jetstream

8 hours ago, Piero said:

one might think that the 24" is actually not that big!

Here is an 8' ladder for size reference, the scope is 8' 9" tall as it sits in the picture..

16 minutes ago, estwing said:

Dont think either of your scopes would look outta place between the Dob mobs..!

Left to right..mapstars 22".Mike's 20".my 18".Steve's 20".

Great scopes Calvin! Awesome shot.

Question, what do you guys use for DSC and how do you mount the "box".

Just think- you could have crosshairs on that screen that move when you move the scope

My finding goes from memory, look at the map then out we go, obviously there are limitations lol! The Sky Commander might impact true dark adaptation though...

Is your red screen emitting polarized light?

As a purist I would have agreed until yesterday. I have problems finding objects such as planetary nebs that are not near constellations or bright stars. Galaxies too. I'm glad I've learned the sky well and the approach I took but for me now the I'm going to use the DSC and might put encoders on the 15" as well. I believe using DSC will help me learn more of the sky, we'll see.

Yes, the 24" is quite big lol!

22 minutes ago, Ships and Stars said:

Hi Jetstream, I use a Cheshire eyepiece for the secondary, and a cheapo laser for the primary, seems ok but the endcap with on/off switch has movement affecting the laser. I'll probably get something better, hotech or similar. The scope is F4. Don't have much time on it yet! Waiting for mid-August to start DSO hunting again. Thanks for the information!!

F4 likes (needs) good collimation.

I am not an expert, Khadder and Menard are...but...

We could discuss the attributes and flaws of different methods- up to you. Or, I can relate my experience with different methods. A sight tube is needed to center the secondary, it automatically sets the offset sec towards the primary and ensures it looks "round" under the focuser. Your cheshire will do this. I do this.

An accurate laser is better than the cheshire/sightube crosshairs IMHO to line up the sec to primary. I use this.

As Ricochet rightly states the primary needs a barlowed laser to collimate, when using the laser method. I use Glatters Tublug so I can watch collimation from the back of the telescope, very nice.

Thing is your Cheshire itself is great for it all... an accurate laser offers an advantage over the crosshairs on the sec IMHO. The Tublug/laser is a great, accurate way to conveniently line up the primary.

HTH.

That is the Sky commander wiring- I need to see what other dob members use to hold the commander box, I can see that ground level mounting might be best- does it ever work good! and simple, just what I need lol!

I have 4 batteries in the mirror box for lots of power, so far no dew heater, just 4 fans and the Commander. Randy gave me a charger, 12v accsy plug, all cables etc. Som,e say nexus is better but I'm satisfied with the Sky Commander.

Will you put encoders on your scope Piero?

52 minutes ago, Paul73 said:

Hi Folks

I’m a bit at sea on the subject of mirror quality. I have read about 1/10 PV, Strel ratio and smoothness. What measure is most important?? Or, just point me to the idiot’s guide!😁

Paul

Paul, there are a few places that give good info, Lockwood is one- a trusted, excellent optician.

http://www.loptics.com/articles/fourlessons/fourlessons.html

Great choice!

Suprax is excellent, the jury is out on quartz for a variety of reasons. I just had a long chat yesterday about glass with someone in the know.. my 15" is Zerodur if I remember. As long as we use good glass, with no residual strain alls well.

We must trust the optician and he/she must have pride in there work. Your specs increased the chance of great specs hugely Piero and I believe your mirror spec might be understated. The real interferometer data is from pictures- I've seen my 15"'s and am told if the lines are straight alls well. That simple.

I don't blame top opticians for not providing interferometer data- it is hard to accurately replicate by others possibly and only promotes "armchair" discussion. For me the real test is under the stars and one simple thing seems to be apparent in good optical system and that is the snap focus. We keep in mind that eyepieces affect this too but you and I have narrowed down some EP's that have great snap themselves. ie ZZ, Docter.

I will be surprised if your new scope isn't a top tier performer. f6 is just so nice..

Dew heaters can cause secondary astig if not installed right - just a heads up and I have zero experience with them.

You can see the boundary layer "crawling" across a mirror if you defocus a bit- boundary fans can cause issues and might blow dust across the face of the mirror. I would try those 3 rear fans first and collimate well.

What do you use to collimate? what f ratio is it?

10 minutes ago, Piero said:

Congratulations, Gerry.

I'm glad that you had a fantastic view. Thanks for sharing this report.

Thanks Piero,

Its all coming together and working Piero, the brighter objects are very bright in this scope- it frames objects like the Swan so well and the Lagoon was spectacular too.

Lots of ideas out there but Mr Greer has done a lot of testing- this site has much good information. Microblur, boundary layer (it doesnt exist when equalized) and much more.

https://www.fpi-protostar.com/bgreer/fanselect.htm

As I report on how my new scope works it might be useful for members to see what spec mirror this is. It goes a true 1/6 PV with high strehl and is smooth (important). The secondary is a very high spec Antares, not sure where that paperwork went.

Ostahowski is a maker who's numbers I believe, Mr Nicols would be another.

I think its important to know that the interferogram is one of many tests that good opticians use- and that the others are important as well. The debate over smoothness can rage on among some, but for me it is an absolute requirement that the mirror is smooth- AFTER COATING as well as (obviously) before.

When asking for a mirror I am flexible as far as f ratio and thickness- this 2" thick 24" was exponentially easier to get the figure right than the thin examples and I'll deal with the equalization issue, so far so good!

5 hours ago, Stu said:

I have a focal length spec but it might be worth checking this for accuracy. I guess this is doable?

