Lee_P

2 hours ago, ONIKKINEN said:

Have you given thought to the darker skies side of things when it comes to reaching a decent SNR? 

If you visit a bortle 4 area (should be reasonable) you can get a vastly superior image in just one night compared to your current multi-week projects from bortle 8. Actually you might get 2 finished targets per trip, depends on how bad in the B8 end you are now.

This one was quite demoralizing to realize for me. I got 2 hours from a B4 area and compared it to more than 6 from B6-7 and, you guessed it, the 2 hours from dark skies slapped the 6 around like it was a cat toy. More effort though, so not a completely fair comparison but nothing is.

For sure, but I rarely get the chance to travel, and have a semi-permanent set-up in my garden that makes set-up and pack-down here easy, but conversely would be a right pain to travel with. Plus, astrophotography from a city is kinda my thing 😆

@Clarkey Could you please do me a quick favour and measure the distance from the back of your RC8 to the back of your astrocamera? I'm trying to work out if I'd need an extension pier to stop the camera-end of the telescope thwacking into my pier... Thanks!

31 minutes ago, vlaiv said:

If you compare two different setups - you should use Aperture^2 * Sampling_Rate^2 formula.

If you compare same setup - native, binned x2, binned x3 and so on - then you can use simplified formula - which is SNR = bin factor, or exposure time decreases by bin factor squared.

These are really the same - in second case, we keep aperture the same - so it can cancel out from equation and we are left with Sampling_Rate^2 part.

Binning increases numerical value of sampling rate and that is why time increase = bin_factor^2.

Hope that part makes sense.

🥴 So am I looking at a reduction in integration time of three or four times..? (Although in reality I'll probably just stick with imaging a target for weeks at a time).

26 minutes ago, vlaiv said:

All correct except one thing - bin x3 will give you x3 improvement in SNR - not x2 which equates to x9 imaging time and not x4.

SNR improvement is related to imaging time by square root function, so x4 imaging time (or x4 number of subs) - gives you x2 SNR improvement, while x9 imaging time gives you x3 SNR improvement.

Since you are binning x3 (joining 9 pixels - which is equal to stacking x9 subs or imaging for x9 longer) - you get only x3 improvement in SNR - or rather, SNR improvement is equal to bin factor.

Ok, does this mean that five hours with RC8 and binx3 is equivalent to 20 hours with FRA400 and binx1 (factor of four); or 15 hours (factor of three)?

28 minutes ago, vlaiv said:

I know this fear exists with people - but that is just because people don't fully understand blur / resolution part of astronomical imaging. Resolution is not there to begin with.

If bin x3 is proper sampling and you wonder what you've lost over having 6000x4000 instead of 2000x1333 - answer is noting. If you want to have 6000x4000 image for say printing purposes or whatever - just take 2000x1333 image and upscale it to 6000x4000. Result will be the same as if you've shot 6000x4000 under original conditions.

Totally, this was my (mis)understanding until you talked me through it.

15 hours ago, Clarkey said:

I think they are generally quite well collimated out of the box. I think many have been made worse by 'tinkering'. My method for collimation is to remove everything xcept the focuser and get the secondary perfectly aligned. I use a TSKOLLI or Reego, but you can use a simple Cheshire for that part. Once the secondary is right I use the DSI method for the primary using a star test.

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf&ved=2ahUKEwi62bCmhJX5AhVMPcAKHcvBBws4ChAWegQIKBAB&usg=AOvVaw1TQ8YAhohC9_gr_IRXBo3x

This works for mine but my focuser and primary mirror appear well aligned as I get no tilt issues. If this is not the case it is more tricky - but the DSI method should still work. My main advice would be adjust it SLOWLY and try to remember what you have done to put it back again.

There is not a lot to say about the scope itself. I do have a home-made secondary dew heater, but I have never had a problem with the primary and dew. Cool down is pretty rapid too as it is an open tube. I have only used it with a ASI1600 which gave a good flat field. With a larger sensor you might need a flattener. I still need to try it with my IMX571 camera to determine if it is OK with and APS-C size sensor. I did change the focuser to a Baader ST. The original was OK in terms of tilt, but it did slip slightly with the autofocuser. I hope this helps. Here are a couple of images with the scope. Just remember I am far from an imaging expert!

