Giles_B

I don't own a SW dob - I own an Orion Optics one which I have dismantled a few times - I'm struggling to understand what the retaining collar is on your OTA.

This could just be differences in build of the telescopes but it would help if you could you send a picture or link.

If it is a part of the mirror cell, there is a small amount of play in the retaining pieces that hold the mirror in place - they hold the mirror loosely in place, and a bit of side to side movement was possible. I understand this is necessary for thermal expansion of the glass, which would fit your conjecture.

Well this is embarrassing, I was out until 1:30am- third night of trying to focus my OAG and I finally did it - yay!

I woke this morning with my other half saying "did you see the lights? tons of Facebook photos of them in this part of Bristol"

Obvs I was rummaging through the T2 adaptors with a torch, not looking at the skies

I work at home more through necessity than choice, but wow, this really makes up for it. First time I've had the solar scope out this year. Amazing lunchtime sun-gazing!

Thanks for the heads up. Looks like full sun at lunchtime in Bristol

I think the "just one of everything" option is unfortunately  never quite achievable - I speak from experience having started with a 10" dob and tried and failed to adapt it to other uses. I haven't got the experience at astrophotography to choose between the refractors you've researched - they all look decent - and a 585MC will be a good camera to start out your journey with, but in the long term expect to end up with a few telescopes that are suited to different jobs.

This can be a bit overwhelming when you start out, but just take things slowly and don't expect to get everything in one fell swoop. Think of it like getting a really good toolkit (or a library if you're more cerebrally inclined) - you can't get the lot all at once, because you only know what gaps you need to fill through experience. But what you've planned will get you off to a great start!

Sadly I have a poor review to post of Optrep.

Bought for astrophotography, it became apparent that my 2nd hand Altair Wave EDF had a rather dirty objective only when I started mono-imaging galaxies on a 1600mm - closer examination showed the objective had a number of smears and smudges on the inner surfaces of the objective, as if someone had taken it apart to remove moisture with a not entirely clean cloth. The smudges were a nightmare to calibrate out with flats so I finally determined the best course was to clean the inner surfaces.

Now this is an FP53 refractor and I didn't fancy my own skills at dismantling the lens and doing a better job than had been done previously. However it took some solid emailing around to find anyone who was willing to do maintenance work on telescope objectives. Most of the key recommendations turned out to have given up the maintenance game. One name that did begin to turn up regularly as a recommendation was Optrep of Selsey. Their website looked like it hadn't been updated for a time, but suggested they might be up to the job. I emailed but received no response. Finally, resolved to do the work myself but then decided what was to lose by phonng Optrep. The phone was answered straight away and the chap on the phone said they could take on the work of dismantling and cleaning the objective for £100 - not cheap, but for a £1500 scope, it seemed worthwhile.

At first things seemed to go well, even if things were a bit old school. I packaged the objective double boxed and sent, as instructed, via Royal Mail Special Delivery. The parcel arrived fine and a few days later I received an invoice for the work by post, which I paid immediately on a Saturday. I expected the objective back early in the week, but time went by a bit. On Thursday I telephoned again to ask if I should be concerned but the chap on the phone said the parcel should be with me that day, although they could not find the tracking number. Okay, I said, and sat tight. The parcel arrived later that day. Phew!

It was not until this afternoon that I opened the parcel to inspect the work. This was disappointing to say the least. The lens had certainly been opened, but this was not the professional job I'd expected. There was plenty of dust visible between the objectives, a fresh part-fingerprint, and a total of five scratches, one at the edge which looked like the lens spanner might have slipped, two short, gossamer sleeks in parallel in the centre of the lens, and two, moderately deep one inch scratches slightly off the centre.

I feel so disappointed, and I'm really not sure what to do. I am so angry I don't think a telephone call would be constructive until I calm down. I don't expect an email to be answered. What do I want? I could ask for a refund, I don't know if it would be forthcoming but really, £100 is not here or there and would by no way compensate for the damage to such a beautiful lens. The only thing I feel resolved to do is let others know my experience. I really would have been so much better off doing this job myself. Some belated research looking for negative testimonies has led me to complaints about a related company "Kay Optical" on a BirdForum, which suggest the proprietor had been unwell and the company was going downhill. It's a shame I didn't research better as this would have given me pause. I'd certainly warn anyone considering using Optrep for telescope work to give it a wide berth.

Lovely report, thanks for sharing your first steps with the hobby

2 hours ago, ollypenrice said:

If your primary objective is to increase FOV then you would be making proper use of a focal reducer.

If you want a better SNR at a given focal length then you can either increase the aperture (by changing the scope, since Aperture Increasers do not exist ) or you can put the object photons you have onto fewer 'Effective Pixels' by binning or resampling.  If you are over sampled, doing the latter comes at zero cost in terms of resolution.

But try taking your 3 focal length images at the linear stage and resample the two longer focal lengths till Thor's Helmet is the same size as it is in the shortest, then compare them as you stretch them.  This is a meaningful comparison and cuts to the heart of the matter.

The reason that talk of F ratio doesn't dissolve into nonsense in the daytime photography world is that, in that world, the pixel size and focal length are constant and only the aperture varies. 

Olly

Thanks Olly.

So, my main object since about February has been to take images of galaxies. I've been doing this at 0.8x because my understanding was that f7.8 would make my targets too dim, even though the FOV would be better for objects of the size of M101 etc.

