vlaiv

17 minutes ago, thekwango said:

coolio - so it's as simple has a that. i was, wrongly it seems, under the impression for the Dob's you'd need a separate mount or a flat levelled table to be able to use them. 

Some of the smaller models (usually designated as table top) need something to put them on, but 6" and 8" are properly sized to be placed on ground.

Here is interesting thread about observing chairs (lot of recommendations) - something that you will need. 6" and 8" dobs are not really suited for standing observing (although they can be used like that, but you will bend your back quite a lot).

Here is a short video (plenty of such videos on youtube) - showing the size of the scope and how it is assembled:

It would be a good idea to browse thru some of those videos to get "the feel" for the size of the scope and what it looks like - both 6" and 8" - it can help you decide if you opt for any of the two.

11 minutes ago, thekwango said:

 

thanks, yea i had read that Dobs were often recommended for beginners and i had looked at those but for me transportation would be a pain. Storage not so much. but i live smack bang in the middle of town so light pollution is a nightmare. it's practically daylight 24/7 front and back of my house! so most, if not all my viewing will require bunging everything in the car and driving (don't have to far though) so i had kind of ruled the Dob out. with something like that i assume i'd need to also bring a stable platform to set it on?

Both 6" and 8" are fairly easily portable with even small car.

Both tubes are around 1m and a bit long (1200mm focal length, but not all of it goes into tube length) - so fit nicely on the back seat lied down. Dob base can fit into most booths. You need an observing chair - look for foldable one or one you can take apart and put together with ease. Eyepiece case and you are all set.

In reality such dob is about as portable as EQ mounted shorter dob - you need place to put tripod and mount and scope. You might not need observing chair for EQ mount - but it is much more comfortable observing while seated down.

For really compact and portable design you need to look at folded scopes - here you will find that you will be limited by aperture even second hand for your budget - like Mak 127 you mentioned - it will gather about x2.5 less light than 8" dob. Mind you, nothing wrong with 5" Mak or SCT - very good scopes and very light for their size - Dobs are often said to be the best bang for the buck - meaning the most aperture at lowest price, and aperture is important for visual (as long as one can manage bulk).

Get yourself one of these used, but even new one is not so far out of budget:

https://www.firstlightoptics.com/dobsonians/skywatcher-skyliner-150p-dobsonian.html

If you can manege the size (in storage and transportation terms), look for 8" version used - probably the best starter (and sometimes lifetime) scopes for visual.

3 minutes ago, sharkmelley said:

You have multiple things going on there!  I think we're heading a bit off-topic but I'll play along.

First of all, all the filters show a very large doughnut with spider vanes.  The doughnut is the same size in each case and disappears when there is no filter.  That indicates reflections off the filters.  The extra length of the light path can be confirmed by using the Dust Donut Calculator (http://www.ccdware.com/resources/dust.cfm). and this will help confirm which other surface is implicated - either the chamber window or the sensor stack.

The centres of those big halos have different offsets to the star causing them.  Either the star is in a different position in each image or the filters are mounted with slightly different tilts to the optical axis.

The H-alpha filter also has a series of increasing rings around the star.  This is usually caused by internal reflections within the filter glass because the AR coatings (if any) aren't too effective at the wavelength of H-alpha.  I'm guessing it's not an Astrodon filter otherwise you need to send it back and complain.

As for the Laue Diffraction off the microlenses, I can't be 100% certain I'm seeing it in the H-alpha image but it is certainly there in the OIII image.

Mark

Hopefully we are not going off-topic, I just found that image online as example of people complaining at mentioned artifact (I believe it was posted on zwo forum, but can't be sure - forgot where I copied it from).

We agree on most points - how certain halos were created, but what I wanted to point out is that "no filter" does not generate much larger (like we agreed roughly x50-100) effect - if anything it seems that there are no artifacts at all related to micro lens reflection.

This leads me to conclude that your description does not account 100% for what is going on - clearly presence of filter and it's reflective surfaces in some way contributes to interference pattern. It is also very indicative that different people get different results based on type of filters they are using. Some even have this effect on R, G and B interference filters.

Don't get me the wrong way - not trying to say that your analysis is flawed and that you are wrong - I'm just questioning if it's the complete picture of the issue. From above, it seems to me that it might be the case that type of filter, it's position in optical train and F/ratio of system all have "a say" in how pronounced effect is, or if it's there at all.