 

Thanks, those are some good images!

I wonder, can you use the top Vixen dovetail as a carrying handle? It's hard to tell from the pictures if you could fit your fingers underneath it to get a good grip. Maybe with some risers? The collimation does sound like a pain, but it holds well once done?

13 hours ago, vlaiv said:

No, you are quite right - binning does not change FOV.

One of things that focal length impacts is of course FOV, but I commented that which you quoted coming from purely sampling rate perspective under assumption that targets you want to image will fit the FOV.

Sampling rate is function of pixel size and focal length. If we keep focal length the same and increase pixel size by factor of x2 - we get the same thing as halving focal length and keeping pixel size the same - in terms of "/px or sampling rate. Similarly if we increase pixel size by x3 (binning x3) we get the same effect as using 1/3 of FL and keeping pixel size the same.

1600 / 3 is roughly 500mm (or 533.333 to be precise) hence my assertion.

RC will be faster than your current setup even if you want to quadruple FOV (double it in each height and width).

OK thanks, makes sense.

13 hours ago, vlaiv said:

When we did "speed" calculations we concluded that RC + x3 binning is about x6 times faster than your current setup. If you image target for 6h with your current setup - equivalent will be 1h with RC.

Four times faster, I think? RC8 binx3 = 84600; FRA400 binx1 = 19510.

OK, let me write my understanding of this binning malarkey and let's see if I've got it:

If I were to use my 2600MC with no binning (i.e. binx1) with an RC8, I'd get an image with a resolution of 6248 x 4176. However, I wouldn't actually be getting useful data across all those pixels because seeing conditions and mount limitations combine to, in effect, "blur" the view. When processing the data, I could binx3. This would make a square of nine pixels into one super-pixel. The plus points of this are that it would increase my SNR by a factor of around four. What takes me 20 hours with my current set-up would instead take five. Or, I could image for 20 hours and get a SNR equivalent to 80 hours with my current set-up. It would also decrease processing time due to the smaller file sizes. The downside is that the image's resolution would only be 2082 x 1392. This isn't that bad though, because the "lost" resolution wasn't useful data. I wouldn't be able to crop in very much with this lower-resolution image, but the RC8's focal length of 1600mm gets me very close in anyway, so that's not a big issue. If I need a wider FoV I could mosaic. 

Is that accurate? The lower final resolution still makes me itch a bit because I'm used to dealing with very high res images (both in astrophotography and regular photography) but I understand that with the proposed set-up it's not really a fair comparison. If I need a higher resolution image, e.g. for printing, I guess I could always use Photoshop's Enhance feature, or Topaz GigaPixel AI, to artificially bump up the pixel count. I just did an experiment doing that with some of my existing data, and it looks quite good to my eye. Enough to somewhat allay my fears about low-res images anyway.

Thanks vlaiv, I definitely owe you a pint! 🍻

1 minute ago, Clarkey said:

As an RC8 owner, I must say I am a fan. In terms of value for money I think they are excellent. As I am sure you have read, collimation can be a bit tricky initially - but once set it does not need much adjustment. I typically bin 2, but in all honesty, with my seeing bin 3 is probably more realistic. Also, although F8 it never 'feels' slow except when compared to my F4 scope.

Thanks, I'd be interested to hear more about how you go about collimating it. I read some threads here on SGL and it seems rather tricky. Any and all other thoughts on the telescope would be most welcome too!

23 hours ago, vlaiv said:

It does not really need to be that expensive.

I would personally go for something like 8" RC + x3 binning - for very nice working resolution of ~1.4"/px.

Speed wise, if we apply formula that you posted, we get:

203^2 * (3 * 3.76 * 206.3/ 1624 )^2 = ~84600

(1600mm FL and 203mm aperture, pixel size is x3 3.76 as we bin x3).

Even if we account for massive central obstruction - things don't change much. This system will be x4 faster than most listed and costs fraction of the price of most of them.

Thanks vlaiv, an 8" RC is a definite possibility, and FLO have them in stock. Your options are sufficiently cheaper than what I was considering that I may be able to get an EQ6-R PRO too, which would make something heavier, like a Sky-Watcher 190MN an option too.

