billyharris72

Not off putting at all - good to learn that AAVSO are able to produce sequences other than those produced by the web tool. My understanding of comparative photometry is also that you don't need to correct for extinction, as you say. But how big a field can you get away with this over? I've seen conflicting accounts, and I suppose it depends on how much accuracy is needed. I've always assumed that accuracy would suffer badly above maybe a one degree field (one degree at 45 degrees alt I think would increase air mass by a couple of percent or so).... It's funny how discussing things with someone can clarify your thinking. I've just realised that I'd assumed this equated to a couple of percent error in the magnitude calculation, when in fact it's a couple of percent error in the extinction, which will be so much lower. So my ideas above may be a solution to a "problem" that doesn't exist. Right- where's that V filter? Billy.

Hi all: Just had a couple of ideas as regards doing some variable star photometry and wanted to get some input from someone with a bit of experience. Idea 1. A regression based photometric model that includes a term for altitude. I've experimented with doing comparative photometry using regression (linear regression on log transformed ADU), doing the photometric measurement in AstroArt and importing the data into R for modelling. That seems to work well, but I was wondering if it's possible to extend the field of view restrictions. My idea was just to use the RA and DEC data from the AstroArt output with Julian date from the FITS header and my Latitude and Longitude and use that to calculate Alt for each object. Then feed the log transformed cosecant of alt into the regression along with ADU in order to account for atmospheric extinction and thus (potentially) widen the usable field. In principle this might work, but my concerns are: a) This will do nothing to manage differential extinction at different wavelengths. How important is that relative to absolute extinction? Would this allow me to work with comparatively distant comparisons (say a degree or so) or is there no real gain. b) How many comparison stars would this need to have a chance of being an improvement on a single variable model? With small numbers of comparison stars I expect it would be much worse due to correlations between the two independent variables - how many comparison stars would be needed before regular tests for multicollinearity would be valid? (I think this one might be the clincher and could scupper the whole idea). Idea 2. Using regression to find variable stars (which might possibly allow enough check stars to make idea 1 work). One issue with differential photometry is obviously finding "proper" photometric comparison stars in a narrow field (the vast majority of variables in the AAVSO database, for example, don't seem to meet this criterion). That set me to wondering what would happen if we allowed any star not clearly flagged as a variable to be a comparison star and managed the error statistically (i.e. by picking maybe 200 stars to build our regression model on). Even if many of the stars had some intrinsic variability, the sheer number of stars would presumably mean the effects on the model should be very minor (we'd have to use diagnostics to manage stars with high leverage). Assuming that the outputs might not be good enough for accurate reporting of magnitudes, I then thought of the following. Say I grabbed a few hundred images of the same field over a year or so, ran photometry as in idea 1 above (with or without altitude as a variable) and then grabbed the residuals from each model and pasted them row-wise into a matrix, could the column in this matrix then be used to identify variable stars and provide reasonable approximate photometry for them? If that worked then a nice compact target like an open cluster could presumably be searched for variables without a huge degree of difficulty. Both just ideas, speculative at the moment, but would be interesting to know what people think. Cheers, Billy.

I'd avoid ebay if possible - lots of good bargains on there, but also the possibility of getting something that's not great. This place is good for second hand - everyone sort of "knows" everyone else and plays fair, so what you'll get will be honestly as described. I've bought a couple of things on UK AstroBuySell and never had a problem - tend to buy only things with photos and make a point of checking the pictures (if they don't have a shot of the objective then I'd email them and ask for one). Generally if you ask people about the item they'll give you an honest answer. If in doubt, you could try to arrange to collect rather than pay and post. That gives you a chance to check the glass and the focuser and make sure everything is in order. Hope this helps. Billy.

That's completely reasonable- Maks are great specialist high magnification scopes but there are better all rounders. A second hand ED80 (or ED72) will do a pretty good job on just about everything. Best of luck in tracking one down. Between here and Astro Buy sell you shouldn't have to wait too long. Billy.

Given that you have said you are interested in lunar and high magnification terrestrial viewing, I think the ED is the only one on your list worth considering. The Startravel range are very much designed for low magnification, wide field viewing. Really bright targets (like the Moon) and high magnification (above about x60 or so) don't bring out the best in it (I'm being kind here). For your stated goals, if you've decided you really don't like the Mak, you'd probably be best looking at either a long refractor (about f10 or so) or an ED type scope. The latter, while more expensive, will give you both wide fields, low CA and high magnification performance in a portable format. I'm intrigued as to what you didn't like about the Mak. Was it just the relatively narrow field? Billy.

Hi all: This is going to sound like a silly question (as I think I know the answer) but I thought I'd sample the collective wisdom on here before I risk another night's frustration (too few clear nights for experimentation without at least checking I'm on the right track)! I've recently purchased the Astronomik LRGB filter (31mm) set as an alternative to the ZWO filters. I'[m using these on an ASI1600. These ZWO filters had been giving me problems - lots of scatter on the blue, and big halos with the Ha filter. I've used the new Ha filter, and despite being 12nm I have to say I much prefer it to the ZWO option - stars are nice and tight with no hint of the ugly halos that the ZWOs were giving me (though there is a tad less contrast). In operation, I've found the Astronomik filters to be fine - no major noticeable issues in the field. Using the focus assist on both Sharpcap and AstroArt I was getting comparable star FWHM with all filters, and things seemed to be behaving well. I finally got round to processing some data (IC405) at the weekend, only to find absolutely horrible chromatic issues, with the stars having very clearly defined, dense, turqoise halos. I also noticed that AstroArt, when set to automatically estimate colour balance, was combining RGB in 3:1:1 ratios. In the individual RGB stacks, both green and blue had markedly larger FWHM values than the red, but also, the background level for red was only about a third of that for the green and blue. My working hypothesis is that this has to do with the response curve of the ASI1600 sensor, and that the larger FWHM may be due to relative over exposure causing the stars to bloat (does that happen like that)? With this in mind, does it make sense to reduce exposure times for blue and green, while increasing for red? Would that affect FWHM, and is getting these (and presumably the background levels) to match liekly to help with the colour fringing issues? Thoughts and advice appreciated. Thanks, Billy.