9 minutes ago, sharkmelley said:

We assume it's the protective cover on the sensor itself because the reflection is from a surface less than 1mm above the microlenses.

Yes we would expect the "halo" caused by this diffraction effect to be brighter with no narrowband filters.  In very rough terms the star would be 50-100x brighter and its halo would also be 50-100x brighter.

Mark

I appreciate your explanation, and certainly think it is plausible. I'm still not 100% convinced however (can't be helped )

What do you make of this image then?

Ha and OIII clearly show patterns and filter presence (large defocused aperture image superimposed over star). Luminance also shows this, and what I believe is mild reflection from chamber window (enough light to make it detectable since it is AR coated).

No filter has no filter signature, only halo that I would say is from chamber window (and some diffraction spikes that are probably from mirror clips and focuser or what ever - since this is newtonian scope). Not sure that there is halo from this effect though - unless I've mistaken halo around star to be from chamber window and it is in fact from this source - but I would expect it to be progressively brighter towards the star?

7 minutes ago, sharkmelley said:

Unfortunately the sensor coverslip on the ASI1600 is not AR coated.  This is why the ASI1600 is notorious for creating these patterns.

I'll briefly explain the mechanism of the interference pattern. Wavefronts hitting the sensor are being scattered in all directions by the 2D array of microlenses.  At certain angles (dependent on pixel spacing and wavelength) these scattered waves will interfere constructively.  This is reflected back onto the sensor forming a regular grid of points where constructive interference is happening.  This is the same as Laue Diffraction from a  2D lattice that occurs in X-ray crystallography.  At each point on this regular grid we see the image of a defocused star.  Typically this forms a pattern of overlapping defocused stars. The diameter of each defocused star indicates the extra distance that the light has travelled and shows beyond doubt that the reflection is off the coverslip and not off any filter.

If you want to understand more then take a read of the Cloudy Nights thread that Carole linked earlier (ignoring the post about the Talbot effect which is another very fascinating diffraction effect but is not what we are seeing here).  There you can also find  some PixInsight code I wrote that allows you to reproduce the geometry.

Mark

Thanks for explanation, and couple more questions if I may - we are talking about sensor protective cover and not chamber front window that is not being AR coated, right?

By saying that it has been shown beyond doubt that reflection is off this surface, I presume distance from micro lens to this cover has been measured and also its thickness and compared to defocused star at certain F/ratio beam (defocus position being twice distance to first or last surface of this cover)?

Also, by saying it is not filter induced, I can expect in pure mirror system and no filters when shooting bright star to have large halo around it with ASI1600 - even more so than in narrow band image (if we look intensity in small part of spectrum being roughly 1/50 - 1/100 of 400-700 continuum depending if its 7nm or 3nm filter, I can expect it to be about 100 times brighter than with narrow band)?

3 minutes ago, sharkmelley said:

The key word here is diffraction.  The exact geometry of the pattern is crucially dependent on the wavelength of the light because it is caused by constructive interference.  The (almost) monochromatic light passing through a narrowband filter produces a very clear pattern.  This pattern becomes smaller as the wavelength shortens.  A broad spectrum filter ( e.g. R,G or B ) will produce the continuous superposition of the different sized patterns produced by each wavelength that passes through the filter - in other words it will be an indistinct smeared artefact.  So the only effect you will see in a broad spectrum image are weird variations in colour in a halo around the star.  There's quite a good example here: https://www.cloudynights.com/topic/565014-new-to-narrowband-imaging-question-about-strange-pattern-in-bright-star-halo/?p=7706969

Mark

I agree with you, but question is where constructive interference comes from? It needs two parallel surfaces to form - and is usually in form of reflected wave that creates out of focus star image on sensor.

It is definitively due to micro lens on sensor because out of focus stars create square / cross shaped pattern.

So it can be between micro lens and sensor cover window, or it can be between micro lens and filter.

