vlaiv

I would go with 28mm extension / combination of extensions. You need to calculate in filters and their optical path, and they usually add about 0.5mm (well I've heard figure of 1/3 of substrate thickness and that is around 0.7mm for 2mm filters, but 0.5mm is good enough).

Also you might find that specs on correct distance are not quite applicable in your specific case (it depends on so many factors, like exact FL of scope that can vary between instruments, ...) so using some spacers to dial in correct distance will probably be good idea. Just start with somewhat shorter distance, and tune it until you are happy with results.

For me it was well worth it. It considerably improved my peak PE, but more importantly made PE curve much smoother and removed nasty spikes when guiding.

Made guiding so much easier. Prior to doing the mod, sub 1" total RMS was really rare, and there were significant elongation of stars in RA direction. Post mod - I'm able to guide 0.5" - 0.7" range (I even managed to get 0.42" one night for couple of minutes - but I attribute this new improvement to additional mods - changed saddle plate and Barlebach planet tripod) and stars are almost always round (subject mostly to other influences like wind and cable snag).

In general it is an easy mod, but you should take care of tension of belts - I had problem with undertensioned belts - improper belt / gear meshing that gave me nasty +/- 2" 13.6s oscillation.

I also "hypertuned" mount by taking it apart, cleaning, replacing bearings with high quality ones.

To answer my own question - it is down to CCDI.

I did have problem with collimation, and something loose in optical train (don't know what it was, but I suspect one of the extension rings for RC 8" that comes before focuser (between ota and focuser) and brings focus in 50mm), so I tightened everything, did a round of primary collimation (I might need a second round, don't think I nailed it 100%, still have a bit more curvature on one side, just by eyeballing subs), fixed secondary collimation after, and star shapes improved considerably.

I then checked it with 5 shots of NGC 6940 and CCDI, and still got results all over the place, so my conclusion is that CCDI is probably not reliable unless one uses high SNR stack instead of the subs, and even then only as a guide.

I've recently taken a shot of M27, and noticed rather funny shaped stars in the corners.

Scope in question is TS (GSO) RC 8", and star deformations are quite unexpected. Some being out of focus displaying astigmatism that I would expect out of RC due to field curvature, others are out of focus without apparent astigmatism (no ellipsis shape, still round with offset center), and some display sort of coma appearance (different corners have different aberrations).

Given that I Roddier tested that scope to 0.94 Strehl I think that optics are ok, which would leave camera tilt + collimation (one, the other or probably both). I decided to download trial version of CCD Inspector just to see what results it would give me.

Results gave me bit of a scare, to be honest. I ran test on 60 calibrated frames, each 1m long exposure, guiding was spot on (OAG, almost no shift between the frames, I think that largest shift when aligning was sub pixel), I did not use dithering, my guide RMS error was in range 0.5-0.8" (very good given HEQ5 and seeing), seeing was good to fair (FWHM measures between 1.9" and 3" with most of the frames being around 2.2") but results for curvature map differ quite significantly between the frames.

I've created a mosaic of curvature maps for first 25 frames (did not want all 60 due to size, mosaic is large as is), and results for individual frames fall in range:

Curvature: 47.7 - 62.5"

Tilt X: 0.2 - 0.7"

Tilt Y: -0.1 - 0.1"

Total tilt: 2% facing left - 34% facing right (with changing direction, and angle)

Collimation: 1.0" - 4.4"

Here is the mosaic to visually show how field curvature is dancing all over the place between the frames

Now the question is what does this all mean?

I would expect results to differ between the frames (don't know, but I would not be worried if values jumped around 5-10%), but range of values just seems too much.

Don't know robustness of CCDI algorithms, but it is possible that target is not suitable for analysis - very rich star field of milky way, it might be that better results can be obtained with in sparser areas of the sky (200-300 stars instead of thousands).

Or it might mean that something is seriously loose in my optical train and shifts with each guide correction?

Could the camera jumping around be the cause (better), or is it one of the mirrors (probably not so good situation)?

Can anyone please help with this?

Please include all precautions to account for carving of Larsen C ice shelf that just happened ....

1 hour ago, John said:

Sorry - I forget that the ED PRO series refractors from Skywatcher are also part of the Evostar range.

My Vixen ED102SS matches the Skywatcher ED100 F/9 that I owned before it for CA and SA correction. I'm not sure that all scopes of a similar spec would do that though - Vixen make very good objective lenses.