However from what you say, I've been misled and as a rule should always be guided by the FOV and not get hung up on the focal ratio - ultimately I'll get the same photons and a better image at the higher f ratios.

Scope is getting a clean at Optrep at the mo (filthy objective , long story) - but it sounds like I have a new project when it comes back!

10 minutes ago, ollypenrice said:

I think it's all about maximizing the useful object photons.  If you want object photons from a wider object then the focal reducer makes perfect sense because it captures them when the narrower FOV doesn't and would require a time-consuming mosaic.

But if the object fits on the chip with and without the reducer, the reducer brings in precisely zero new photons. What matters now is what you do with them.

Olly

 

That's a helpful exchange for me. Both points of view seem to make sense, even if you disagree. My primary objective is to increase the FOV, but it's interesting to think about increasing the SNR - I'll be interested to see if I get less noisy images as a result, although I'm unlikely to spend enough time looking at the same object with different set ups to prove this one way or another.

Just now, Adam J said:

I generally think that 0.6x reducers only work really well on small chip cameras like a IMX533 or smaller. 

Adam

Can you explain why - because of the small image circle? square stars? pixel size? Or just personal preference because of all of the above?

Focuser should do it - the reducer is designed to fit a 2" and has a T2 connector, and requires 55mm of backspacing thereafter - so all eminently doable. Square stars we can fix , but my worry was that the image circle being much too small to be useful, but it sounds like this won't be a dramatic problem - or at least it's worth a go to find out.

Thanks - yes, definitely moving in the direction of a punt!

hmmm, that's tempting. Not sure if I'd obsessed with round stars - maybe if I'd noticed them... I suppose I could always create synthetic stars on Siril if it really got to me.

Is there any formula that will give me my image size based on my aperture and focal ratio?

I have an f7.8 Altair Wave 125EDF which I combine with the recommended 0.8 reducer for astrophotography with a ZWO 1600mm - this gives me a focal ratio of 6.24

Today a Altair Lightwave 0.6 reducer has popped up on ABS, and I'm tempted to buy it at the price so I have another focal length (a fairly fast f4.6) to play with. But I'm concerned the results might be disappointing - I expect that there will be some vignetting due to the increased reduction factor (is this what you call it?) but will this be a lot given my current image train, and are there any other problems I should be alert to?

I'd be grateful for thoughts and advice.

As a fellow beginner, problems with guiding sound very familiar to me

Mine have stemmed from -

PA being out (from the mount being knocked - who knew this could lead to huge errors...)

RA being unbalanced (due to change of target / scope orientation revealing Z-axis problems...)

Calibration (I now routinely calibrate at the start of guiding)

On 21/04/2024 at 22:47, adyj1 said:

If you do manage to achieve this I suggest keeping it to yourself. Robert Brown has had many requests to provide ASIAIR compatibility for myFocuserPro, but has not done so because he says he'd have ZWO lawyers on the phone... 

That's a cheek - the Asiair is reputedly full of uncredited open-source code!

Wow that looks great!

Or wifi VR goggles 😁

Yes, so nice to have these clear nights - it will be the 3rd in a row I'm out. And more clear sky on Friday and Saturday, although I won't be able to do much galaxy imaging with the full moon...

I agree it is really interesting the connection I can form with "my" data. I'm just getting into imaging having come by way of a SeeStar, and ended up with a set up based around the ASiair. The fact that the ASiair will capture the data according to a programme means I don't have to sit at the telescope for long once everything is up and running, which really suits my family life and means the telescope is getting lots more notional time under the stars, despite the awful UK cloud. But I find the process of setting up and taking down the equipment  makes me feel really connected to the data I'm capturing (most of it not premium data at all thanks to me being such a novice!), and i get much more out of it than using the Seestar (which really felt much too easy with no real set up needed).

I have philosophical training and I've always though John Locke's theory of property rights - that things become "ours" because we "mix" our labour with them - was pretty silly, but now I am actually starting to wonder if this is what is happening with astronomy data! 

Thanks, interesting to hear about the 5Ghz WiFi issue.

For the record, the Lindy cables seem to have done the trick for my set up.

1 hour ago, Clarkey said:

Have you watched the Robin Glover video about AP. Well worth a look if you haven't.

 

Thanks - I hadn't watched it but will do so tonight

Many thanks. As you say, the exposure time does seem very small. I'd previously been experimenting with 20s L exposure and 40s RGB, but thought I should try to be a bit more scientific. The experiments are a bit rubbish as tracking was off, but they should still be useful to check the histograms and see if I can find a good compromise between the science and the practice!

I'm delving into the dark arts of LRGB imaging with a 1600MM Pro and trying to work out optimal exposure times, guided by this post https://www.dpreview.com/forums/thread/4377640

Having taken some test shots I've determined my SQM=18.5 [Bortle 7]

Using the calculator linked to in the post, I've determined my sky electron rate in e/pixels/s for LRGB

L filter = Sky Electron Rate 8.99 e/pixel/s
RGB = 3 e/pixels/s

However, when I look at the technical data for the 1600MM I get either a base read noise of 1.2e or a graph that plots read noise in e-rms against gain (below). I can't work out how to relate the e/pixels/s number to e-rms, and the more I read the more confused I'm getting - could someone walk me through an answer?

Thanks John, I'll give that a try