Sensor cover window is AR coated, while filters are interference blocking filters - one is coated to pass as much light as possible - other is created to pass narrow band of light and reflect everything else - key emphasis on word reflect

Simple experiment could show if this is related to cover window or filter - just move filter further away - if pattern grows in size and gets dimmer and possibly change intensity - it is related to filter. Another clue that it might be related to filter is to observe OP image and example that I posted.  My filter is mounted very close to sensor, and my guess is that OP has regular filter wheel and filters a bit further away from sensor - based on size of reflections and distance between first and second order reflections - mine smaller and denser while in OP image larger and further apart (indicating greater distance between reflective surfaces).

24 minutes ago, sharkmelley said:

Don't bother changing spacing of filter etc. because it won't make any difference.  It's a diffraction pattern generated by the microlenses and a reflection off the sensor coverslip so the position of your filter and field flattener won't make any difference.

The pattern is most obvious with narrowband filters, especially H-alpha.

Mark

If filters play no part in it, then similar artifact would be visible in red and lum channels as well (those pass Ha wavelength as well), but would not appear with different wavelengths of light - sensor to cover distance is fixed and interference happens when there is matching between wavelength and distance of reflective surfaces.

Here is H-alpha shot of mine (ASI1600 + Baader H-alpha 7nm, pure mirror system):

Effect is barely noticeable on mag 7 star

Same star with OIII filter:

Much larger effect

And I have not seen this effect in any of my broad band images - even with very strong stars.

As above it is interference between filter or focal reducer / field flattener and micro lenses on ASI1600.

Not everyone gets those, and it usually happens on bright stars. If you can, try changing distance of your Ha filter with respect to sensor - either a bit closer or a bit further away. You might also try removing field flattener or changing a bit it's distance (I know it will cause trouble with flat field, but just for testing purposes). This effect is probably dependent on spacing as well as angle of light cone coming in on sensor. Maybe some combination of spacing and light cone angle will lessen or almost remove effect.

5 minutes ago, kbrown said:

I'm sure it'll work as the calibration figures out which way it should do the corrections...

Yes, you are right - it is rigid connection between the two (there better be ) - and going in circle - catching up is in opposite direction on sensor but in same direction along the circle.

Interesting.

I do have one question - will it work as a guide scope?

Raising this question because I can't get my mind straight right now if there will be impact of it sitting on the opposite side. Both main scope and guider rotate in same direction, but mount moving ahead in RA on scope side will mean lagging behind on guide side (or will it?) - wonder if calibration is going to take care of that.

I have that GSO (Revelation) x0.5 1.25" focal reducer, and here are some things that you should know about it:

1. Focal reduction depends on distance between focal reducer and sensor - greater distance, greater reduction - use this in combination with above diagrams - start with close distance and small reduction to see if you are getting usable field at all - then extend distance up to point where it still works.

2. I had to reverse lens in mine - it was set one way at factory but I found that for imaging it works better by reversing the lens in holder (don't ask me how I got idea to try it out - probably read something somewhere). You might want to try which way it gives you better image, by undoing retaining ring and just flipping it around. Use some sort of soft cloth to handle the lens - don't touch it with your fingers to avoid staining it. You can do this at prime focus of scope with your guide camera to avoid any OAG related complications.

I'll try to explain with diagrams, first just reducer physical length:

Regular diagram:

Diagram with reducer in place:

As you see, in order to focus both guide camera and imaging camera at the same time, because you added focal reducer before guide camera and thus pushed it back, you need to add optical path between OAG and imaging camera.

Next diagram will try to explain additional distance needed because of the way simple reducer works:

This one is trickier to understand because reducer bends rays - this is why we need inward travel of focuser for simple reducers like mentioned two element 1.25" x0.5 reducer. Actual sensor will lie where bent rays meet to be in focus, but at the same time imaging sensor needs to be at a distance equal to where guide sensor would be without bending of the rays - further out.

Hope this makes sense

Forgot to add, if you can, guide with ASCOM driver and higher bit depth, rather than native drivers and 8bit.

It will not work (at least I think so, below is why).

All it can do is shrink available field onto smaller region of sensor, but ultimately field of view is limited by pick-off prism and more importantly opening in prism holder.

I currently guide with a bit larger sensor - ASI185, which has diagonal of ~8.6mm at 1600mm FL - F/8 beam. I get vignetting on this setup, here is screen shot:

Other things that might interfere with using focal reducer are:

- do you have enough back focus to fit body of reducer between sensor and T2 connection of OAG? Additional distance must be mirrored between OAG and main camera as well. Focal reducer moves focus point inward - this means another change in imaging sensor position vs OAG (not sure if this will bring it back forward or does it need still being pushed further out - probably this second point).

