Forum Replies Created
-
Posts
-
Tony,
My comment to Steve was in relation to his f/7 apo refractor. For an f/10 SCT, I think you will definitely need a focal reducer with the Alpy.
The issue of focal length and positioning of various makes of focal reducers is a bit of a minefield. I have seen several figures quoted for the focal length of the Meade 0.63x focal reducer and even that there are two versions on the market, one made in Japan and one in China with different focal lengths. I have no idea if that is true. In my experience, the best approach is to believe nothing and measure it yourself. To get a reasonably accurate value, put the focal reducer at the back of any scope, point it at a distant object on a bright day and measure the distance from the position of the lens in the focal reducer to the point where the image is sharp. Call this F2. Do the same without the focal reducer and measure the distance from where the lens was in the focal reducer was to the new focus. Call this F1. It will be larger than F2. The focal length of the focal reducer is then FL=(F1*F2)/(F1-F2). If you don’t get a sharp image without the focal reducer you may need to rack in the focuser a bit and try again.
The focal reduction factor RF you have just measured is F2/F1. It will vary as you rack the focal reducer in and out. Every focal reducer is designed to operate optimally at the position where the reduction factor RF is the quoted value, 0.63 in the case of Meade. If the measured focal length is FL, the optimum distance from the lens in the focal reducer to the image plane, F2, in this case the slit of the Alpy, is given by F2=FL*(1-RF) = 0.37*FL. You may need to insert spacers to get as close as possible to this optimum spacing.
As a general comment, you want to keep the Alpy as close to the scope as possibly to reduce the possibility of flexure so go with the focal reducer with the shortest focal length provided there is enough space between the focal reducer lens and the Alpy slit to achieve the optimum spacing. Needless to say you should also use a high quality achromatic focal reducer in a situation like this.
Hope this makes sense. It is based on my experience and has worked for me.
David
Hi Steve,
My inclination would be to run your setup without a focal reducer to avoid adding more chromatic aberration and see how that works in practice. That also avoids the problem of having to get the spacing just right for the focal reducer. As a general rule, I like to keep things as simple as possible consistent with getting good results.
David
Hi David,
I also have an SX H694 and run this at -20C all year round. It seems to hold that temperature within a few tenths of a degree, even on warm summer nights.
Regards,
David
Hi Robin,
Nice work. The results you are getting with your modified Alpy will make an interesting topic for discussion at the spectroscopy workshop next month. It would be interesting to discuss with Shelyak whether they will consider making this modification an option for customers.
Cheers,
David
Hi Steve,
It is going to be a full weekend of talks and practical sessions, hopefully with observing in the evening if the weather cooperates, so staying there would offer the best chance to be fully engaged. I will contact you directly to discuss.
David
Nice result. It illustrates well the extended response of the Alpy at the blue end of the spectrum.
David
Well done Robin. You are showing the way in spectroscopy – as usual!
To avoid any misunderstanding in the light of the current offer on the Alpy it is worth emphasising that you were using a non-standard Alpy which you had modified yourself rather than the one which is supplied by Shelyak. It would be interesting for people here if you could explain the difference in performance you achieved by exchanging the grism.
Thanks,
David
Hi Nick,
Once the Ridley Grant applications have been reviewed after the closing date we will supply Shelyak and the CCD camera suppliers with the names of those BAA members whose applications were approved and they will be able to order the equipment with the stated discounts. Does that answer your question?
David
Hi Tony,
Like Andy, I now use ISIS for processing my spectra including calibration and producing spectral profiles to send to the ARAS database. It does have a fairly steep learning curve but the effort is repaid many times when you discover the range of functions for working with spectra which Christian Buil has built into it. There are lots of “how to” pages on Christian’s website about use of ISIS although I have never found a simple index to where they all are. It’s a bit of a treasure hunt to find everything but he has written up most of its functionality somewhere. The originals are in French but quite a lot has been translated by either Christian or Robin Leadbeater and Google translate makes a passable job of the rest.
I still use VSPEC http://www.astrosurf.com/vdesnoux/ which is free and useful for quickly looking at and comparing spectra and for most of the basic processing functions. I do find it a bit prone to crashing but I used it a lot in my early days.
I have used Astroart http://www.msb-astroart.com/ for many years for controlling my cameras and this also has a very good slit guiding facility which will happily keep the target in the slit for long exposures.
I’ve just posted details on the Forum about the BAA scheme to help members acquire an Alpy spectroscope if you decide to go in that direction. All the Shelyak spectrographs will deliver good results, you really need to decide whether you are more interested in seeing the whole spectrum which you get with an Alpy or LISA (higher resolution but more expensive) or in studying specific lines at much higher resolution with the LHIRES.
Don’t worry about asking questions, that’s the way to find out!
David
Hi Steve,
This is not a BAA Observing Section in the normal sense as these are oriented towards the observation of particular objects or classes of objects. Rather it is an observing technique in the same way that photometry is a technique which can be used to observe any object. So I don’t see it being a BAA Section in its own right but rather we are trying to encourage its use in any of the Observing Sections where it can be practically useful.
There will be an announcement in the April Journal of an initiative to help members who are interested in getting into spectroscopy.
David
