Yes this is a really useful thread, dare I say 'award winning! 😁
I've heard from one technical expert how bad Orion Optics can be, having to recoat a new expensive mirror! And another industry supplier saying pretty disappointing things about their tactics. Just rumour of course, but enough for me to be glad I've never used them.
At these longer focal lengths, once off focusing at start is not possible. Temp changes cause the focus point to vary too much
My CDK17 has a carbon fibre truss. Once the mirrors have equalised in temperature its focus does not shift hardly at all. In fact the next night it will still be almost exactly in focus much like RCOS used to promote with their scopes. Still in focus the next night.
So focus shift may be true of some scopes but not the CDK or RCOS in my experience. I still check and refocus after a substantial temp shift and I can get a slight improvement but its a small gain. An FSQ shifts the most of any scope I have used. The AP Honders shifts a bit as well but not that much and it seems to be fairly predictable and temp compensation of Sky X seems to keep up with it.
Greg's experience with telescopes confirms what theory predicts. From what I understand, it is easier to focus larger apertures and at slower focal ratios. The most difficult telescopes to focus precisely are fast telescopes with small apertures.
My 105mm CFF definitely requires refocusing every 1 degree Celsius at f/4.5, ideally perhaps every 0.5 degree to maintain perfect focus. So it is best to use temperature compensation at that focal ratio and with this telescope, instead of frequent refocusing.
At native f/6 the same telescope needs refocusing every 1.5 degree change in ambient temperature.
I'm yet to test whether larger aperture telescope also has a larger critical focus zone...
The size of the CFZ is determined by the focal radio as it is the steepness of the light curve. The size of the pixels also has to be taken into account as this is the area in which the CFZ occupies.
When you have a larger slower telescope you typically have larger pixels and at higher resolution you are never diffraction limited. Take what Mike and Trish have, a 20” CDK with a 16803. When the seeing is less than perfect the CFZ aid basically made larger via atmospheric blurring.
4-5” F/5 refractors really struggle because their CFZ is the determining factor of perfect focus where as a CDK can largely be determined by the seeing conditions.
Yes, I forgot to mention the size of pixels also needs to be taken into account. Small pixels in a smaller sensor are not necessarily easier to handle...
As Colin says, The CFZ is directly related to the f ratio.
The Airy Disk diameter is:
2.44*lambda*F/D (micron) where F is focal length, D is aperture and lambda the wavelength of the light being studied.
A 200mm f8 telescope gives a diffraction disk of 11 micron, a 400mm f4 it would be 5.5 micron. To catch the "lucky seeing" the CCD pixel size needs to be 1/2 to 1/3 the diffraction disk.
To maintain the Raleigh limit (1/4 wave), the CFZ is = 4 *lambda*f ratio^2 micron. Based on green light 550nm examples:
f15 gives 495 micron
f10 gives 220 micron
f8 gives 141 micron
f5 gives 55 micron
(The tolerance at UV wavelengths is 1/2 that of Ha)
- the average human hair, for comparison is 70 micron!)
There is IMO an improvement on the CFZ equation that takes into account the seeing component, see here: http://www.goldastro.com/goldfocus/ncfz.php
I can vouch for the logic, it correlates with what I have observed with a C11 and a CDK20.
For imaging, yes the seeing is King!
The original CFZ as is mentioned, was for visual and Airy Disks.
This new version just acknowledges the reality of seeing conditions.
another 5 cents worth Andy - most of which has already been said.
You don't need a big scope. The linked composite image shows M83 taken with scopes from 8.2mVLT aperture down to Codemonkey Lee's 0.12m Chinese refractor. The big scopes do better, but the VLT clearly does not produce 5000x more information than Lee' scope - the differences in detail can reasonably be explained by the seeing - 0.7arcsec on average for the VLT and maybe around 2 for Mt Lemmon and Lee's system. And of course, nothing comes remotely close to Hubble with no atmosphere. http://astrob.in/314502/0/ .
ie, the atmosphere is the primary resolution bottleneck for scopes above about 0.1m aperture (unobstructed) and going bigger may increase sensitivity, but does not get sharper results in typical Australian conditions. A central obstruction changes things a bit, so you could aim for something around 200mm as a minimum aperture.
