Cake

On 11/09/2020 at 00:01, JamesF said:

My gut feeling is that the simplest solution would be to make an adapter to fit the OTA and accept a new focuser of a common size.  In some cases they may be readily available, but otherwise it might mean looking around to see if you can find someone who can machine one from aluminium or delrin or something like that.

I actually did just that some years back to allow me to fit an aftermarket SCT-style focuser to the back of a PST:

I chose the thickness of the adapter to make sure the image would come to focus somewhere useful.

James

Now that is a really interesting idea. I wasn't too keen on any attempt to modify the tube itself, as it's my only telescope at the moment and I don't want to do silly things to it, but aside from having it manufactured this has no downsides. Considering the OTA does have decent 4" lens, retrofitting it with a decent focuser would still be cheaper that getting a new 4" goto refractor, even if it's after getting a big newtonian dob.

9 hours ago, Geoff Barnes said:

Hmm, sounds like a wonderful opportunity to treat yourself to a nice shiny new telescope @Cake

(Hope you can afford one!) 😁

That's a backup plan, yeah. I'm just still hoping to be able to get full mileage out of this one, though.

8 hours ago, Ricochet said:

So? There is no need for a new focuser to fit in the same way as the current focuser, all it has to do is fit and provide the correct amount of extension to allow you to focus. If all other focusers fit into the tube then you need to measure the inside diameter of the telescope tube and find a focuser that fits that diameter.

Note that's that exactly what I did, I posted all the dimensions in the OP. The problem is that so far finding the focuser flange dimensions is exeedingly difficult and this telescope won't just take them as standard, hence this thread.

8 hours ago, Ricochet said:

Teleskop Service are probably your best bet in terms of the number of different focusers that they stock. However, focusers are reasonably expensive and if your telescope has a plastic focuser perhaps it has other parts that are of similar quality in which case it might be more worthwhile changing to one of the 102mm Bresser achromats, which are available from FLO for £219, if you have a mount that is suitable (and if a 102mm achromat is the type of telescope you want).

You are absolutely right that the focusers are expensive. Most cost more than I paid for the telescope in the first place, goto mount included. As for the OTA, the rest of it is reasonably decent and they just seem to have really cheaped out on the focuser (and the red dot which mounts to it). That seems to be the main thing Meade went on to improve in the NG models because from reviews in seems that those focusers are pretty much the same as skywatcher ones, the rest of the OTA and the goto stand are fairly similar to mine.

I guess I'll just try to improve this focuser as much as possible and maybe diy motorise it, and save the money for a 10" dob.

2 hours ago, banjaxed said:

Not sure of the size but the Skywatcher focuser upgrade fits the same way as yours and it fits the 102, 120 and the 150 models. If you get in touch with FLO I’m sure they will help.

Is this what you mean? https://www.firstlightoptics.com/skywatcher-focusers/dual-speed-2-crayford-focuser-for-sky-watcher-refractors.htm

Because that's the problem, it seems to go inside the tube, mine slides over it. But if FLO offers advice, that's great to hear.

EDIT:

Looked it up, this website actually provides dimensions https://www.365astronomy.com/dual-speed-2-inch-crayford-focuser-for-skywatcher-refractor-telescopes.html

This focuser cannot be mounted on my telescope without extensive modification.

I have an old (not NG) Meade StarNavigator 102. The focuser on it is rubbish even by my complete newbie standards, as it's plastic-on-plastic, rough sliding (no space for teflon skids or such) rack-and-pinion. Shakes the entire telescope when used. I have taken it apart, cleaned it up and lubed it up. I know I can attach some extensions to the wheels to improve it somewhat, but that's a half-measure that I want to only leave as a last-ditch option if nothing else is worth the expense.

The problem is, I don't know whether I can get a better focuser which would fit instead of it, as this one slides over the tube and none I've seen online do.

To make things a bit easier, I'm attaching some photos and here's some dimesions:

Outer tube - outer diameter 94mm, inner 90mm, 2mm wall

4mm gap

Inner insert - outer 82mm, inner 70mm, 6mm wall

All screw holes 12mm form the end of the tube

For good measure, the inner side of the focuser collar is 93-4mm (duh), and the actual tube is 55mm.

If there is no easy replacement, would I at all be able to mount an affordable motor (because I won't spend a lot of money on THIS focuser) like the Skywatcher one on the underside, as per photos?

5 minutes ago, JamesF said:

Yes, I believe both rules of thumb originate from considering the same physical properties of the telescope so it makes sense that they'd be in agreement with each other.

James

I've checked where I've read that again, it was actually 3x the Dawes limit, which now seems less suprising than it seemed to me.