The ED100 Skywatchers are very close to Takahashi scopes of a similar spec. The TS 102 F/7 might get close but without trying one I don't know whether it would match the Skywatcher ED100 F/9

It's not just about the ED glass element of course but the figuring, coating, mounting of the objective and the match of the glass of the 2nd element that creates the overall performance.

 

No worries, I also had to double check if it really is branded Evostar.

I'm not hung up on whole FPL-51 vs FPL-53 (or FCD-1 / 100 for that matter ) thing, I just reckon that it is easier to properly figure lens of higher F/ratio (since we are discussing doublets here) by using glass with best index / characteristics (that being FPL-53, flourite would probably fare better, but I suspect price would match that). I simply do not know enough on topic to even know if ED100 level of correction (or very close to it) is possible in F/7 design using FPL-53 / lanthanum combination.

2 hours ago, Peter Drew said:

May I be the first to help you spend your money? . Tough one to call on paper, if the TS one is as good as they claim then I would go for that one to best suit your purposes/ The extra that the TS costs could soon be swallowed up by replacing the focuser on the Evostar if it proved necessary.

Yes, it's a tough one, isn't it? Out of listed "down sides", only one that actually makes any real impact is CA handling / optical quality, and no info on that apart what is written on TS website. I was hoping that someone would come along and offer their firsthand experience, but I realize now it's a slim chance, because it is relatively new product (have not seen it before listed, and I do scan items on their website from time to time), it certainly differs from already existing TS ED 102 F/7 with FPL-51 element and 2" dual speed crayford focuser (that one is same optics as Starwave 102 ED, possibly Lunt 102 ED)

2 minutes ago, John said:

Amongst my weaponry (Python - Spanish Inquisiton ?) I have an excellent Vixen ED102SS refractor that has an ED doublet lens that uses an FPL-51 (probably - Vixen didn't specify) ED element.

It's a really good all round instrument which can deliver low power views up to 3.8 true degrees in extent as well as handling magnifications of 220x when the seeing is good. While it's not totally colour free in the way that my Tak 100 F/9 Fluorite is, the Vixen's control of CA is far, far better than any Evostar achromat refractor that I've used, even the F/10 ones.

If the TS 102 F/7 can get that sort of performance it would make a good all round scope

I might not have been clear on that but achromatic refractors are not considered as contenders for this one, I was thinking of Skywatcher EVOSTAR ED100 Pro - F/9 ED doublet with FPL-53, and not achromat doublet one - at F/10. By all accounts ED version is virtually color free and excellent performer. There is, as I already mentioned, FPL-51 F/7 scope from several vendors, but from what I've read in terms of reviews on internet, it looks like it is easily beaten by ED100 pro on color correction and planetary performance.

I think that TS102 F/7 with FPL-53 should be ahead of any fairly good 4" F/7 with FPL-51, but that is solely based on my previous experience with TS80 apo (which is truly apo, and I've tested it to Strehl 0.98 in red, 0.94 in green and 0.8 in blue using Roddier test and OSC camera - probably not the best way to conduct such testing since I've used single focus position and real star subject to not the best seeing possible, and my Roddier skills are dubious at best, but I got assurances that any testing error would give worse results rather than falsely present better optical quality). The question is of course, is my assumption correct, and if this glass indeed has required optical quality to compete with ED100 PRO.

I plan to replace my ST102 F/5 with a decent all rounder for visual at some point (not so near future, but hopefully by the end of this year).

This is my list of "requirements / wishes":

- close to apo (ED) / apo performance, good optical quality, CA minimal to non existent for visual. ED doublet preferred to triplet due to price, weight and cool down reasons.

- 4" class (can go bigger, would not go smaller, so 100mm+)

- relatively light weight up to 5kg for ota, it will be mounted on AZ4 (can hold up to 6.8kg)

- Ok focuser, does not need to be anything special, good 2" crayford with fine focus will do.

- Capable up to x250 power on planets (with right eyepieces, plan to use TV N-zoom 3-6mm, 7mm TV delite for planetary role, would like to avoid barlow/powermate combos if possible) giving good sharp views

- Capable of up to 3 deg TFOV with right eyepiece (something in the ES 34/86 or ES 30/82 class eyepiece for that, depending on actual scope) - would like to fit most of M31 in the view, also other wide field targets

- High bang for the buck is a + of course.

- Shortish tube - it will be sort of travel / grab'n'go scope, but it does not need to be "airline travel" size, it needs to fit in medium car boot with room to spare.