The focal length should be long enough for the pixels to extract all of the detail left in the optical image, after it is blurred by the atmosphere - with your CCD, about 1.5m will do the job in 1.5-2 arcsec seeing. You could go longer, but would not get significantly better detail - and you would need to spend a longer time to get an image (eg going from 1.5m to 3m would increase the imaging time by a factor of 4x).
Given that you might want something with >100 mm clear aperture (~200mm+ if obstructed) and 1.5m fl, that will work on an EQ6 - you could consider:
- an 8 inch f8 RC with field flattener (will take a long time, but give best resolution)
- a vastly expensive APO refractor
You could consider a shorter scope and use drizzle to recover some of the detail lost in sampling. Then suitable scopes would include:
- an 8 inch f5 Newtonian with coma corrector
- a 10 inch f4 CF Newtonian with CC (on the weight limit for the EQ6)
- a 7 inch Mak Newt (also a bit heavy)
Whatever you get will need a good digital focuser - my CF f4 Newtonian needs refocusing (by SGP) about every 10 minutes in typical conditions. The Newtonians will need at least a 3 element coma corrector and the Skywatcher (a bit better mechanically at this size) or GSO tubes will require stiffening around the focuser to hold your heavy camera - some sections cut from a cheapo tube ring can be glued and screwed to the OTA to do this.
I reckon that a 200mm Skywatcher steel tube f5 Newtonian (drizzled) with the Skywatcher CC and Moonlite stepper focuser should do the job and be close to your budget. The Skywatcher optics should be quite good enough out of the box. Sounds like a fun project.
wow that's great useful advice Ray, thanks for posting.
+1 above - thanks Ray, that's an excellent summary - appreciate the time you took to post this (I'm a big fan of your galaxy work!)
Quote:
Originally Posted by Shiraz
...
1.5m fl, that will work on an EQ6 - you could consider:
- an 8 inch f8 RC with field flattener (will take a long time, but give best resolution)
Yep, was heading down that path, but missed out on one in the classifieds still, they're not too expensive new at around $1k, and 1200mm is great, but f8 is VERY slow, maybe even too slow to get sufficient data in one night at a dark site (I don't often get to one, so I would want to make the trip worthwhile)
Quote:
Originally Posted by Shiraz
- a vastly expensive APO refractor
I really do like refractors, but a quality 120/130mm +/- 900mm FL, well - yes big $$$
Quote:
Originally Posted by Shiraz
You could consider a shorter scope and use drizzle to recover some of the detail lost in sampling. Then suitable scopes would include:
- an 8 inch f5 Newtonian with coma corrector
- a 10 inch f4 CF Newtonian with CC (on the weight limit for the EQ6)
Yes, probably the most sensible, logical solution Although cumbersome and prone to wind issues
Quote:
Originally Posted by Shiraz
- a 7 inch Mak Newt (also a bit heavy)
I'm very curious about these. Around $2.2k for the MN190 or there's a Russian one for sale under $2k in the classifieds, f6 too - hmmm no diffraction spikes either ...
Quote:
Originally Posted by Shiraz
Whatever you get will need a good digital focuser - my CF f4 Newtonian needs refocusing (by SGP) about every 10 minutes in typical conditions.
Cheers Ray
MMM, this is new territory for me - I typically set & forget focus at the start of an imaging run with my two refractors. Motor focus will require imaging with a PC (I currently use a mac for everything, I've never used a PC )
But you certainly have given yet more food for though, many thanks indeed
MMM, this is new territory for me - I typically set & forget focus at the start of an imaging run with my two refractors. Motor focus will require imaging with a PC (I currently use a mac for everything, I've never used a PC )
Probably a lot variables in that, mechanical and environmental. One guy who uses a TS ONTC newt says he hardly even needs to refocus during the night.
IMO you can get some decent results using a moderate sized refractor with small pixels... it's certainly been my approach and, not to blow my own trumpet, but I think my images hold up well to much larger, much harder to manage and potentially much more expensive scopes, even if it is well within the "yawn" zone as some might put it.
I've been eyeing off the ASi 178mm-Cool as well. Very small sensor, but only 2.4 micron pixels that I think will do well with smaller galaxies on moderate sized 'fracs; it's relatively cheap too and because of the small sensor size it'd be less demanding on optics.
