rl

There may be a couple of points to make here...you don't tell us what cameras you are using which may invalidate some of my comments. I'm not sure what experience you have but your question possibly implies you are just starting out with astrophotography. A guidescope only has to work....by which I mean that all you need is one star in the field with a good enough signal-to-noise ratio on which to guide. Once you're guiding properly there is not much point in fitting a bigger guide scope. There are a number of parameters to trade here; aperture can be traded for exposure time to some extent (improving signal-to-noise) , which comes at the cost of update rate on the mount. If you're imaging in the milky way then a very small aperture (25mm) may well be enough. If you are pointing at the more barren parts of the sky then the bigger the better but 50mm aperture covers most situations with reasonably short exposure times (0.5 to 2 seconds). As regards focal length, a longer focal length will allow better detection of guiding error for a given camera pixel size, at the cost of smaller field of view and hence choice of guide stars. Software like PHD2 can calculate the errors down to a small fraction of a pixel so the guidescope focal length can be a small fraction of the main scope. To cut the waffle and get to practical options; Your main scope has a f/l of 500mm; I would expect either a 30mm aperture 120mm f/l or a 50mm 200mm f/l to give good results with normal guide cameras assuming the mount is aligned on the pole reasonably well. Half the secret is minimising the amount of work the guiding system has to do in the first place. Mechanical stability to eliminate flexure will be more important than the exact aperture. A word might be in order to manage expectations. Assuming you are starting out in astrophotography, a ST102 ain't the ideal scope for several reasons. The Chromatic Aberration will be severe and worse than seen by the eye since a camera will be sensitive over a greater range of wavelengths. The field curvature will also be severe which will show up if you're using a DSLR. But if it's what you've got then have a go...we all have to start somewhere and it will still let you see stuff in greater detail then the naked eye will ever show, even with a few purple haloes. Just don't spend too much cash on kit that can't be used on a better setup in the future.

Well done! I am extremely impressed....

Best of luck with this. It's trying something on the limit...in a way I enjoy this sort of thing much more than pretty pictures. It's a bunch of fun trying even if you fail. Do you have any estimates on the brightness dip and the photon statistics? Please let us see the results even if there is no obvious transit signal.

https://www.baader-planetarium.com/en/downloads/dl/file/id/483/product/3004/mpcc_mark_iii_instruction_manual.pdf The MPCC has a thread on the front which accepts 2" filters (the thread for 2" filters is actually 48mm). That's all there is to it! RL

How important the spacing is depends on how critical you are of coma in the corners of your image. Ideally it needs to be +/- better than 1 millimetre. If you ever use a full-frame camera it gets more critical. For me using the standard parts has always worked to my satisfaction but I'm not a pixelpeeping addict. The Skywatcher 0.9 coma corrector (CC) is designed for 55mm spacing from the back face of the CC to the image sensor. For this you will need a M48 thread diameter adapter whIch screws into the back of the CC and presents the Canon style lens bayonet fitting to the camera. These are different from the standard lens adapters which have a 42mm diameter thread. https://www.firstlightoptics.com/adapters/astro-essentials-m48-camera-adapter.html The distance is made up of about 44mm from the front face of the camera to the sensor plane inside plus the 11mm thickness of the adapter. FLO and others sell the adapter; it's mentioned in the CC blurb. You can screw in a 48mm threaded 2" filter into the front of the CC and you should be all set. Sometimes filter threads aren't very clever and it will only engage for 1 turn or so; it's not unusual and not normally a problem. Don't forget to show us your first M42...

My best shot: If you ask really nicely OO might make you a 6" f/4 Newt. They made me an F/6 nonstandard version. It's a bit of a cheat but there is probably someone else doing the same thing out there. These used to be quite common as wide field scopes. Team that with the ES coma corrector (mag *1.05) gets you to 630mm focal length. Add a 28mm Nirvana gives you a magnification of 22.5 and a field of 3.64 degrees, exit pupil 6.67mm, and the stars should be reasonably tight to the edge.