You can temporarily install the mirror and secondary, collimate (doesn't need to be that great) and then use a light source (I've used the moon) to find the focal position. You can use a piece of bristle board mounted on a moveable bracket- or with help, someone holding it- just hold it way from the focuser until the moon (or source) focuses on the board and measure this from the focuser.

Then measure from focuser to sec, add this number and then measure from sec to primary- add it up and there it is.

When doing this it helps to install the mirror higher than normal- pushing out the focal position away from the focuser. I'm sure there are more fancy options to do this but its worked for me.

Last night the sky gave avg conditions with the MW showing a bit of structure and the split in the southern portion was easily visible. Cleardarksky rated the NELM at 6.3. Seeing seemed pretty good.

I'm still shocked this morning!

M17 was an absolute treasure in the eyepiece, unbelievable. Sometimes views grab you and stay with you- this was one of them! Large, bright, showing mottled green (UHC) and structure in the object, the neck area was one of them. This object was VG in the 20mm Lunt HDC but the 17mm Nikon HW stole the show, hands down. Upping the mag using the 14mm HW option diminished the object- fine tuning these objects is crucial IMHO...

speaking of which- I inadvertently had the 14HW in trying IC1318- which shows great in this scope, well the 14mm just killed it..so important to try different things to see what works and what doesn't.

Jupiter is very l;ow obviously but the 24" gave great views up to 275X+ a bit using the Zeiss Vario zoom, same as @Piero. Very good contrast, detail, and color- I just can't wait until its higher.

This was my first use of DSC- thanks Sky Commander! very accurate, many new (to me) objects found including some great planetsaries such as NGC 7008- the Fetus nebula- I has structure showing and once my excitement settles down I'll fine tune these objects- this view of it was an excellent beginning!

What a fantastic 1.5 hrs of observing!😀

ps- who put that bright galaxy with its glowing core and nice elongation next to M13?

Not the one asked but all I can say is that I'm extremely pleased with the TSA120- it will not show more than my dobs though- last night around 275x I had a remarkable view of Jupiter with the 24".

However, the refractor will give very pleasing views and some of them offer seemingly endless mag (TSA120 etc) so when seeing is good you have a great option at your disposal. To me the fracs give an aesthetically pleasing planetary view and a good mount (thanks @Stu) with tracking gives a big advantage IMHO.

50 minutes ago, Stu said:

Thanks Gerry. Yes I hadn't thought of that but think I could do.

Any advice on working out where I need to place the primary vertically? I guess I need to work out where my focal point needs to be relative to the focuser then work backwards as Peter suggests?

Holy cow Stu! I just got in -1:30am here- what a night! M17 stole the show... unbelievable.

Do you have the focal length of the mirror?

If so calculate "L"- set the focuser in the middle of its travel and measure the distance from the top of it to the center of the secondary. Subtract this off of the true primary focal length and then set the cell triangles this distance (from center of sec) plus the thickness of the mirror, should work. When doing this set the collimation bolts in the middle of their travel too.

Eagerly waiting reports Stu!

12 minutes ago, estwing said:

To use that scope...I'd take the risk.!!!;)

I'm in too!! at 100kg lol!

Great Stu!

Can you use a square on the top of the mirror box with the hole to define the hole edges below? once done you can locate the cell so the mirror is lined up with the mirror box opening(?). Maybe using a cardboard "mirror?

Here is the 15 f4.8" and 24" f4.1 for size comparison..

3 hours ago, Paz said:

That's a big scope! I'd be interested to know the focal ratio, and also the weight of the mirror box. I've picked up a 16" box in the past which must be a lot lighter but even that was tough.

I easily lift my 15" Astrosystems box, mirror and all. The 24" goes 220lbs or so with the cell, bearings, mirror box and 80lb mirror. I can lift it all once off the rocker but need help if on it- we just did that to adjust the encoder pivot. Thankfully a friend happened by for a coffee.

The mirror is f4.1 at 2499mm fl.

8 hours ago, niallk said:

I didnt get what you mean about the 'not closed' mirror cell - any chance of a pic please?

Thanks Niall, the mirror cell looks like there is little ventilation, but there is... it is calculated out I believe and the 15" with similar design can hold over 700x on lunar when seeing is excellent. The front on these are open. Very, very little stray light affects this scope. If I take it down again and with the mirror out I'll post pictures, for now I just want to observe lol!

8 hours ago, MSammon said:

Want to get better with my current one first though.

Great idea, I would concentrate on collimation and cooling- IMHO you need a cheshire, very easy to use. All scopes need to be equalized in temp - how long do you put your scope outside before observing?

When collimating with a star test very high mag is needed- just using the sec shadow isn't good enough IMHO.

The contrast loss from miscollimation and heat can be staggering..

2 hours ago, MSammon said:

Problem is it’s just basically a bright ball of light for Jupiter. Saturn is okay, a bit bright still perhaps.

Just curious- what do you collimate your scope with?

2 hours ago, Piero said:

We can talk in more detail if you like.

Sounds good- and it might be informative for others to banter here a bit- such as...

Can you tell me the measurement from the center of the sec to the focal plane (roughly)? or to the top of focuser? I'm always curious as to where makers set the fully illuminated field vs central obstruction %. I'll run Mels calculator, both my 15" and 24" have nicely illuminated fields and run 20% and 19% roughly.

More interesting stuff- this graph shows the effect of 1/4 wave spherical aberration (unobstructed) vs perfect optics with 32.5% central obstruction. It tells us that obstructed telescopes need good optics and that less than perfect unobstructed optics give no where near the performance expected. Note the similarities in where the sag occurs in the graph and corresponding frequencies.

All these things will "stack up" reducing performance available. Personally I like obstructions less than 25% and good optics from either obstructed or unobstructed scopes.