I've been reading up on binning, and think I'm starting to get my head around it, but am coming stuck with a comment you made earlier in this thread: "RC has 1600mm. Bin your pixels x3 and you have effective focal length of about 500mm (which is increase over your current 400m)." How is it that binning reduces effective focal length? I'm imagining that a binx3 image would have the same field of view as a binx1, but a much lower resolution -- but I'm pretty sure that analysis is wrong!

Thanks vlaiv and raadoo, you're both legends and are being very generous with your time in helping me. I'm going to digest all your comments, and read up more on binning. This is also what ONIKKINEN suggested earlier in this thread.

37 minutes ago, raadoo said:

If getting something speedier than the FRA400 is top of your list,

Well it's not that I definitely want something speedier, more that I'm a bit reticent to get something much slower 😁 

On 21/07/2022 at 21:28, vlaiv said:

Sharper is closer but the problem is that people don't really think this way.

We think of being closer in terms of FOV, right?

Above image of M13 is not very close in, right?

How about this image:

That is close in, right?

Same image. It was taken with 80mm scope. Sampling rate is 2"/px - same as your image.

When it is zoomed in 100% do you feel it is soft and too much zoomed in by looking at the size of stars? I think it has good sharpness - and this allows for it to be cropped and looking much "closer in".

Thanks vlaiv, that's a really useful example.

I'm understanding that perhaps I should bite the bullet and get a new mount to improve guiding and allow more options for an OTA. An EQ6-R PRO should fit my DIY pier and so be a relatively straight-forward upgrade. It's been mentioned before that with my current mount and sky conditions, a FL of 700mm is likely a realistic limit. I wonder, how is this worked out? What would be a realistic FL limit on an EQ6-R PRO? And how much better would this new mount be compared to my Orion Sirius EQ-G? The EQ-G's manual says that its resolution is "0.144 arc sec (or 9,024,000 steps/rev)", while the EQ6-R PRO is "9216000 Counts/Rev., approx. 0.14 arc-second". Sounds basically the same..?

Something else I've learned from this thread is about the "speed" of an imaging system. I just want to check I'm getting this right... If I calculate aperture squared multiplied by image scale squared for the FRA400, I get 19,510. For a much bigger telescope -- the Askar 130 PHQ -- the result is 10,177, i.e. 52% of the FRA400's speed. In real terms, would that mean that I'd need twice the integration time from the Askar 130 to get a comparable SNR from the FRA400? It's hard to get my head around, considering the slower telescope has a significantly larger aperture.

Thanks again to everyone helping with all these questions! 

38 minutes ago, vlaiv said:

Well, yes.

I often advocate at least 6" of aperture in order to push down to 1.5"/px and 8" for going below that value.

However, given above spot diagram - I think that you will see a difference with 107mm scope.

Not because of aperture size - but because of better looking spot diagram.

First thing to note is that spot diagrams are given in micro meters and not arc seconds. Here longer focal length gives a bonus because it makes error in micrometers smaller in arc seconds (same thing as being closer in with longer FL - but in opposite direction).

~4.9um RMS spot diagram on axis for FRA400 is really 2.53" RMS

~3.16um RMS spot diagram on axis for 107PHQ translates into 0.87" RMS

So spot diagram alone is x2.9 times smaller. That will improve things as far as detail goes.

Mind you - adding focal reducer will most likely skew spot diagram and make RMS bigger again. I would use it at native FL.

This won't really get you closer in to your targets, but it will get you sharpness. Given that you use OSC camera, I would do super pixel debayer and effective resolution would then be 2.07"/px - but I think it would be "true 2"/px".

Blimey, this is intense! So, the PHQ107 would get me sharper views, but not "closer"; and would be about 0.6x the speed of the Askar FRA400 (calculated earlier in this thread) thereby necessitating longer total integration times? That doesn't sound like a great deal for a £2500 'scope, compared to the Askar FRA400's ~£1000 price tag... It does seem weird that increasing the aperture by such a large amount doesn't really yield much in the way of any benefits, and it's actually down to the quality of the optics that there would be an improvement 🤯 

And is it the case, flagged earlier, that ~700mm is an effective limit on focal length for any system if I stick with my current mount?