What is the focal length of your main scope? What type is your main scope? If it's a Newt or SCT you might be better off with an off-axis guider. A 50mm guide scope generally works fine with small/ medium refractors. Look for mechanical rigidity; how is it coupled to the main scope? A non-rotating focuser can be helpful but in many situations you only focus these things once in a blue moon if it's a permanent fixture. Many 50mm finders can be modded with an adapter to work as guidescopes. Personally I've always liked the Altair ones. https://www.harrisontelescopes.co.uk/acatalog/altair-starwave-50mm-finder.html?gclid=CjwKCAiAh_GNBhAHEiwAjOh3ZGuN1dVzVuu1TrS__QR0tQC9pLy__aJVVywJQapFvKIcLfxXdIoEghoCsggQAvD_BwE if you can cope with bigger a secondhand ST80 makes an excellent guidescope.

Given your other half is generously encouraging you to spend more is it worth investigating a GT81 or similar and an APSC SLR camera? I appreciate you've just bought the 485 .....It's a sort of "in the middle" camera with a medium area 11mm by 6mm, no cooling and very small pixels. It would do well on the smaller DSOs, better on solar system stuff. But secondhand 1200D cameras come up fairly frequently standard or modded for between #100 and #250. Still no cooling but they offer a lot more FOV. With the reducer the GT81 works at about f/4.9 which is pretty quick. As you specifically mention doing visual as well, the 80mm scopes do show a lot more than the 60mm ones. 60mm is great for portable but a bit limiting if it's your main visual option at home. Which are you more likely to keep hold of in the long run; the scope or the camera?

I would defer to the imaging experts on this one but I don't think that is a bad image considering the aperture and integration time involved. There is quite a lot of structure visible in the nebula. The pulsar is one of two very close faint stars in the middle; they are not quite split. The Crab is quite a small deep-sky object to image well with a small scope. Everyone is going to say "get more data" which is statistically true for random noise. But there is a law of diminishing returns that sets in; you have to keep increasing the integration time out of proportion for the same step of improvement. 20 mins is a bit short. A couple of hours on one object is usually feasible in one session for most people leading normal working lives! Technically, the signal/noise ratio is a Poisson distribution if you're interested in the maths. the signal/noise improves as the square root of integration time. Was it taken on the transparent night with poor seeing last weekend (10th dec?). Good seeing is worth a lot. Mine taken on that occasion with an 8" scope look a bit better but not much, with 2 hours data. Your stars look a bit blobby but that might be down to over-enlargement. Did you use a Bahtinov mask?. If you're guiding with PHD2 you can see the guiding errors and how they compare to normal. I can't see too many hot pixels so presumably you did a dark frame subtraction. But there is some noise on the left side that might want some sort of gradient correction. I think the colour balance is not far from the truth. The colours in the Crab are quite subdued unless you force the saturation in the processing. I use Siril which does colour balance by adjusting the colour ratios to match the spectra of stars in the image. It plate-solves the image and goes to an online catalogue of spectra before tweaking the colours for the best spectral match. Siril will do the gradient correction mentioned above as well. And it's free (but you are at liberty to make a donation!) There are some real experts on this site ( I am not amongst them!) Hopefully one of them will chip in and give you chapter and verse concerning resolution and pixel size....but I don't think you are far wrong in that respect. We could use some more info on how the image was obtained. Guiding, focussing, Seeing, processing et cetera.

Failed! I think my image is left-right swapped compared to yours. First attempt with Siril. 8" Newt 1 hour data, Deep Sky filter, QHY183C. Not enough data; too much noise: The pulsar is just about there but no clear shock waves! There might be something visible with very low contrast just to the right of the pulsar.