What about fixing problem of guide stars in another way? Things that you can do to guide on fainter stars:

1. Make sure your OAG is focused properly. Although you can guide on slightly defocused star, this is not a good thing if you want to use faint star for guiding. You want star light to be concentrated in smallest possible area to maximize star profile and SNR.

2. Position prism as close to light beam aimed at the sensor - just barely avoiding prism shadow on your imaging sensor. This means rotating prism so it sits next to longer sensor edge. Further out in the field stars become more distorted due to different aberrations like coma or astigmatism - look at difference between stars in above image - in right part they are more concentrated than in left part of the field. You want to pick up stars in the least distorted part of the field - again has to do with star light concentration into smallest region and SNR

3. At longer focal lengths you can use a simple trick because with small pixel guide camera you have enough precision - bin your guide camera output. It does not matter if it is true CCD hardware binning or CMOS software binning - it will increase SNR of stars and make it possible to guide on faint stars.

4. You can always increase guide exposure (up to mount limit), if you are used to guide at 1s or 2s exposures and can't find guide star - why don't you try 3s or 4s guide exposures? Sometimes in poor seeing I go as long as 6s or more with my HEQ5 (it is tuned and belt modded, and that is above max exposure that I would otherwise use on it but if seeing is poor, guide performance is not going to be the best possible anyway).

4 hours ago, Stub Mandrel said:

Interesting link.

With the SW CC and 130P-DS (f4.7) my curvature map is almost identical to the ES one in that link, except my curvature is only 12% rather than 26% and the corner stars are as round as a round thing. Strange that many people prefer the Baader?

Could it be that the 'economy' Skywatcher CC is the best of the bunch? Does suggest it might do a good job with the 200P-DS?

Not sure how big the weight difference is between 200P-DAS and 150PL, but the 1200mm 150PL guides well on a HEQ5.

 

 

 

F/ratio plays significant part in performance of CC - SW CC is good for F/5 scopes, although I'm not sure how much SA there is with that F/ratio.

Scope used in comparison of coma correctors is F/4. 8" F/4 has focal length of 800mm - and that puts it in right spot for 1"/pixel with ASI1600. F/5 8" with x0.9 reduction will provide 900mm focal length - that is 0.87"/pixel with ASI1600 and I would personally rather be on north side of 1"/pixel than below it - that would mean ASA x0.73 CC - and that one will not provide fully corrected field for ASI1600. It will for ASI183, and I think that would be really good combination, but then again I would bin ASI183 as well for effective resolution of 1.36"/pixel rather than leaving it at 0.68"/pixel.

There is characteristic signature of SA on star shapes - most SCTs have such stars (SA depends on wavelength with SCTs, and also with primary/secondary distance that changes when you focus) - almost "button" like rather than point like - smooth vs sharp so to speak.

After reading posts like this one:

http://www.astrofotoblog.eu/?p=856

Not really convinced in perfromance of coma correctors and fast newtonian systems.

Granted 200PDS is not that fast at F/5 - but slower the newtonian - longer the tube and harder on the mount and guiding it is.

19 minutes ago, ollypenrice said:

Why are you keen to add resolution via focal length and then reduce it via binning? It really doesn't matter whether you fill the frame or not. What matters is how many pixels (single or 4 pixel superpixels) you put under the object's image as projected by the telescope. It is this which determines the object's image's final size at full size (1 camera pixel = 1 screen pixel.) *

Earlier in the year I did a comparison in Astronomy Now between a 14 inch scope (2.4M FL) with large pixels and a 150mm scope (1M FL) with small pixels on galaxies. I found that the level of resolution and the size at which the images could be presented on screen was effectively the same. It seems that the considerably greater optical resolution of the larger scope was not translating into more final details. In both cases the mount was running with an RMS of less than half the image scale.

Personally I'd look for a scope with a FL which will give you about an arcsecond per pixel or a tiny bit less and make it the kind of nice simple design that you know you will find productive. I've struggled with one of those RCs with a guest and got nowhere. The theory is one thing but this example did not behave according to the theory.

Olly

*Not the best sentence I ever wrote! 