4 hours ago, vlaiv said:

It is not easy to summarize the differences.

I can give it a go, but I'm afraid it will be all but "summarized"

- in same conditions 107mm of aperture has potential to out resolve 72mm of aperture. How much? That really depends on other conditions as these don't add up linearly. I'll give you few examples (good seeing, excellent mount, then medium conditions and poor conditions). Mind you - this is for diffraction limited optics (we need to discuss in some detail what I mean by this).

In 1.5" FWHM seeing and 0.5" RMS guide error

72mm aperture will produce 2.46" FWHM which corresponds to 1.54"/px

107mm aperture will produce 2.17" FWHM which corresponds to 1.36"/px

In 2" FWHM seeing and 1" RMS  guide error

72mm aperture will produce 3.45" FWHM which corresponds to 2.16"/px

107mm aperture will produce 3.26" FWHM which corresponds to 2.04"/px

In 3" FWHM seeing and 1.5" RMS guide error

72mm aperture will produce 4.89" FWHM (3.05"/px)

107mm aperture will produce 4.75" FWHM

-------------------------------------------

72mm aperture performs 13.36% worse, 5.83% worse and 2.95% worse with respect to FWHM size depending on conditions (from best to worst).

If you want to really take advantage of larger scope - you need to have good mount and good skies and difference will be significant.

Another thing to notice is that improvement in sharpness is not proportional to aperture size - double the aperture and you won't get double the resolution, so even 150mm aperture wont help much in poor conditions.

Mind you - above are theoretical values for diffraction limited aperture.

Most scopes that we use for imaging are not diffraction limited! Most of them will in fact produce more blurred image. Primary optics is almost always diffraction limited in center, but we almost always use correctors of some sort - like coma corrector, field flattener, focal reducer - you name it.

These often make optics perform under diffraction limit. Take look at Askar FRA400 spot diagram:

RMS radius is 4.873um in center of the field and growing as you move away from center.

As a comparison - RMS of Airy disk of 0.662" for 72mm of aperture, which at 400mm focal length gives 1.28um RMS (0.516"/um for 400mm).

That is x3.8 larger Airy disk because you are using quintuplet astrograph instead of regular refractor.

Note that in average conditions we calculated expected FWHM of ~3.5" for diffraction limited optics - and we measured about 6.75" FWHM in your image. How come that we have such huge difference?

That is down to having optics that is not diffraction limited. If I do another calculation for same parameters (2" FWHM seeing and 1" RMS guiding) but this time instead of 72mm I use 19mm of aperture - which gives x3.8 larger airy disk - I get expected FWHM to be:

6.66" FWHM star profile - which matches what we measured in your image.

What does this mean for 107mm vs 72mm comparison?

Well, we can't really tell until we see spot diagram of 107PHQ and we run some calculations. Will there be some improvement - I think so.

Probably most improvement will come from larger aperture coupled with very good field flattener with excellent spot diagram. Alternative is to use telescope with longer focal length that has flat field without additional corrector and already good spot diagram and to bin those to get to your target resolution.

 

 

Thanks vlaiv for the thorough answer, as always. I appreciate your efforts to "summarise!"

Here's the spot diagram for the PHQ107. Does it help with the comparison with the FRA400?

You say that to take advantage of a large aperture telescope, I need a good mount and good skies. Well, I'm stuck with the skies I've got. But I could consider a mount with a higher tracking accuracy and larger weight capacity. (Not sure which one, exactly..!) The FWHM percentage improvements you calculated don't seem that significant, so should I conclude that if I want something that will be a decent step up, I should be getting a new mount *and* a large aperture telescope?

Thanks again...

29 minutes ago, ONIKKINEN said:

The resolution will depend on your seeing conditions mostly, if the mount plays along nicely, Vlaiv has gone into that in detail.

But the speed will still increase, even if you dont bin/keep debayering using interpolation. Your focal length will increase by 1.87x but aperture area increases by 2.2x so it will still be a little bit faster in terms of integration times. Your subexposures will need to be a bit longer, but swamping read noise in bortle 8 is not difficult even with narrowband filters so this is something you dont have to worry about either.