Problem is this is England...Last night was the first night in weeks where it was worth setting up for anything more than a quick look. Transparency was great but the seeing was rubbish. So bad as to cause new threads to appear on this site on just how bad it was. It showed up on PHD2 with guiding errors just over 1 arcsec. It's normally less than half that. Anyway, I got 1 hour with a 8" Newt and a Deep Sky filter (OSC) and another hour with a L-Enhance. I should get it processed over the weekend.

I'm imaging the Crab right now...just came in for a coffee, leaving the kit chugging away when I saw your post. You've definately caught a hint of it. Very impressive. Certainly something to check for when I do the processing...

A word in favour of the 12" option, as most opinions are going the other way I might just play Devil's Advocate. I'm at retirement age and live under Bortle 4 skies according to CO. Perhaps one rung above average on the fitness ladder. I have a 14" OO dob and several small refractors (GT81, SDHF75, ST120). It takes me the same amount of time to set up the Dob as it does the refractors on a Vixen portamount. Certainly it takes a lot more calories to lift but setting it up is no great inconvenience. Both are kept in the garage. Some of the refractors are triplets; the cooldown time is just as long as the big dob. And the deep sky views through the 14" are predictably better than the small refractors. Generally planets too. I think I accept that there will come a time when I can't handle the 14", and it's probably not that far away. But until then I'll get as much use out of it as I possibly can! The other thing about the 14" is that for the same magnification as the refractor, the exit pupil is a lot bigger which helps with floaters. Common sense has to apply; no way is it worth a hernia, but if you can hump the thing around I'd go for aperture while you can! And an observing chair is a very good accessory. And a 10" might be a good compromise. A more valid question might be ..can you lift it without knocking the collimation out of kilter? Maybe that's the real convenience argument for refractors Good luck either way.

I believe there is a precedent; Clyde Tombaugh is supposed to have used a 12" f/10. It may well be similar to the scope in the wikipedia article photo which looks like it might be f/10: all the photos I can find of the scopes he made show incredibly long tubes. https://en.wikipedia.org/wiki/Clyde_Tombaugh

I use an ASI120MC very successfully. The MC-S version supports higher frame rate over USB3 useful for planetary but not essential for guiding; pointless if the PC can't keep up. GPCAM2 is another good choice, as is QHY5_2. And all have the ST4 guide port which connects to the mount delivering the corrections. 30mm at 130mm FL should be ok. 50mm at 200mm will theoretically see and correct smaller guide errors with fainter stars. Given the choice I'd go with 50mm but it's not a dealbreaker if you really need the more compact guide scope and your main scope is fairly short. 30mm certainly ok for camera lenses.

30mm will do. 50mm goes fainter.. In theory a mono guide camera will see fainter stars but in practise most people don't see much difference. And a colour guide cam can make a good planetary camera. Choosing one with a good frame rate and a frame buffer can stop USB lockup issues when doing lucky imaging. A lot of 30/50mm finders can be adapted to take a guidecam. There is an adapter for this.

It's a bit off topic but anything you can do to make things easier is time/money well spent. The learning curve can be very steep. Just practising with the kit in daytime can be helpful so that all the software fires up and talks to the hardware on cue.

