rl

Possible extra issues starting AP with a big newt.

• Collimation. Not just the usual visual collimation. The focal plane should be perfectly square to the camera sensor and not tilted. The problem gets more difficult with faster scopes because the focus blurs more quickly as you go away from the focal plane. . Some focusers allow you to set the "squareness".
• You will have to guide. OAG usually necessary to handle differential flexure, but typically harder to adjust and limited choice of guide stars compared to a guide scope.
• Mirror flop during meridian flips. Your OAG will help a lot in this regard taking out slow changes but it won't cope with sudden movement of the main mirror
• Focus tends to change more with temperature. Not specifically a Newt issue but small refractors seem to do better in this regard. Pricier Newts come with carbon fibre tubes for this reason. 
• Big tubes are more susceptible to wind. Not as bad as a 6" f/10 refractor though...
• The longer focal length makes finding targets more difficult if the mount alignment isn't on the money. The big sensor in a D300 will help here. 

None of it is particularly difficult to get right once you know about the potential issue..it's just more to go wrong when you're coping with learning all the other stuff. 

Best of luck..let us know how you get on...there is always plenty of help on this site.

I've got the Skywatcher f/4 aplanatic coma corrector which I think is the same as yours from the description. Baader do a useful variable spacer #2956929 which covers 20-29mm and works fine on my OSC camera...assuming your camera focal plane is the standard 18mm deep, and you're not having to squeeze in a filter wheel as well.  The OAG is 11mm thick so the spacewr is set to 55-18-11 = 26mm. I think they do other ones as well. Just tweak for best results. 

The learning curve for astrophotography is steep, painful and expensive!

Personally I'd get the 200PDS and keep the load lighter on the mount,and put the difference in price towards the extra bits you will need. You will also need to buy an off-axis guider setup and guidecam, and a coma corrector. The spacings are a bit critical so do some research before randomly choosing the bits hoping they will all bolt together and just work. 

Many good shots have been taken with the 300D...mainly 20 years ago when it was state-of-the-art. You might want to find a SLR with live view facility which makes focusing much less of a pain. I think the 450D is the earliest with this option. 

Don't forget the usual way in to AP is with a short focus 80mm refractor for lots of good reasons. You can just dive in with a big Newt but you will hit lots of separate issues to be solved all at once. But it can be done...

Just had to look that one up. That's a big boy's big budget coma corrector. It's almost insulting it to stick with 62 degrees...Is it worth looking at a Delos or Ethos?

Can't really go wrong with either. Pentax has wider AFOV, is less bad value for money. Delite has possibly better edge correction in fast scopes and flat field plane which may be a plus depending on what coma corrector you're using, if any, at this high magnification. 

I like the Delites very much, especially the lockable eye guard, but in this case I'd take all the AFOV I could get just because it's a dob at high magnification. Although I guess it depends how much of that field is still good at F/4......John is probably the oracle on this one. 

I've found the statement to be essentially true. Polar alignment is good to better than an arcminute if done with care. On my AZ-EQ6 I simply align on whatever star is easily available near the celestial equator  and after that virtually all objects in the same E/W hemisphere are on my camera somewhere even if it's not exactly in the middle....I'm using a QHY183 at 900mm focal length which is approximately a degree across.  

The only time things sometimes go wrong is after a meridian flip. Stuff moves in a Newtonian.

I bought mine in a fit of frustration when I could not get the mount to align properly, and fully expected a bunch of buyers' regret the following morning. It's certainly very expensive for what is basically a webcam and a bit of code, but I've found the results far more reliable than using the handset and polarscope.  The software has the nice characteristic of doing one thing and one thing only, and doing it well, which makes it comparatively bulletproof. The few crashes I've had have been down to too many other things on the same USB port. 

Mine won't be on ABS any time soon....but anyone reading this thinking about buying one might do well to investigate the finder/ guidecam/ sharpcap option before splashing the cash. Having said that. it is nice to have it set up and ready to go without reassembling the finder and realigning all the time. First world problems....

It's worth keeping this in context; the star in question is Deneb which is 1st magnitude and very blue..the B-V colour index is +0.1 all of which means a lot of blue light. It's a tough test for any doublet system when you're trying to bring out faint nebulosity. 

Cooling the camera allows longer exposures because the dark counts drop by half for every 7-10C cooling. But the maximum capacity of the pixels remains unchanged.....if a pixel saturates out at 20,000 captured photons at 20C it will still saturate at 20,000 photons even at -20C. Once a pixel is full that's it....you're only option is to allow in less light. 

I guess this is the  reason people invented triplets...the colour correction is better, especially for photography.

I'd try halving the sub time just to see if it's some sort of saturation issue on the camera. A luminance filter that cuts off sharply in the violet might help.

That's a nice shot seen in context.

Assuming the Bahtinov mask is matched to the scope then the focussing should be spot-on..in which case then yes, you're seeing residual CA, which is a little bit strange because it's not the fastest 80mm doublet out there. The slower ones usually have CA better controlled.  Is yours the original Starwave 80ED? I note they now advertise a Starwave 80ED-R which claims "better colour correction in the blue regions of the spectrum" which probably tells you something.

https://www.altairastro.com/starwave-80ed-r-ed-doublet-refractor-telescope-466-p.asp

Difficult to say from a screenshot, but that looks like over-exposure to me. You might have some focussing isssues as well. 

Is the image a crop or the whole frame? What camera and exposure time were you using? What is the target? Were you using any filters?

An ED scope with FPL53 is unlikely to have objectionable chromatic aberration unless there is something seriously wrong with it. 

Some doublets do show minor CA occasionally referred to as blue bloat..but that looks a bit excessive! 