Thanks! Re: speed, earlier in this thread it was said that "You can work out a measure of the “speed” of your current system- it’s aperture squared multiplied by image scale squared."

FRA400 = 72^2 * 1.94^2 = 19,510
107PHQ = 107^2 * 1.035^2 = 12,264

Given that method, the 107PHQ is 0.63 (let's say 2/3) the speed of the FRA400. But you're saying that the 107PHQ would be a bit faster. So there's something I'm missing, or not understanding. For clarity, and I think this is what you're saying, the subframes would be slower, but the total integration time would be a bit faster?

45 minutes ago, vlaiv said:

AstroImageJ gives quite different results. Something else seems to be off

Average FWHM seems to be around 3.5-3.6px.

FITS header of that file is also very strange - it gives:

Pixel size seems to be reported correctly, but focal length is set to 749mm and hence pixel scale is 1.0356"/px?

Do you plate solve or did you manually enter 750mm FL for some reason? Is this image from that 400mm scope?

If image is from 400mm and all else is correct apart from wrong FL reported - then your FWHM values are about ~6.75", which means that you should be sampling at about ~4.2"/px or image should be about 2.2 times smaller (it is over sampled by factor of x2).

Indeed - if we reduce it to 50% (a bit less than it should be - but easy to do in order to remove bayer matrix), and crop it to 100% "zoom level":

It still looks good and sharp (above is just linear stretch - white/black point to show what is there).

Aaaah, the plot thickens. That's odd. I use an ASIAIR Plus for my imaging, and the FL there is correctly stated as 400mm (or 402mm or something, from memory). It uses this info to plate solve correctly. I've no idea where 749mm comes from -- unless it's a sign from above that I should get the 107PHQ, which has a FL of 749mm 😝 

I've got a fair few telescope options to choose from. I appreciate the benefits of going for something like a 6" reflector, but fear it would be a bit fiddly and, for me, not as fun to use. The MN152 looks appealing, but hard to get hold of now. As for refractors, I'm still drawn to the 107PHQ (FL 749mm, f/7). I haven't quite got my head around speed and binning and the like. So, with that telescope in mind, would you be so kind as to summarise the differences I'd get between it and my current FRA400 f5.6? I *think* I'd get "closer" views, would see more detail (e.g. separating close stars), but would need longer integration times to reach my desired SNR. Or maybe longer subframe lengths and the same total integration... I'm getting in a muddle and am not sure of anything 🤣🥴 

Thanks vlaiv!

1 hour ago, The Admiral said:

Just to muddy the waters further, have you looked at the Altair triplet scopes? I had a Wave 102ED triplet and that gave some nice results. I see that there is a new 130mm triplet, which with the reducer gives about 700mm effective FL at f/5.6. Might be a bit heavy though. These scopes might exist as other brands of course (i.e. TS APO range for example). They all come with an optical bench test results sheet.

Ian

Good shout, the 115 looks pretty tasty.

16 minutes ago, raadoo said:

At the end of the day, though, we're not professional astronomers, we're just passionate about the crazy stuff that's up there in the sky. So, if you under or oversample your images, is that the end of the world? Did any of us that started out by pointing a DSLR attached to a modest camera lens up at the sky - with or without a tracker - even know what sampling was? Did that lessen the fun or awe or just child-like giddiness when you see that first image of Orion or Andromeda or the Moon and you feel like you trapped lightning in a bottle? And sure, we into imaging are often gear-heads of the highest caliber and can't help but dive deep into all the fun technical details of this hobby, but I for one won't let technical limitations, theoretical or practical, get in the way of having plain old fun. And if I'm lucky, I might also learn a few things along the way.

Totally agree, 100%! (Although I'm very grateful to the experts here on SGL who assist with my understanding of the technical aspects!)