I agree with you on that one - aim for final resolution, but this is where I found RC to be the best match.

Let's say that we want ASI1600 and 1"/pixel resolution with 8" aperture on HEQ5. That means focal length of about 800mm. There are couple of options to go here:

1. F/4 Newtonian with native pixels, or F/5 with reducer CC

2. F/8 RC with binned pixels.

For option 1, we need good CC. I'm not sure that I've found CC that will correct coma to good degree but not introduce SA or something else, over large enough field. Best CC that I've seen in terms of correction would be ASA x0.73, but it has 17mm corrected circle (or there about). It also means F/5 scope - and that will add its own complexity in terms of bulk being put on HEQ5. Somehow I can't seem to find good 8" Newtonian candidate for 1"/pixel - maybe I just have not searched enough.

With option two we have nice compact design that sits well on Heq5. It is flexible in terms of resolution / focal length as there are reducers for it - x0.67 or x0.75 (I would say that any reduction past x0.72 will not work well on this RC and asi1600 due to corrected field - x0.67 reducer can be "spaced" to x0.72 - x0.75, but there is also very good reducer - Riccardi FF/FR - x0.75).

Binning ASI1600 in software is exactly the same as using camera with larger pixels and having larger read noise - one of 3.4e read noise - still better than most CMOS cameras out there in terms of read noise. Only drawback for RC is FOV, and for galaxies it is of course less important.

So if we go by aperture at resolution (around 1"/pixel), I think RC is very viable if not the best option in 8" class to be mounted on Heq5 (with given camera).

Being true mirror system it has other advantages - broader use as astronomical instrument - for spectroscopy, photometry and astrometry - I mean it is most widely used by professionals in scientific role because of its characteristics.

59 minutes ago, evil_yoda said:

Hi vlaiv,

I've managed to find myself a used carbon fibre RC8! Cloudy nights for a while longer I suppose.

Not got it yet, but it's totally standard with the factory focuser etc.
I assume you upgraded the focuser on  yours, and was wondering what you upgraded it to? Doesn't look like the standard focuser has a threaded attachment for the imaging train.
Might be a while before I can upgrade the focuser, but it's always fun to make an upgrade list

Matt

 

Hi, yes, I ended up upgrading it precisely because of lack of threaded connection. Stock focuser was otherwise quite usable - never had slippage issues with it, but I try to keep my imaging train light. I plan to use that monorail focuser on my SW Evostar 100 F/10, just need to get suitable adapter to it (OTA tube to M90).

This is the one I upgraded to:

https://www.teleskop-express.de/shop/product_info.php/info/p6970_TS-Optics-2-5--Rack-and-Pinion-Focuser---holds-Acc--up-to-6kg---travel-53mm.html

I also have this extension before focuser, screwed into OTA:

https://www.teleskop-express.de/shop/product_info.php/info/p2773_TS-Optics-50-mm-Extension-Adapter-for-M90x1-thread.html

At the time of my purchase it was not included into delivery (I have TS version - regular one, not carbon fiber), but from what I see, TS includes it now with their RC units.

I also added rotator and suitable thread adapter (because need to mount 2" filters and reducers):

https://www.teleskop-express.de/shop/product_info.php/info/p9781_TS-Optics-360--Rotation---Thread-Adapter---M63-to-M68--M54-and-2-.html

+

https://www.teleskop-express.de/shop/product_info.php/info/p6400_TS-Optics-Adapter-from-M54x0-75-to-M48---T2-Focal-Adapter-for-M54x0-75.html

+1 for RC 8" - I have such setup and very pleased with it. I also bin x2 in software for around 1"/pixel. It is on edge of HEQ5 capability - in terms of guide performance, you need nicely tuned mount to be able to guide at around 0.5" RMS total.

Scope hold collimation very well - I setup each session and I've collimated scope only twice (well, one could say that it was single collimation, because I did not get it spot on in first round, so I had to repeat the next day).

11 minutes ago, iapa said:

pegasus actually have a diagram for their l-bracket on their web sight.

You could start with that...?

It has multiple slots for the focuser side

For an RC, can you 'lock' the mirror, and add a crayford inline with optics? - I did not you think you are at weight limits, so, may not be viable. 