Using your figures, a back-of-envelope calculation suggests light at 500 nm would be shifted by 1.5 nm for the "local" galaxy and 10 nm for the others...not sure whether that would be visible or not. Probably a younger stellar population. 

Very nice shot. I've always wanted to take this picture myself. NGC7320 (foreground galaxy) is definately a bit bluer than the members at 300M light-year. I wonder if that is down to the extra red shift?

15 minutes ago, tomato said:

This one gets my vote, what with speculum mirrors that tarnished before your very eyes, and surely the most hard to use, restricted movement mount ever devised, plus siting it in the middle of Ireland, I truly marvel how Lord Rosse and co-workers ever managed to see anything at all. 

 

 

I can see a few plusses though...

No problems with planning permission..

Even an ES 30MM with coma corrector won't upset the balance..

No meridian flip...

No barlow lens required..

The gantry up to the eyepiece has a certain je-ne-sais-quoi..

Bausch and Lomb Criterion 4000. by a country light-year.  Mid 80's vintage, built without any regard at all for quality control, optically an utter dog of a scope.  Diffraction patterns like a thumbprint. Strangely the mechanics we actually quite good. 

 I kept mine for 10 years knowing I could not sell it on with a clear conscience.  Eventually I binned it down the local tip and actually relished the sound of breaking glass as I chucked it in, knowing I had an excuse to go and get something decent! Put me off SCTs for life.

I'm actually looking at this post in isolation....my own kit runs from a laptop PC switcher power supply that gives 12v at 8.3 amps. In view of its intended application all the safety stuff is catered for...it's double insulated and has no ground of its own, but the output side has the negative grounded through my own wiring. All you need is a regulator to the camera voltage.

I'd try a decent switcher first. Preferably a flyback or forward converter if you're really worried....a buck converter has the same issue as a linear in that a failure of the series transistor will be catastrophic. But having said that I have designed loads of buck and linear regulators for oilwell applications that work in incredibly harsh environments ( to 200 centigrade) and have never had a problem in literally thousands of production units. It's important to keep a perspective here..a good design will be very hard to break, either linear or switched. And a commercial unit made by Astec or Traco run within its ratings that sells by the million and has been in production for a few years will be a good design by now. And a crowbar will protect in the unlikely event of a failure.

Just don't buy the cheapest!

Following up on a different point about adding a diode in series for reverse protection, you do indeed get 0.5 to 0.8 volts lost depending on the current draw. This can be halved by using a Schottky diode. 

I've always done this and the volts drop has never been a problem. I've never understood why Skywatcher don't just do it themselves. The mount is normally specced for 12v, a car battery charged will give about 13.5....the loss is already taken care of until the battery goes flat at 12v. If you're designing your own regulator, setting the output to about 13 should be ok for everything. Just make sure the diode current rating is high enough to handle slew currents with the mount a bit off balance. If it looks like the diode loss really will be a problem I can show you a trick with a transistor that protects without the voltage drop.

I should have added "beware laser collimation tools".......the principle looks great at first sight but unless a lot of geometrical details are correct it's easy to get the wrong answer with them. I've always found a good Cheshire eyepiece to be more reliable.

Too late now!

Is there a local friend or society than can walk you through it?

It's fairly common for Newtonian scopes to arrive with the collimation slightly out. They just don't travel well....part of life with a Newt is learning how to twiddle the knobs and screws to get the best out of it. It's not that difficult after you've done it once...look upon it as the astro equivalent of learning to ride a bicycle!

Before touching any of the screws find out what stars look like on a well-collimated scope and see how yours compare. A bit of reading up front will pay dividends..don't dive in first before you're sure of what you're looking for.  And then make the adjustments in small steps. 

The main mirror actually needs to be very slightly loose or the locking screws will bend the shape. The secondary supports are often a bit flimsy in order to obstruct the minimum amount of light, and to minimize the faint "cross" effect caused by diffraction. 

There are articles on this site of how to do it. 

Collimation is one reason why a lot of people prefer refractors...they are pretty much adjustment-free. But Newts show a lot more per pound of investment once you've learned to get the best out of them . 

You may well find Jupiter and Saturn look even better with the scope collimated..

That looks to me like it's out of focus, and out of collimation. At focus a star should be like a bright point of light too small to see the middle, no black dot. At low/ medium magnification it really is just a point.  

As the magnification goes above 100  you might start to see a bright central dot with a few very faint rings due to the wave nature of light (called a diffraction pattern).

Out of focus the black dot should be in the centre; it's due to the secondary mirror blocking light. 

It's worthwhile reading up on collimation. 

Buck converters and flyback converters are cheap (reasonably), very efficient, widely available..lots of plusses. They still suffer slightly from output noise at the chopping frequency which might get in to the kit and cause banding issues on images....might be worth looking at some filters just to make sure what goes in to the camera is really clean. It's the one area where a linear regulator might still have a place.

I always use the metal box versions which helps contain noise issues. 

Mounts and dew heaters don't care about any reasonable amount of noise. 

I know, I know....I just couldn't resist it. I'm hardly free of refraction affliction myself!

Very nice collection...add them all up and you've got about the light grasp of a 12" Newt....

Quote

Can you confirm the Luminos's viewing comfort without eyeglasses?

Louis, Azrabella

That's the puppy. The eye relief is not enough to use with glasses, but without glasses I find it rather comfortable. I can see the whole field easily without my eyelashes going anywhere the glass. The eye relief will be whatever Celestron say it is....

There is not a lot that's any good at 82 degrees for under a ton new.

Having just bought a secondhand 14mm Delos to replace a 15mm Celestron 82 degree, you would be welcome to try the 15mm on a sale or return basis if that helps. I use f/4 or f/4.5 scopes and it's not the best but it should work a lot better at f/8. 

Beware weight and balance issues; your f/8 dob might be more suceptible due to its extra length.

RL