10 hours ago, raadoo said:

@Lee_P

I’m in the same boat as you - got an FRA400 paired with a 183 and am looking for an apo in the 700 - 900 focal length range to pair with a 2600. I’ve not yet reached a decision on what I’ll move to next but maybe my shortlist will help you narrow your search down or even prompt others to share their thoughts:

Refractors

• TS 115/800 - €1500 - affordable and gets a lot of praise from knowledgeable folk (e.g. @vlaiv).
• ES FCD-100 CF 102/714 - €2000 - Optics should be good and that carbon fiber means your mount has an easier time swinging the rig around.
• TS CF 102/714 - €2000 - Looks solid from a mechanical point of view, with all the right imaging accoutrements and each one gets tested before shipping.
• TS 106/700 - €2200 - FCD-100 should perform really well but it’s pretty new and untested.
• ES FCD-100 127/952 - €2700 - Bigger brother to the 102 is as light as a 4” aluminium scope but hey, bigger aperture. Do keep in mind that it’s pushing close to a meter in length for the scope alone (oversized dew shield extended).
• Vixen AX103/825 - €2900 - Vixen often get overlooked and no one knows what glass they are using but its pseudo-petzval design is quite appealing.
• Askar 107PHQ - €2900 - A thing to note about Askar’s Petzval scopes is that there will be some variability in quality. My FRA400 is no lemon but it does show chromatic aberrations across the field, whereas yours is a better corrected unit, from what I’ve seen in your images.
• APM LZOS 100/800 - €3900 - A lot of dosh but by all accounts should be an amazing optic even though it’s quite slow at f/8.

Reflectors

• Skywatcher 150PDS/750 - €410 - Cheap as chips and a large community of modders means you should find help for any issue. And f/5 is going to be more forgiving of collimation errors.
• Vixen R200SS/760 - €1400 - Despite the thick vanes and mediocre focuser, this one has a neat party trick in that it can become an 1120mm f/5.6 scope with the use of the Extender PH. And you can switch out the tube for a carbon one later down the line.
• Boren Simon 8”/568-800 - €2200 - This one’s interesting because it’s a carbon tube and can play double duty as an f/2.8 or f/4 scope with the use of a TSGPU Coma Corrector.
• TS 8” ONTC w/ FeatherTouch ~ €2900 - You do get a fine scope for the money, but be prepared to wait a while to get it and at this price you better love newts.

I’m intentionally leaving out RC’s from this list as you mentioned wanting to faff about as little as possible (a sentiment I echo), which kind of only leaves newtonians on the table for the focal lengths you’re after.

For what it’s worth, I’m personally leaning towards the ES102CF, as it seems to offer the most for one’s buck. I’d spring for the 127 even, but on my narrow balcony I’d probably end up crashing the scope against a wall. Alternatively, the R200SS and it’s party trick makes it appealing for next year’s galaxy season without moving to an SCT.

Did I miss any other potential candidates?

Slightly off-topic: Sharpstar just dropped news about their upcoming Z4, which is a 100mm f/5.5 refractor. Maybe they’ll follow up with a 120mm f/6? Strange naming though; not sure what the 4 means. Or why they needed another refractor in this focal length / f ratio to compete with their own 94EDPH, 100QII or FRA600?

This is a brilliant list, thanks! I've spent some time this morning looking through them all. I think the two that appeal to me most are:

• ES102CF. This has a good specification, and a like how light it is. Would it need a flattener though? It sounds like it can take a ZWO EAF but longer bolts are needed.
• Askar 107PHQ. This was my original thought for an upgrade and it's hard to shift it out of my head. I hear you about the quality control issues, but FLO bench test all their Askars, which means they should pick up on any lemons.
   
• And the ES MN152 is an interesting proposition that may be the best of all worlds. Maybe hard to get hold of though!

Have you been following the other posts in this thread? It's interesting to hear about the limits of sky conditions and the like, and vlaiv's comments around how there may not be much gained by having a new telescope rather than just cropping in on the images we're taking already. (Although you and I are using different cameras).

12 hours ago, GalaxyGael said:

Image scale as mentioned above is a good guide, and if your skies are bortle high then go down in scale from your 1.9 "/px  to roughly double your average guiding rms so that you get all the resolution your scope gives you for given skies.