I agree with Peter - it's an aluminium rail, and it's going to get a lot more dinged up with use. I don't have a single scope where metal isn't showing on the dovetail and it's not a problem. The main thing is that there is no mechanical damage and the clamp grips the scope securely. Personally, I'd keep it, so long as it works. Billy.

This. I'd say all of the eyepieces you've listed there are respectable enough performers, especially on a slow scope like Mak. I have the 25mm Ex-Cel and the equivalent SW Plossl and there is not much between them (the Ex-Cel has a slightly wider field and is maybe a touch better at the edges, the Plossl has the edge for control of glare and stray light). If you're determined to spend some money, maybe increase the budget a little and go for the Explore Scientific 68 degree range. They're good value, and you might pick one up second hand in the right price range. I'd try to look through a wider eyepiece before you go above this, especially if you wear glasses. For me, seeing all of a sharp, well defined field stop in a single, comfortable view is an absolute must in an eyepiece. (If someone gave me an Ethos I'd sell it and buy some nice Plossls, because I genuinely prefer them). Others love them though, and I can understand why - it's personal taste. But at that end of the market I'd advise try before you buy. Billy.

Hi Geoff. I have the minitrack, but I'm not sure I would benefit from adding a polar scope. While the minitrack can image up to about 300mm, I think it's fair to say it's really designed for wider field use - the tracking (at least on mine) at 200mm+ is usable, but not great, and improved alignment won't help with that. If you're using, say, a 50mm lens then the sight tube is more than accurate enough. That said, i find it awkward to use. I do have a green laser that I use when aligning my main scope for AP, and find that shining that through the tube makes alignment much easier and more comfortable. Depending on the issue you're looking to solve that might be an option. Billy.

My take on this would be that you should get some benefits from the preprocessing, but not others, once you have scaled the images to match. Assuming you choose to upsample the binned images your calibration frames won't help with fixed pattern noise at the individual pixel level - you could try downsampling the luminance and see what happens. In terms of each type of frame. Flats - should be fine. I've seen people denoising or median blurring flats, and I don't see this causing problems (you wouldn't do it for scientific imaging obviously). You might lose a little accuracy, but it won't matter in practice. Darks and bias will have info on the bias and dark current level and distribution (e.g. if one side or corner has stronger unwanted signal), but might need to be scaled to your image depending on how it's been binned (if the ADU is the pixel average rather than sum then no need). I don't use Pixinsight but believe Pixel Math is what you want here if it's needed. If you bin the light frames you'll obviously lose resolution, but might be an option for plan B - this approach should allow the calibration frames to deal with hot pixels as well. Overall, maybe not the ideal workflow but we can only work with what we have. Experiment and see what you come up with. Billy.

Great post Dave - many thanks. I've been meaning to have another go at this with the ASI1600 but one thing that has been bugging me atmospheric correction. Do you have a view on how wide a field you can get away with in comparative photometry without doing corrections? I've seen some sites that suggest half a degree, but trying to find variables with decent range of comparison stars in a half degree field has proven tricky. My own attempted workings would seem to suggest that, for anything above 45 degrees, a field of two degrees would probably be okay, as the error would be only a few hundredths of a magnitude, but I could be wrong and have not seen this anywhere else. Thoughts appreciated. Billy.

Focal ratio will be relevant if you go down the frac route (and don't want spend a fortune) due to chromatic aberration. It won't matter much for a Newt. On axis an f5 Newt is pretty much as sharp as at f8 or f10 (collimation becomes more critical, but is still easy with smaller mirrors; one definite advantage of the Heritage is that you can look into the focuser while collimating, which makes things incredibly fast and easy). This would also be a another plus for the Skywatcher 150P Dob, as that's f8, and at that ratio collimation is not fussy at all. A really good refractor will beat either of the above scopes on double stars, but nothing within a £200 budget will match either. Billy.

Indeed - this would be even better if the budget could stretch. Somewhat brighter and better image is one thing, but the other thing to bear in mind is that it can sit directly on the ground (the 5 inchers tend to need a table, which is invariable awkward and less stable). You will sacrifice a bit of portability, but the 150P is not a heavy setup and is easily transported by car. As a backyard scope it would be a great choice.

Hi there. The open tube design might be an issue if there is a source if incident lighting nearby, like an awkwardly placed streetlight that is basically shining onto your mirror. If it's just a case of more "ambient" light pollution and washed out skies then it won't make any difference at all. In either case though, the Heritage 130 will do the business - it's easy enough to make a shroud to cover the open part of the tube (I used some black flexible foam sheeting from Hobbycraft, but anything opaque with a matt black finish will be fine. I think for the money there isn't a better scope out there. If you really want a closed tube, Bresser does a 5 inch Newt on a Dob base that looks pretty good, but is about £180, so a bit pricier. As to whether it's as good as the Heritage, I'd be surprised if it wasn't very similar indeed - 5 inch mirrors are not that hard to make and performance is unlikely to vary much. Billy.

Thanks Alan. I've email FLO - fingers crossed that will at least help. Billy.