I think my images hold up well to much larger, much harder to manage and potentially much more expensive scopes, even if it is well within the "yawn" zone as some might put it.
And They do!...sad really, the elitist hubris is quite laughable..?..
I agree, your images are great and I certainly dont yawn when looking at them....
Thank you for sharing your experience Lee - your images certainly are great and inspire me and many others
Galaxy imaging is something I would like to do in the future, so this discussion is of great interest to me.
I put together a simple excel spreadsheet in an attempt to roughly estimate which system would be optimal for galaxy imaging with the seeing we have in coastal Australia.
It looks like my camera's small pixels are always limited by any smallish optics slower than f/6 until we get to 140 mm aperture and above, when seeing will become limiting factor. So for my current camera, either a 250 mm f/4ish Newton or 140 mm f/6ish refractor would do for galaxy imaging, with Newton being significantly faster but would require collimating.
Those two might be my options since I feel that in the future we will see more new sensors with smaller pixels that will most likely will be more affordable than sensors with large pixels.
Yep, was heading down that path, but missed out on one in the classifieds still, they're not too expensive new at around $1k, and 1200mm is great, but f8 is VERY slow, maybe even too slow to get sufficient data in one night at a dark site (I don't often get to one, so I would want to make the trip worthwhile)....
Just on this, I have been chasing ARP14, a tiny little galaxy, from Sydney with an RC8 (1628mm, not 1200 per your earlier post) and an older ST2000XM with a 7.2 pixel size. Its doable but needs patience. This was after 6 hours and I need to get more colour data so definitely a WIP, noisy little devil.
But this would be a struggle if the plan is to do single nights at a dark site. It may work because you get all those dark sky benefits but given that the usual rules apply (alignment issues, chasing collimation in the field, etc,) it might be a struggle, unless you have your setup process nailed completely.
Theres comments about refocussing but these things can hold focus and collimation all night so that's a plus.
Good luck with the search. I have to say, hunting down the unusual galaxies can be addictive
Longer focal length instruments suitable for imaging get expensive quickly. Wouldn't it be nice if we can just have one OTA in the medium focal length range and just change the cameras to suit the targets. I guess with smaller pixels the area covered by each pixel becomes smaller rendering each pixel less sensitive. My question is, what is the current sweet spot in terms of smallest pixel size that will still have sufficient sensitivity? I am really tempted by the QHY183 (or the ASI equivalent) as it has 2.4 micron pixels and reasonable FOV at 6MP.
Just wondering if there is any trade off in small pixel size? Does it change anything other than arc seconds per pixel?
Shallower wells but also usually less read noise. However, in spite of having lower read noise, dynamic range is generally lower with smaller pixels, so more shorter exposures are needed if we want to control saturation of stars. The biggest advantage of small pixels on a small chip for me is substantially lower cost of the entire imaging apparatus.
As I understand, 2.4 micron pixels are in most cases best matched with fast telescopes (f/5 and faster).
Shallower wells but also usually less read noise. However, in spite of having lower read noise, dynamic range is generally lower with smaller pixels, so more shorter exposures are needed if we want to control saturation of stars. The biggest advantage of small pixels on a small chip for me is substantially lower cost of the entire imaging apparatus.
As I understand, 2.4 micron pixels are in most cases best matched with fast telescopes (f/5 and faster).
Can the problem with lower dynamic range be mitigated by HDR techniques?
Shallower wells but also usually less read noise. However, in spite of having lower read noise, dynamic range is generally lower with smaller pixels, so more shorter exposures are needed if we want to control saturation of stars.
That's what I suspected. I guess the trade off then is in image processing time?
Is it possible to get the same depth in an image with many short exposures vs fewer longer ones?
still, they're not too expensive new at around $1k, and 1200mm is great, but f8 is VERY slow, maybe even too slow to get sufficient data in one night at a dark site (I don't often get to one, so I would want to make the trip worthwhile)
Although cumbersome and prone to wind issues
Around $2.2k for the MN190 or there's a Russian one for sale under $2k in the classifieds, f6 too - hmmm no diffraction spikes either ...
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