With 2600mc, 650 mm is about 1.2 "/px and 760mm hits about 1" /px. If you look at your average star fwhm (maybe asi studio fits viewer has a star size button now), divide that value by 1.4-1.6 as a guide for your best seeing, and that might help decide the focal length and image scale. 

I'm guessing around 700mm will be your limit, any longer and you may not see the benefits compared to a crop ay 700mm. 

Then, I would decide aperture for brightness and some resolution benefit, and then the size and weight and use all these criteria to narrow down. 

And the there are spikes. If you don't like them, the Mn190 is great if you have the mount payload capacity and the right pixel size, or a long, expensive heavy apo. The TS 130/910 is very good, and with 0.79x reducer is about 715mm or so at f/5.5. Perfect in my view. 

A 6 inch imaging newt, decent one is a good option, such as the ontc range from ts which are outstanding. 

Or an epsilon 130d with the 1.5x extender to give 650 mm at f/5 ( there is one available, hint hint, about to put mine on the block). The f ratio for objects that will now span across more pixels compared to the FRA5. 6 is partially moot once you are above the noise and plan do long longish integrations. Its longer, but does t scale with ratio of squares of f ratios unless the focal length and pixel scale are the same. So slower f ratios are no thin to be feared unless its f/12 or something! 

TS also have fcd 100 106mm apos with beefy focusers at f/ 6.something. 

Lastly, you are osc so binning in software is an option for ~1200mm mm at bin 2, giving you the same pixel scale as 600 mm with bin 1 on the same camera. 

 

Thanks, this is very helpful. It does sound like around 700mm is my limit; maybe a bit more on very steady nights. The MN190 and TS 130/910 are a bit heavy for my mount, I fear. An Epsilon 130d with 1.5 extender is an interesting proposition, and not something I'd considered. Do you know if it would fit a ZWO EAF? I'm not so keen on diffraction spikes, but could be persuaded if all my other boxes are ticked. Given that I image a single target over many weeks, would I need to consider the OTA's orientation to keep the diffraction spikes in the same positions? (If that question makes sense).

11 hours ago, ONIKKINEN said:

Sounds like you would hate a newtonian, maybe pass that idea for now. Newtonians might be the furthest thing you can get from a petzval plug and play APO so probably not a good idea!

With the 107PHQ you could BINx2 to reach the (more or less) same resolution as your current setup gives you, so now you have the same working resolution but with more aperture = speed increases and you might find that you can get away with shorter total integrations (but with smaller FOV). But since you are using an OSC camera its not quite so simple, as using debayering methods other than superpixel debayering results in basically an upscaled image. So if you debayer the subframes using some method that does interpolation, you are basically artificially getting to the quoted 1.9'' per pixel resolution your current setup at a glance gives you. The camera samples the sky at half this rate in reality, so the real data is at best 3.8'' per pixel. If you then bin that already upscaled 1.9'' image to 3.8'', you dont get the full benefits of binning that a mono camera with the same sensor would. Its better than not binning at all but does not improve SNR by x2.

You can test this with your own data to see that it really is like this with OSC cameras. Resample one of your processed images to 50% using some quality preserving resampling method and then resample that back to 200%, so to the original resolution. There will be no change in the fine detail level as that level of detail was not captured in the first place.

Ok, thanks. So what if I didn't bin (i.e. BINx1) with the PHQ107? Better resolution but slower speed (0.6x my current?)

2 hours ago, 900SL said:

Having just gone through the same process and candidates, I've decided to get an ES 152 MN. Now waiting for them to arrive in stock. 

It is a tempting option. But annoying if they've just been discontinued though! Do you know if they fit a ZWO EAF? And does it come with a camera rotator?

10 hours ago, vlaiv said:

What units are those?

Pixels or arc seconds?

Arc seconds, I think... Attached is a FITS file if you'd be so kind as to check!

NGC_3372_Light_600_secs_2022-03-24T21-00-32_001.fits

10 hours ago, vlaiv said:

Btw, your stars are rather elliptical with ~0.7 eccentricity.

Do you have big difference in RA and DEC guiding performance? Usually if you have elongation in X axis (RA aligned to X and worse guiding performance in RA due to backlash and good balance) it is hard to spot in image.

Thanks, I never noticed. Something for me to investigate further!