Now, from all of the above, first / best option that comes into consideration is SW Evostar 100 ED, and so far it has been my first and only choice. But I recently stumbled onto this:

http://www.teleskop-express.de/shop/product_info.php/info/p9868_TS-Optics-PhotoLine-102mm-f-7-FPL-53-Doublet-Apo-with-2-5--Focuser.html

And that got me thinking which one of these two would be a better choice?

Compared to Evostar, TS 102 F/7 brings to the table:

- better focuser (I have TS80 Apo which I use for astrophoto, but would like to replace for visual with something more capable, it has the same 2.5" focuser and it is a good one).

- Shorter focal length - easier to go wide field, while still been able to go to x200+ power with mentioned planetary eyepieces

- I think that fit & finish will be overall better (don't have experience with evostar, only what I've read online, but C&C tube rings on my TS80, and retractable dew shield are rather good).

- overall shorter length - easier for transport.

- I might even use it for astro photography if it proves capable in that regard.

On the other side, there are couple of things that put it behind of evostar:

- heavier - 4.5kg vs 3.7kg (not sure if this is a real negative thing, mount will be able to carry it, it is not much heavier to carry around).

- more expensive (200e - that is about the amount of money for a decent 2" wide eyepiece)

- Being F/7 doublet, there is real concern that CA won't be handled as good as in evostar, although TS on their website state that it is color free (FPL53 + lanthanum element) and they even market this scope in their photoline brand - intended for astro photography.

- Unknown optical performance compared to tried and tested evostar. I do think it should be rather good scope optically (TS80 really is), but there simply are no reviews confirming this - at least I did not manage to find any. How well will it behave on planets?

A penny for your thoughts ...

Not sure about this, but I think that spherical would be a problem with mirror distance / spacing if they are collimated properly. If not, wrong spacing can amplify astigmatism which is caused by miscollimation.

Don't know if this is going to be much of a help, but I did collimation of my RC8" - visually. It only needed collimation of secondary (quite a bit) - but I managed to do it under 15 mins. I collimated on in focus star image by inspecting airy disk / first ring - until it was more/less perfectly concentric. Still haven't tried it on larger format sensor (4/3 is hopefully coming soon), but for 9mm circle it's perfect.

Should be normal - but only far from optical axis - it's signature of astigmatism (inherent in RC design). Depending on your sensor size it should be noticeable in the corners, but not near the center.

I'm by no means expert in either conducting nor interpreting Roddier test, but I did two of them in order to check optical quality of my imaging scopes.

First is TS (GSO) RC 8". Test was performed last summer, camera used was ZWO ASI 185mc, under real sky. Seeing was ok, but not great. I've heard that quality of test does depend on seeing, but in general obtained results are either that good or better - not worse.

This makes me very happy since Strehl for this scope comes out to be at least 0.94.

Test was conducted as per instruction manual for Win Roddier, and with use of that software (again, as I've said, I'm no expert so I followed the manual to the letter).

Here is result of the test (screenshot):

For the second test, conditions were not that great, a bit worse seeing than in case of the first test, this one was conducted in second half of October, 2016. This time I did deviate a bit from manual. I shot defocused star again with ASI185mc but used R, G and B channels of stacked frames to make three separate tests - one for each channel. OSC camera is probably not best choice for such work - I guess mono cam with filters would be better way to do it (there is some overlap in bayer response curves - over all three channels). Also I suspect that B result is most heavily impacted by not so good seeing, still I'm happy with results as well. Even in conditions of not so good seeing and with OSC camera, I got "diffraction limited" performance in Blue - Strehl 0.8. Green gave 0.94, and the Red was best with 0.98.

My main concern with this scope, given it has traveled quite a bit in the Post - it was sent to the wrong country, and I had to wait 3 weeks for it to return to Germany and then be shipped again to correct destination - was that of possible damage in transport. I was really pleased to see that collimation was spot on, both visually and in tests.

Here are results in Blue, Green and Red (again screenshots):

I actually wanted to do proper reviews for each scope, but unfortunately I did not have good weather / time to do extensive work with them - only couple of recording sessions with each - in less then satisfactory conditions.

From these tests and those few imaging sessions all I can say is that I'm really pleased with quality of the scopes.

All comments are welcome, and I hope someone will find this information useful.

Ok here it is:

1. original FOV orientation

2. After 180 around RA axes

3. After 180 around DEC axes

4. Actual line DEC rotates around - it's angled to what it should rotate about because RA and DEC are not 90degs to each other.

