the OTA.

This evening I completely flocked the tube, a fairly major undertaking but it all went very well and I'm happy with the result.

The scope was supplied to me with two secondary mirrors, a 50mm and a 60mm. The 50mm is best for ultimate contrast but won't fully illuminate a 2" eyepiece. For the moment I am going to refit the 60mm on the 4 vane spider and have ordered a curved vane secondary support from Protostar for the 50mm.

Once received and fitted, I hope to have a top notch planetary newt. 300mm f6, 1/11th wave optics, sub 3% obstruction by area, flocked tube and curved vane spider for eliminating diffraction spikes.

The mirrors are due back this week, probably won't fit them until next week some time but I will update this thread with pics and further info as I proceed.

Stu

glad your mirrors are back and in great shape. now the most important question are you taking it to sgl 10

[Uplooker]

glad your mirrors are back and in great shape. now the most important question are you taking it to sgl 10

If so, I think Stu will have to spend more money to change his car, not sure a 12" scope and all his other bits and bobs will fit in the BM. Happy to be proved wrong ;-)

Ian

Sent from my Fone

[John]

Your 60mm is a fantastic size for this scope Stu offering a very small CO for this aperture. I like OOUK's extension adapter ( focal point) as it allows for a bit smaller sec size, the 60mm will be the best of both world in this scope IMHO. Below a certain size of CO contrast loss is minimal. I use the 63mm sec on my 10" and this scope has VG contrast if I may say so myself.

Bigmak your newly coated high spec mirrors are going to be "wicked" on the moon and the planets...

My 300mm F/5.3 has a 63mm diameter secondary (minor axis) which is 21%. A good compromise size I guess as I use the scope for both high power and wide field observing. It's the size that OO currently still fit to their 12L newtonians as standard.

[Stu]

If so, I think Stu will have to spend more money to change his car, not sure a 12" scope and all his other bits and bobs will fit in the BM. Happy to be proved wrong ;-)

Ian

Sent from my Fone

Unfortunately you are completely correct Ian. The 300 just doesn't fit in my car at all!

I'll bring the Canopus and if at all possible the Vixen, plus a few other solar bits and pieces.

[Stu]

My 300mm F/5.3 has a 63mm diameter secondary (minor axis) which is 21%. A good compromise size I guess as I use the scope for both high power and wide field observing. It's the size that OO currently still fit to their 12L newtonians as standard.

Thanks John. I'm going to see how I get on with the 60mm then when I get the curved support have another play with the 50 as a planetary 'special'

[jetstream]

My 300mm F/5.3 has a 63mm diameter secondary (minor axis) which is 21%. A good compromise size I guess as I use the scope for both high power and wide field observing. It's the size that OO currently still fit to their 12L newtonians as standard.

Now that I've learned a little about scopes I can see why you own this telescope John- it offers no real compromises and is good for everything, from 1.25 -2" EP's, planetary, DSO and coma is well under control @ f5.3. If ordering a dob again I would choose your scope over mine. Great choice John.

[Stu]

A few images showing progress.

Flocking.... Not too tricky because the tube is a reasonable diameter but it's long! Getting to the centre was tricky and involved the use of a wooden spoon to smooth it out!

An exciting looking package!

Lovely mirror!

Don't drop the phone!!

Installed in the scope

All back together again

Just the collimation to sort now. I've fitted a new focuser so hope this doesn't cause issues. I had a tinker with centring the secondary but didn't manage to sort it so that's a task for next week.

I know what it should all look like, the VX10L is bang on in all respects, Mike did a great job fettling that scope!

Stu

[jetstream]

Great flocking job Stu. The sec is so close just gotta, pull it up toward the front of the tube a squeak. Without starting a huge "what collimation tool is better debate" I have to say that the Catseye sight tube and separate Cheshire are great, easy to use and well built. Less busy visually than a combo tool. You would be properly collimated in about 3 minutes with a set on your first try, seriously. No need for an autocollimator even though I like mine and the challenge it presents.

This scope, once cooled will be amazing on everything!

[Stu]

Cheers Gerry. My collimation skills are not what they should be, although much better than they were (ie non existent!)

I have all the tools I need, sight tube should help me sort the secondary. I'll get there!!

[John]

Good work Stu

I've just done the tube wall opposite the focuser and and about 10" above the primary with mine.

Now that I've learned a little about scopes I can see why you own this telescope John- it offers no real compromises and is good for everything, from 1.25 -2" EP's, planetary, DSO and coma is well under control @ f5.3. If ordering a dob again I would choose your scope over mine. Great choice John.

Thanks Rob. When the OTA came up for £250 not too far from me I could not believe my luck

The 12" dob is my lowest cost scope !

[Stu]

Good work Stu

I've just done the tube wall opposite the focuser and and about 10" above the primary

John, I wish I could just have done that but my OCD just wouldn't allow it!! :-)

[John]

John, I wish I could just have done that but my OCD just wouldn't allow it!! :-)

I have OCD on eyepieces but not so much on scopes, oddly enough !. Probably got the priority the wrong way round

[mapstar]

When's first light Stu 

[Stu]

When's first light Stu

Next week Damian. I'm a bit busy Tuesday/Wednesday but will have a go at collimating it on Monday so hopefully it will be ready for next weekend

[Stu]

Well first stab at collimation done, I think it's reasonably good. Not totally sure my secondary vanes are in line but I seemed to need to move them to get the secondary centred. Will have to see if there is any play in the Focuser position I guess?

Anyway, I set it up using a sight tube and Cheshire and then did the last bit with the laser and it seems pretty close for the moment. Enough to first light it later in the week anyway.

[Stu]

Well, just managed a very sneaky and unexpected first light and I'm pretty chuffed I must say.

Jupiter poorly positioned over the neighbour's house, but still very nice views.

Using the Leica which gives a very nice range of x101 to x202 in this scope, the resolution was clearly a step up from the 10", even though that is a cracking planetary scope.

Quick look at the moon too, showing plenty of detail, a couple of nice rilles on display.

Still a little work to do on collimation but basically there, as hoped.