23 minutes ago, vlaiv said:

There is really little difference between the two - I mean cropping current setup and using longer focal length telescope.

Difference is in ability to actually achieve certain resolution - which depends on guide performance, seeing and aperture size (in non trivial way).

First thing you should do is to measure what you've been able to achieve in terms of resolution / star FWHM with your current setup.

My guess is that you'll find your stars to be in 3.5" to 4" FWHM range.

In order to say properly sample at something like 1.5"/px - you'll need about 2.4" - 2.5" FWHM stars. This requires decent seeing and at least 6" of aperture if not more.

Here's a FWHM analysis of the subframes that went into my latest image:

55 minutes ago, Clarkey said:

How about a 115mm triplet such as a TS photoline? There are a number of 'clones' too. Probably the limit for an HEQ5 but could be used at native FL or with a reducer.

That could work, similar to the Askar 107PHQ I'm pondering.

11 minutes ago, vlaiv said:

@Lee_P

What is it that you are hoping to achieve?

400mm FL with 72 giving you somewhere about 2"/px is very good combination for nebulae. If that is your interest - I don't really see what you intend to improve upon.

There is certain relationship between quantities that you need to consider:

- Size of FOV

- Sampling rate

- Image pixel count.

You don't really have too much wiggle room as far as sampling rate goes. You are already at 2"/px - and realistically - you can go down to 1.4" - 1.5".  You can certainly sample at higher resolution - like 1"/px or below that - but there is really no point in doing so as aperture sizes, seeing and mount performance won't allow you to go lower.

This puts some constraints at above dependence - you can't shoot large detailed FOV if you have certain amount of pixels at your disposal. Longer focal lengths will reduce FOV and at some point you will have to start to bin your pixels in order to maintain sampling rate (and ultimately speed - defined as total imaging time to reach certain SNR).

That will in turn result in image with less total pixels.

For example - you can happily image with 8" aperture on HEQ5 class mount if it is in form of compact scope - like EdgeHD 8" or 8" RC

Both of these scopes will have very large focal lengths - but you can still image at around 1.5"/px with them. EdgeHD will have 2000mm of FL which you can reduce using their reducer to something like 1400mm - RC has 1600mm. Bin your pixels x3 and you have effective focal length of about 500mm (which is increase over your current 400m).

Cost is reduction in FOV and your images will have something like 1500x1200 px if you don't do mosaics (btw doing mosaics negates aperture advantage as you need to spend time on each panel - no free lunch).

In any case - question is - what you hope to change compared to your current setup?

Hi vlaiv, thanks for your input -- invaluable as always! I guess in non-technical terms, I want to take "closer up" images of DSOs, primarily but not exclusively nebulae. With my current wide-field set-up I can get a large nebula in a single field of view, which is good, but I find I'm wanting to zoom into specific areas more. I can crop in, but then the image resolution and quality go down. If that makes sense? Perhaps I've been thinking about it too simply, but my logic was "get a telescope with a longer focal length to get closer in, but also a larger aperture so it doesn't take months to gather enough light to make a decent image."

It would also be good to get sufficient signal-to-noise ratio faster than I'm able to currently, but that's secondary. I'm quite patient 😁

1 hour ago, catburglar said:

You can work out a measure of the “speed” of your current system- it’s aperture squared multiplied by image scale squared. In your case 71^2x1.9^2=18,200…if you do the same calculation for your new options then you’ll get an estimate of how much faster or slower it’s going to be…So a 6inch newt at approx 1.2arcsec 150^2x1.2^2=32,400…So about 1.8x faster than your current set up…if you bin pixels that will change your working resolution, but provided you use the correct resolution for your bin factor I think this calculation should still give you a good guide.

This is so so helpful, I can't thank you enough!

So, going through those calculations I come up with the speed of three options:

Askar FRA400 (as a baseline): 19,510
Askar PHQ107: 12,264 (0.63x the speed of the Askar FRA400)
Explore Scientific MN152: 26,008 (1.3x the speed of the Askar FRA400)

That's a point in the MN152's favour. 

I thought that the focal ratio was an important factor in the speed of a system though, have I just had the wrong end of the stick all this time?