Diagram shows exaggerated angles just to make it obvious. So 1 - initial FOV orientation, 3 - final after flip - clearly FOV has rotated.

Just repeat diagrams with DEC not being 90degs to RA - it will show rotation.

1 minute ago, moise212 said:

If you have the offset and then you try to center to the same FOV, you need to rotate the axes. Hence the rotation.

You should be able to experiment this by pointing to horizon during the day and center on the same point, on both sides of the mount.

Well if you have proper polar alignment and RA and DEC are orthogonal (I edited my post above, I think it is plausible explanation for field rotation - RA and DEC not being perfect 90 degs), any offset will be corrected with motion in DEC and RA - these do not create FOV rotation.

39 minutes ago, ChrisLX200 said:

another is if the DEC axis itself is not orthogonal (perfectly @ 90deg) to the RA axis.

Yes, I see what you mean, still not sure if that would result in frame rotation. It might, but if we reason that 180deg rotation in each axis is flipping FOV (in either vertical or horizontal direction), not sure if it would lead to FOV rotation more or less than 180 degs. Pointing error will surely be present - GOTO would suffer no doubt. Center of FOV might end up in wrong place after flip - offset by some RA and DEC amount but not sure about rotation.

Lets think of it this way. Instead of having tube pointing only in one direction let's consider tube that is hollow and points forward and backward - empty OTA for sake of argument, we are looking at central line - line of sight of OTA both forward and backward. 180 deg rotation around any axes that crosses OTA central line will result in flip. Yes, indeed - if we flip on two axes that are not orthogonal we will end up with rotation that is not perfect 180. This is possible solution to problem.

So if RA is not orthogonal to DEC - frame rotation will happen!

This case is not even hard to imagine. On my HEQ5 when I did adjustment of backlash and general maintenance - at certain point in procedure DEC axis is adjusted only on one side of RA (next to the worm, the other side, down where weights bar is is held by bearing in place), by moving couple of millimeters back and forth - enough to deviate from 90 degrees.

Also - rotation of FOV for 180 degrees after flip is just apparent rotation - it's not real rotation so it can rotate a bit more or a bit less - its two times mirror image - once in vertical, once in horizontal - two flips combine to give same result as 180 deg rotation. This is due to 180 degs RA and DEC movement when performing meridian flip.

I gave it a bit more thought and here is my argument:

Properly polar aligned setup:

Any movement in either RA or DEC or combination of those will not introduce FOV rotation. Cone error (up down) is movement in RA. Sideways movement of OTA is DEC. So anything that can happen to scope is combination of RA and DEC if properly aligned. Except tube / focuser / camera rotation.

So possible causes of field rotation would be: improper clamping of tube - so it tilts left right (looking down the tube with mount being at the bottom) under gravity when lying on its side (just before and after meridian) - but I guess this is not case - easily spotted, one would feel ota being loose on a mount. Any kind of focuser rotation, camera rotation due to gravity and uneven distribution of weight in configurations west / east of pier.

And of course - polar alignment. We know that field rotation is problem for long exposure if polar alignment is not good. So to me this is obvious reason for field rotation. Mind you, although points before and after meridian flip are close on sky, they are 180 degs apart in both RA and DEC when meridian flip is performed.

Not sure that I'm convinced with this argument.

Cone error - in essence same thing as pointing to a different part of the sky. Only important in the terms - goto / mount / computer thinks that scope is pointing to a certain place, and the scope knows it's pointing somewhere else. After meridian flip, frame will be rotated 180 degrees and point where scope is pointing will be "rotated" 180 degrees - meaning it will flip cone error to other side - it will not contribute to FOV rotation - only offset - this is corrected with RA/DEC offset (which does not rotate FOV).

Orthogonality of camera being X/Y orientation to RA/DEC? - no impact on meridian flip as it rotates FOV by 180 no matter how it's oriented - if it's 30 degs to "horizontal" (being RA) - after 180 deg rotation it will continue to be 30 degs to horizontal.

So it has nothing to do with actual exposures, its just scope orientation pre/post flip?

Yes, I'm thinking about small error, also depending on polar alignment error - it could be present on one side and almost none on the other side - look at two circles - place where they intersect - tangents are at the angle, further away from that point tangents are more and more parallel - 90 degs from intersection - tangents are fully parallel.