47 minutes ago, JamesF said:

I'll belatedly add another vote for the "focal ratio approximately five times the pixel size in um" rule of thumb.  That's where I've been for some time now.  If you can find it (I can't, for the moment), vlaiv posted a very informative piece on resolution and planetary imaging some time back that I meant to return to at some point before Mars came out to play this year.  As it happens, the weather has made such study largely academic thus far.

James

Interestingly, when researching this I've seen someone mention another rule of thumb which is "3 pixels per arcsec on the sensor" (which you can calculate based on this http://plato.acadiau.ca/courses/phys/1513/optics.htm), and if you calculate both, they do seem to be more or less equivalent in my particular case. For a 3um pixel and 102mm aperture, that's an ~f/18, so just slightly longer than the five times.

EDIT:

I've checked again, it's 3x pixel per Dawes limit.

Thank you for your replies

12 hours ago, CraigT82 said:

The Dawes limit itself is based on separating point sources of light, and in planetary and lunar imaging its common to resolve extended features well below the Dawes limit (the rille in the Alpine Valley, and the Encke gap in Saturn's rings are two examples) because they are linear features rather than point sources.

Thanks, that's the kind of stuff that isn't obvious from just theory.

6 hours ago, Cosmic Geoff said:

Try it in practice, and you will find that the atmosphere is the main limiting factor, though the effect of 'seeing' will be less extreme with a 4" than with, say, an 8" telescope.  Jupiter and Saturn are currently low, so from the UK you will be trying to image through a lot of unstable atmosphere, which also causes some chromatic dispersion.  Often a Barlow, though theoretically advantageous in most cases, gives no improvement in practice. The use of an atmospheric dispersion corrector (ADC) is advised.

It is worth investing in a decent planetary camera rather than the cheapest, as the difference in performance can be significant.

I think the seeing and my use of the telescope are the most important factors here. I'm thinking of prime focus as an improvement on EP projection to a smartphone, no more no less. I'm not an avid astrophotographer hunting for the best viewing conditions, so sub-optimal seeing will be the limiting factor. I needed to know the physical limits of the telescope so that I wouldn't exceed them, I don't need to push them. I'm not willing to splash out on even mid-range equipment at this moment. My whole setup (the telescope with it's standard bits plus a cheap 4mm plossl) have cost me only £160, a decent camera and ADC would likely double or triple that. So I'm willing to spend £40 more on a cheap camera and something on a half-decent barlow, but I don't think of imaging planets as priority enough for more.

Alternatively, I might not spend anything more and stick to EP projection if a cheap prime focus camera is just too poor of a tool. I would like to hear your opinion on that.

Is smartphone on a EP (I currently have a 25mm and 9mm 40deg afov whatevers that came with the telescope and a 4mm/48deg afov plossl) better than a cheap prime focus camera, like the SVBONY 105?

EDIT:

I specifically mean the quality of the image, as I did go to https://astronomy.tools/calculators/field_of_view/ to comapare the possible fields of view as in the attached pictures.

5 hours ago, geoflewis said:

For planetary imaging you should aim for a focal length (FL) of x5 camera pixel size, hence for a 3um px camera, your sweet spot FL will be ~F15, so with your 102/800 (F7.8) scope you should add a x2 barlow. An ADC will help, but you can probably get away without that with such a small scope. Good luck.

4 hours ago, Gfamily said:

If you are imaging, you can use software to simulate the effects of an ADC. 

Registax6 and Autostakkert have an RGB Align option that will recentre the three colour images after stacking.

http://www.ianmorison.com/combating-atmospheric-dispersion/

Not tried it myself, but Ian generally knows what he's talking about.

Yeah, I don't mind a barlow, but if I can avoid getting an ADC, that's awesome.

Hi all, I'm very new to astronomy as a whole and need someone with practical experience to tell me whether I actually understand the theory.

I've recently bought on the cheap an old Meade StarNavigator 102 refractor, 102mm/800mm (not NG, as reviewed here). It's physically in good condition and I'm happy with what it is, but I understand it will have limitations and I'm just not sure if I actually know what they really are. Specifically when it comes to imaging the planets.

I've read up a bit about how prime focus photography works and from what I gather this 800mm focal length telescope with no Barlow will give a 0.77 asec/pix image on a 3um size pixel camera (I'm specifically thinking of the SVBONY 105 because it's cheap, I'm on a budget and I care more about learning how it works before I start spending real money).

On the other hand, I believe I understand that the absolute physical limit of resolution for this telescope is its Dawes limit, which (assuming Wikipedia isn't lying to me) would come out to ~1.14 arcsec. In other words, each pixel apparently already catches the telescope's smallest resolvable detail?

Do I undertand correctly that adding a barlow of any power would not have any more effect on the amount of detail than simply cropping the image, i.e. just make it bigger? Altough I imagine that the more pixels per arcsec, the better the signal/noise ratio of what is being captured, but that's a tangential issue.