The Star Adventurer will serve you well with a DSLR and short lenses, many award-winning shots have been taken with this setup. But sooner or later you will want longer focal length and more resolution which will involve a scope. Scopes are better corrected compared to camera lenses for the application in hand assuming you have the field flattener or coma corrector. At this point you have to consider weight and portability, because for astrophotography the starting point is the mount. Is it going to be used just at home, or set up regularly in some farmer's field? You need to add up the weight of all the kit you're likely to want including an OTA, camera, filter wheel, guidescope et cetera. A popular starting point for deep sky is an 80mm ED refractor (typically 4.5 kg with rings and field flattener) DSLR (700g), guidescope (500g) guide camera (300g) so you're looking at 6kg for a OSC rig, 7kg with a filter wheel for mono. For a beginner I's stick to one-shot-colour (OSC) but this is a contraversial point. You mention planetary; I have seen good results with 80mm scopes but something larger with a bit more resolution will be more satisfying. If you're serious about AP then the HEQ5 mount is about the minimum that will allow you to carry this sort of weight and get consistent results bearing in mind that the practical limit for most mounts is about 1/2 to 2/3 of the visual weight. If portability is not an issue then something in the EQ6 class will keep you going for years with a wide variety of scopes and accessories. You can get results with an EQ3 or basic EQ5 but you may well find the percentage of successful subs is low. Given the UK weather it's worthwhile having decent kit just to maximise your use of clear nights. The mount will need to be capable of guiding corrections in both axes, usually via a ST4 port or pulse guiding. Full goto is not strictly necessary but makes finding objects a lot easier. Sometimes just getting a small faint target on to the camera chip can be a real chore. After the mount is sorted you will very probably want to sort out the guiding for longer deep-sky subs. Not so important for planetary lucky imaging. In most UK locations a filter is very beneficial. There are special clip-in filters to suit Canons but you will need to get another one if/when you trade up to a cooled astro camera. Secondhand Canon filters go for less than £100. You're asking the right questions. A lot of people (including myself) start out buying any old cheap equatorial mount, scope and camera as and when bargains become available and regretting it because the setup was not planned as a whole!

Agreed....the reason to buy the best optics you can afford is to make the most of those rare occasions..or take the scope to a decent site.. And the knowledge that if the scope is'nt performing the problem is likely be something else which might be fixable...

OO optics can be extremely good, especially the long focal length variants with more accurate mirrors and small central obstructions. I'm using 3 of their 1/10 wave mirrors...6, 8, 14 inch and they're all stonking performers. Bear in mind that the long tube lengths can become unwieldy if undermounted. How they compare against the cheaper Chinese offerings depends a lot on the local seeing. If the atmosphere imposes a 2 arcsecond limit on detail then you won't see much difference. On a really good night they do start to pull ahead a bit but it's not dramatic. The difference probably gets less with increasing aperture as the atmosphere and other factors tend to be the limit more often. If buying new bear in mind that the depreciation on a new OO scope will be very heavy if you decide to sell it on. There seems to be an unwritten law that the price of any secondhand Newtonian is set by the equivalent Skywatcher model....not really too much of a problem if you intend to keep it a long time but seriously painful if you can't get on with it after 3 weeks...

I see the same symptoms running a QHY183 with Sharpcap on an I7 with 16gb under windows 7, USB3 connection with only the camera on that port.. Sharpcap just freezes and up pops a non-specific error banner telling me the only option is to exit. Drivers are up-to-date. I've yet to find a satisfactory solution. I'm about to move to W10....

Excellent advertisment for the Redcat...and probably the filter!

Is the focussing tube racked in far enough to intrude on the light path? How about the clips on the main mirror? Are the 2 extra spikes of the same intensity as the other 4?

To be honest I'm not sure where you go from here. The FSQ85 would be the dream widefield astrograph for most people. I thought being a quad Petzval design the flattening was built-in? If you add an extra flattener the curvature will simply go the other way. I've seen some good reviews of the 5-element Askar and they're all checked in the UK before sale.

I did a whole bunch of astrophotography with Ektachrome over a 12-year period from 1985 - 1997. By chance I came across the original edition of Michael Covington's book "Astrophotography for the Amateur " in a bookshop in Muscat, Oman which got me hooked on the AP side of things. I used to do my own E6 processing in the bath ..used to love the excitement as the wet film came out of the tank...it would be milky for about an hour until it dried hard...revealing the results of the night's labours. Manual guiding was a real chore...keeping the crosshairs on the star for a 20-minute exposure was a real test of endurance on a cold night. My job took me to several of the world's best observatories at the time. I made a clock drive that would work in either hemisphere so I could take shots with a 50mm lens in Chile and South Africa. I'm inspired to sort through the detritus of my astronomy life and find a few of those slides...