You can't lock the mirror since it is not movable. Problem with 6" and 8" models is that mirror cell and focuser attachment are in one piece. There is tilt mechanism for focuser - it is "squared in" by design and manufacture. This as a consequence has a problem if too much weight is hanging of the back of the scope (focuser included) - as it will move the cell together with the mirror.

I've checked my scope and one of the collimation screws was indeed a bit loose (1/8 of a turn) - but I think I now wrecked collimation - I forgot that smaller screws are collimation screws and larger ones are locking screws so I tightened up smaller screw (instead of just making sure cell is locked by larger screw). Will need to check it under stars.

Don't know if I'm over weight limit, really should not be - using ASI1600, OAG, and filter drawer - all of those are fairly light components (in their class). Putting lightweight step motor and bracket shouldn't really be a problem.

I've decided on "serial" configuration, and made sure both fine focus knobs are removable - and indeed they are, simple set screw. Fine focus shaft looks like 4mm (or maybe 3mm I need to finally get myself a caliper one of these days) - and this seems to be problem on its own . Checked all local retailers and they have elastic shaft couplers in every possible combination except one that I might need (3-5mm or 4-5mm, neither is available, every other combination under the sun is in stock).

32 minutes ago, Thalestris24 said:

Hi Vlaiv

I wasn't suggesting you buy one, lol! I was just showing you how mine is put together - it's quite simple, and as I said, probably not difficult to duplicate. It's just a geared stepper motor, L- bracket and controller. If I'd had the ability to do the mechanics I might have tried to make one myself at the time. For one thing, they charge 100 Euros just for the L-bracket on its own which is nothing to make, if you can do such things.

Louise

But I did consider buying one at first - until I calculated total costs and saw that it is quite easy DIY solution - very nice for a small project in winter months when weather does not cooperate.

At the moment I think I'll look into building L bracket - lego way - out of two or more pieces. Retailers that deal with stepper motors and "robotic" accessories have generic mounting brackets - so maybe I can figure out some way of fashioning suitable L bracket - bolt here nut there ...

1 hour ago, steppenwolf said:

I have to say that I prefer a non-belt driven connection if possible and use a direct connection with the drive shaft and focus shaft in line as this way, there is no sideways pressure on the shaft bearing:-

More I think about it, more I'm in favor of "in line" approach vs belted side by side configuration. That would mean removing both fine focusing knobs, but I guess that would be ok as it's "non invasive" operation - I can mount them back without much hassle.

1 hour ago, Thalestris24 said:

FWIW my usb focus motor has gears and a direct coupling to the focus shaft:

As you can see, the motor is held in place with a simple L-bracket. It would be easy to duplicate the hardware but some effort to write an ascom interface, I think. I imagine it has a simple usb-serial interface with some sort of microcontroller and stepper motor driver in the little control box. There is an external temperature probe which plugs into the box (with the blue manual control buttons). It does 8000 steps/turn and is powered from an 8v dc mains adapter. My version handles  ~1.5kg load - there's also a ~4.5kg version, I think

 

Here is motor from other side (you can see the end of the geared shaft that goes to coupling):

Hope that's useful

Louise

Thanks for that info. I did look at usb focus as an option, but I can't justify the cost at the moment. My dealer has kit listed at around 180e without holding bracket, brackets are around 85e and additional motor is around 90e

That would total to about 570e with VAT and customs fees - pretty steep. I believe that I can motorize both focusers for quarter of that price if I choose DIY route.

I just realized that I will not be as straight forward as I thought.

Although both focusers are of the same name - TS 2.5" - they differ in fine focus knob and travel per turn. On RC I believe that fine focus knob will have to be replaced with either pulley or coupling, depending on mounting position (below or to the side). On TS80 APO fine focus knob already has proper grooves that I can use to put belt on.

I was hoping to leave focusers "intact" and just mount motors to them, but it appears that I will need to mod at least RC focuser.

I've found good "summary" resource on this one that helped me understand differences between mounting options:

https://astrojolo.com/astrolink-4-0-mini/focusing-stepper-motor-solutions/

According to that page, coupling to fine focusing shaft is pretty good option. Did some calculations and it looks like there will be enough resolution with 200 step motor and fine focus reduction to be able to go as low as F/4 - which is enough for my needs. Now I just need to find where to source pulleys, belts, couplings (I can do it online, but would rather like to inspect items for suitability prior to purchase).