Contributing to this would be guide scope alignment. Are you guiding with guide scope or OAG? Also what is length of your exposure? If long, check for field rotation inside frames. Make sure that guide scope is aligned well with main ota. You can easily do this by slewing to a bright star and checking fovs of both cameras - imaging and guiding - star should be centered in both.

No, I was wrong, it can't be due to mount being tilted. Only way to get angle between frames is due to polar alignment, or lack of it. One way to explain it would be - misalignment is such that on east side circles are aligned almost ok, while on the other side they start do diverge.

Just a guess here, what would happen if mount is not perfectly level? if it's tilted to one side? could this be a cause for slight rotation of field pre/post meridian? If mount is a bit tilted to west or east then meridian line on sky and meridian line of mount will be at a slight angle.

Now when I come to think of it, yes, this might be a reason - sky / stars flip around sky meridian and sensor flips around mount meridian, on one side - east of meridian - consider them aligned, but on west side frame is rotated with angle 180 degs + difference between sky meridian and mount meridian. Well frame rotates 180 degrees but stars inside frame rotate a bit more. I made a mess out of explaining, let me try to make a diagram.

I think it is viable option for public outreach.

Take for example following setup:

Projector, white screen, fast system with matched cmos of new generation (low read noise order of ~2e) and appropriate software

Proposed setup:

F/4 newtonian 8" or 6", HEQ5, ZWO ASI185 / 224, laptop and software to autoguide while stacking short exposures and doing stacking and auto development in the background - I think most bright objects can be viewed in under a minute and even less - with exposures in range of 1-4s  (software takes exposure, corrects guiding for next exposure, aligns and stacks frame and does basic stretching to display faint parts) - yes it will be noisy in first couple of exposures but by the end of the first minute - I think one will have decent image - probably better looking than image at eyepiece to inexperienced observer without dark adaptation - and it can be shown to bunch of people at the same time.

All sorts of things come into play when choosing guiding setup.

It is not only focal length that counts, it is also the size of pixels both in imaging and guiding equipment. With increased focal length of guider scope you get more precise guiding provided your mount is up to it and the skies allow but trade offs are: Smaller field of view to pick guide stars from, longer exposure needed to get good SNR on guide star. Many people use off axis guiders that operate on same focal length as imaging train but use guider cameras with smaller pixels.

I have read somewhere that you should aim for 1/3 - 1/4 in arcsec/pixel ratio between imaging and guiding setup if using guide scope. For example if using scope of 800mm focal length and have same pixel size on both imaging and guiding camera you would like to stay above or equal to 200mm focal length for guide scope. You can guide 1000mm focal length scope with 160mm guider provided that your imaging camera has twice as large pixels as guide camera.

Sensitivity of guide camera and level of read noise also play part in this equation because of way that guide programs work.

I'm in favor of fast guide scope with large FOV. This can have benefit of having multiple guide stars to guide in the same time. Not sure if this is implemented in any of guiding programs but I feel that it would be of great help in beating the seeing - perturbation of star position due to seeing is rather local so if average drift across multiple stars is taken - one expect for seeing effects to be smaller.

According to ZWO, Gain of 350 should give you the least read noise for that model. Any gain setting above 200 is good - aim closer to 200 to get better dynamic range. Set exposure time as low as possible to get at least 50% (you can go lower than that but at expense of SNR), higher values are better (like already suggested 75% of histogram, provided you don't clip histogram or have too long exposure for given seeing). I usually leave gamma at 50 (it is only digital so no real benefit, and certainly so for planetary imaging), white balance at 50 for both blue and red (again no benefit since it is digital control and can be adjusted later after stacking). One thing to do as well - there is brightness setting (at least for 185mc, but I guess it is same for 224), when you decide on exposure length, cover telescope (as if taking dark frames) and look at the histogram. If it is clipping to the left, increase brightness setting. Minimum values of your dark frames should be just above 0. Do take at least 256 dark frames and process your recording in pipp (to do dark frame subtraction - this will remove both dark and bias signal).

For DSO there is no real benefit in going over 135 with Gain, so keep under. For shorter exposures stay above 60 for gain, for longer exposures you can go below 60 with gain. There seems to be a switch in read mode around gain value of 60 that has effect on read noise.

Last tip - don't use bias frames - take as many dark frames as you can at exactly same settings - without touching any of controls - just cover telescope with a cap.

It seems that these sony cmos sensors have some internal calibration that is applied whenever you change either gain or exposure length.